The Basics of SARS-CoV-2 Transmission
(Previously An Introduction to SARS-CoV-2)
[Last Updated March 21, 2021]
The emergence of a novel coronavirus in late 2019, identified as SARS-CoV-2, has resulted in a global pandemic accompanied by an unprecedented public health response. This brief review of the properties of SARS-CoV-2 and how it is transmitted outlines some of the evidence that currently forms the basis of the evolving public health response. This document has been updated from previous versions published in April, July, and November 2020 (previously titled “An introduction to SARS-CoV-2), and January and March 2021 to reflect new findings and provide additional information about the virus that may be relevant to the public health response. The evidence presented below is based on current knowledge on dominant variants known to be circulating. As new evidence and new interpretations evolve, this document will continue to be updated.
SARS-CoV-2 genomics and emerging variants
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the coronavirus responsible for the illness COVID-19. Coronaviruses are genetically distinct from viruses that cause influenza. They are enveloped, single-stranded RNA viruses whose surface is covered by a halo of protein spikes, or “corona.” Other coronaviruses that have caused significant and lethal outbreaks in the past 20 years include SARS-CoV-1 and MERS-CoV that caused SARS and Middle East respiratory syndrome (MERS), respectively. Phylogenetic (evolutionary) analysis has helped to establish that SARS-CoV-2 emerged in the human population in November 2019. Since then, continued analysis of the genome in COVID-19 cases from around the world has identified small mutations that can be used to track the evolution of the virus.
The rate of mutation observed for SARS-CoV-2 is significantly lower than influenza, but similar to other coronaviruses.1-3 Thousands of mutations in the SARS-CoV-2 genome have emerged over the course of the pandemic, with the dominant variants shifting over time. Monitoring has been ongoing to inform how variants are spreading geographically and whether emerging variants are a cause for concern.4-8 The variants that are cause for most concern may:
- spread more quickly,
- evade natural or vaccine-related immunity,
- cause more severe disease,
- evade detection by available tests, or
- are less responsive to treatment.9
At the time of writing, three variants of concern, which all emerged in the latter half of 2020 and commonly referred to as B.1.1.7, B.1.351 and P.1. are being closely monitored by public health officials around the world.10
The B1.1.7 lineage, initially detected in the UK in the late summer of 2020 has become dominant in the UK, and is widespread in much of Europe and the US. As of March 16, 2021 close to 4000 cases had been detected in Canada.11 B.1.1.7 is more transmissible than previously circulating variants and there is evidence of an increased rate of mortality.12,13 The increased transmissibility is believed to be due to mutations to the receptor binding domain of the spike protein that makes it attach more readily to host cells.10,14,15 There is also emerging evidence that persons infected with the variant may carry a higher viral load and are infectious for longer.16-18 The deletion or replacement of certain genes in B.1.1.7 are believed to be responsible for the failure of certain types of test kits in detecting positive cases.
The B.1.351 lineage, initially detected in South Africa at the beginning of October 2020, has become dominant in that country and has also been found in other countries.19,20 As of March 16, 2021, 238 cases had been detected in Canada.11 Similar to B.1.17, B.1.351 shares a mutation (N501Y) that is thought to confer greater transmissibility compared to previously circulating variants.21,22 Further study is needed to understand whether the variant causes more severe disease or increased mortality, but B.1.351 appears to be more resistant to neutralization by antibodies, inferring that natural or vaccine induced immunity may be less effective against this variant.23,24 A recent study found that immunization with the Pfizer and Moderna mRNA vaccines confers less immunity against B.1.351 compared to earlier variants. B.1.351 does not share the gene deletions of B1.1.7 and therefore seems to be more readily detected by most tests.10
The P.1 lineage, first reported in Brazil in December 2020, and soon after identified in Brazilian travellers in Japan, has spread rapidly throughout South America and has been detected in Europe, the UK, the US, Canada and elsewhere. As of March 16, 2021, 71 cases had been detected in Canada.11 P.1 also shares the N501Y mutation with B.1.17 and B.1.351 that is thought to confer greater transmissibility and P.1. also shares a mutation (E484K) with B.1.351 that is thought to be associated with evading natural or vaccine induced immunity.9,24,25 Further study is needed to assess the impact on disease severity and mortality.
Two other variants of concern have been identified in the US (B.1.427 and B.1.429), which are estimated to be about 20% more transmissible compared to previous variants. There area also three variants of interest (B.1.526, B.1.525 and P.2) being monitored for signs they may become variants of concern.9
Research is ongoing to understand how the evolution of the virus in different geographies is affecting transmissibility, severity of disease, ability to evade detection, or vaccine-induced immunity, or susceptibility to therapeutic treatment.3-8,14 Relating genomic variants to health and epidemiological data can help to inform the public health response, vaccine development and the design of therapies.1,5 The potential for increased transmissibility emphasizes the need for increased compliance with existing public health measures such as mask-wearing, physical distancing and limits to social gatherings.
Symptoms and severity of disease
The most frequently reported symptoms of COVID-19 can include new or worsening cough, fever or temperature ≥ 38°C, shortness of breath or difficulty breathing, fatigue or weakness, loss of appetite and loss of smell and/or taste.26 Less frequently reported symptoms can include sore throat, body aches, dizziness, headache, nausea, vomiting or diarrhea. In some severe cases, the disease can result in lethal pneumonia.26-28 Among children, the reported symptoms are similar to those of adults but may be less severe, and abdominal symptoms and skin changes or rash may be more commonly reported.27 The severity of disease and range of symptoms can vary from person to person, with some people experiencing no symptoms or very mild symptoms.26,28 Some more serious manifestations of illness may be due to an immune response to SARS-CoV-2 rather than due to infection. In some patients an intense immune response results in a hyper-inflammatory reaction that can result in more severe outcomes.29,30 The elderly, the obese, smokers, and immunosuppressed persons and those with pre-existing conditions including diabetes, hypertension, heart disease, or cancer are at the greatest risk of requiring hospitalization or dying from COVID-19.31-33 Persons with conditions that involve multiple comorbidities, such as Down syndrome, may be at heightened risk of COVID-19 related hospitalization or death.34 Some groups may also be disproportionately affected by COVID-19 as a result of existing health inequities related to socioeconomic factors.35
As of March 9, 2021, about 7.7% of persons with COVID-19 in Canada have required hospitalization, of whom about 17.6% required admission to intensive care and 3.2% required mechanical ventilation.11 This is a reduction in the proportion of cases requiring hospitalization or admission to ICU compared to earlier in the pandemic. Research is ongoing to help explain the relationship between the viral load (the quantity of viral particles per unit of bodily fluid in the infected person) and severity of disease.36 Patients with a higher viral load appear to experience more severe symptoms and shed more virus over a longer timeframe than mild cases37 and a higher viral load has been found to be associated with a higher rate of mortality among COVID-19 patients.38
Duration of illness and long-term sequelae
The duration of illness ranges from about two weeks for mild cases to between three and six weeks in severe to critical cases. Long-term symptoms (sequelae) that persist beyond six weeks have been observed in some patients, with some referring to this as “long Covid”.39 Age, chronic health conditions and obesity have been found to be significant predictors of persistent symptoms and those who have been hospitalized may experience symptoms for longer.40,41 Persistent symptoms may include fatigue, cough, breathing difficulties, headache, joint pain, and many of the other common symptoms listed earlier, with the majority of those suffering long-term symptoms experiencing more than one.40,42 Some people have also suffered damage to the heart muscle, scarring of the alveoli, endocrinological and metabolic dysfunction, neurological effects, strokes, and seizures. 41,43,44
The case fatality rate for COVID-19 differs around the world and across Canada and relates in part to case identification and to local epidemiology. Not all cases are identified so incidence of diseases may be underestimated and the associated fatality rate overestimated. The case fatality rate for Canada as of March 16, 2021 was reported as about 2.5%, with most cases (67.6%) and fatalities (78.7%) occurring in Ontario and Quebec. The highest case fatality rates have been recorded in Nova Scotia (3.9%), Quebec (3.5%), Manitoba (2.8%) and Ontario (2.2%).11 No deaths have been recorded in P.E.I. or the Northwest Territories at the time of writing and only one in each of Nunavut and the Yukon.
COVID-19 in children
COVID-19 is less prevalent in children as compared to adults and children infected with SARS-CoV-2 experience less severe symptoms.45-49 Evidence also suggests that children may be more likely to be asymptomatic compared to infected adults, but further study is needed to understand transmission pathways in children.50 As of March 16, 2021, 17.1% of COVID-19 cases in Canada were persons 19 years of age and younger, accounting for just under 153,000 cases.11 Of these, there were 767 hospitalizations, 113 admissions to ICU and 6 deaths in this age group. Case data for various provinces indicate that the incidence of COVID-19 cases in children under 10 is lower than that of teens (11-19).51-53 Children under one year of age and with underlying conditions may experience more severe illness than other children, but the case fatality rate for children is much lower than for adults.48 In very rare cases, children with COVID-19 have developed pediatric multisystem inflammatory syndrome in children (MIS-C), which can include symptoms of fever and inflammation, and can affect cardiac, renal, respiratory hematologic, gastrointestinal, dermatologic, or neurological systems.54-57 Children who do experience more severe symptoms have shorter hospital stays, decreased requirement for mechanical ventilation, and decreased mortality compared to adults.54
Rate of transmission
The basic reproduction number for a contagious disease, or the R0 value, estimated at the beginning of an outbreak, indicates the number of secondary cases that can be infected by a primary case in a population with no underlying immunity, vaccine, or preventive measures. Where R0 is greater than 1, the number of infected persons is likely to increase. Over time, the effective reproductive number (Rt) changes as more people are infected and public health measures are implemented to contain the spread. Factors such as the emergence of new variants with different levels of transmissibility can affect the Rt. The goal of public health interventions is to bring the Rt below 1, which would indicate that the outbreak intensity is declining and will eventually die out.58 Monitoring the change in Rt can help to evaluate the effectiveness of public health measures.
For SARS-CoV-2 the preliminary World Health Organization estimate of R0 was 1.4-2.559 with subsequent research estimating the mean R0 at 3.28.60 This suggests that every primary case at the beginning of the outbreak could potentially infect about three others. The Rt is an average and can vary depending on the location and patterns of local transmission over time.61,62 The rate of transmission can also change with mutations in the genome and may be higher for emerging variants.61 Estimates of Rt can not easily account for secondary cases that are asymptomatic unless these cases have been detected in the population through widespread testing.63 The Rt for Canada near the beginning of the pandemic in March 2020 was estimated to be > 2. Following widespread public health measures to prevent transmission, the Rt dropped to < 1 from about the end of April to late June 2020, followed by fluctuations above and below 1, up to the end of July, with an upward trend from mid-August into the autumn, with the Rt remaining above 1.64 As of March 16, 2021, the Rt was stable or decreasing in most provinces and territories, with the exception of Alberta and Manitoba, where there are indications it is likely increasing.65
Routes of transmission
SARS-CoV-2 is thought to infect a host cell by binding to ACE-2 receptors that are present on tissues throughout the body including in epithelial cells of the airway, lungs, intestines, kidneys, and blood vessels, etc.29 The virus replicates predominantly in the tissues of the upper respiratory tract.66 SARS-CoV-2 is primarily transmitted via prolonged close contact with an infected person. The vast majority of COVID-19 outbreaks have taken place indoors and are most often associated with close contacts in the home environment, or other indoor spaces where there is a high density of people and an extended period of contact.67-70 Most transmission appears to be due to exposure to the respiratory droplets and aerosols of an infected person.71-78 Other routes (e.g., fomites) may be possible but are not considered to be major routes of transmission.
Transmission via respiratory emissions
Forceful respiratory actions such as coughing, and sneezing can produce a burst of droplets and aerosols that range in size and could present an exposure risk when near an infected person. Evidence from animal studies has shown that transmission due to close contact is likely to be more efficient than indirect transmission over longer distances.79-81 A susceptible person is more likely to encounter large droplets (e.g., > 5-10 µm in diameter) that have not fallen to the ground, or concentrated bursts of aerosols when in close proximity to the emitter.82,83 Large droplets are thought to travel less than 1 m before dropping to the ground, leading to the 2 m physical distancing practice that has been adopted for limiting the spread in the general public.78,84-86 Current evidence has shown that measures to protect against the spread of respiratory droplets, namely physical distancing and mask wearing, have led to a reduction in cases.87,88
Respiratory emissions produced by less forceful respiratory activities such as heavy breathing, speaking, singing, shouting, or laughing are mostly aerosols < 5 µm in diameter. Transmission via respiratory aerosols may be an important route of transmission.82,89-93 Aerosols can remain suspended in air for longer than large droplets and be transported over larger distances by ambient air currents.71,89,94 Under experimental conditions, SARS-CoV-2 has been found to remain viable when airborne over short distances for several hours and in field studies, viable virus has been isolated from air samples at distances greater than two metres from a COVID-19 patient.73,95,96 Transmission via respiratory aerosols could be occurring in settings where they accumulate in poorly ventilated indoor environments where there is a high density of people, and extended duration of contact, allowing for transmission beyond 2 m to occur.68,83,97 Control measures for this type of transmission may rely heavily on reducing crowding, reducing the duration of interactions in indoor spaces, and ensuring good ventilation.98,99
- See more from the NCCEH on transmission risks in different settings including Indoor spaces, Outdoor spaces, Multi-unit residential buildings, Choir or Performing arts settings, Encampments, Shared laundry facilities, Outdoor dining, Outdoor urban spaces, and Carpools.
- See more from the NCCEH on measures for reducing transmission risks including Masks, Face shields, Physical barriers, Air cleaning technologies, Ventilation and CO2 sensors
Transmission via contact with surfaces
Contact with contaminated surfaces (fomites) followed by touching of the eyes, mouth or nose is another possible mode of SARS-CoV-2 transmission, although it is not considered to be the main route. Fomites can become contaminated by deposition of droplets, aerosols, sputum, or feces, either directly or by cross-contamination by touching an object with contaminated hands. Surfaces that are frequently touched by many people (high-touch surfaces), such as door handles, or faucets may be more important in fomite transmission compared to objects or surfaces that are only touched incidentally and less frequently.
The risk of transmission through contact with fomites is not well understood and could depend on the initial concentration of viable virus, its viability on a specific surface over time, and the quantity of virus transferred through touching of the eyes, mouth, or nose. Several studies have measured the persistence of SARS-CoV-2 on common surfaces under experimental conditions.95,100-102 The virus appears to remain viable for longer periods (one to seven days or more) on smooth hard surfaces such as stainless steel, hard plastic, glass, and ceramics and for shorter periods (several hours to two days) on porous materials such as paper, cardboard, and textiles, although viability may be dependent on other factors such as temperature.95,100-107 Survival time on copper, aluminum, and zinc is low (a few hours).95,102 Experimental study of the persistence on skin found that SARS-CoV-2 remained stable on swine skin for up to 96 h at room temperature (22°C),104 whereas a study using human skin found that viable virus was only detected for up to 10 h at room temperature (25°C).108 There are fewer studies that have detected viable virus in real-world settings, where variation in environmental conditions such as temperature, ultraviolet radiation, and humidity can all affect viability.100,109-111 Observational studies have detected viral RNA on a wide range of surfaces in settings where persons with COVID-19 have been present, such as hospitals or quarantine rooms.112 Most of these studies did not attempt to culture virus, so it is not known whether viral detections represented sources of viable virus in many cases. Hand hygiene and routine cleaning and disinfection of surfaces reduces the likelihood of contact transmission.113-119
- See more from the NCCEH on Hand sanitizers, Cleaning and disinfection of household surfaces, Air and surface disinfection measures, use of Disinfectants and sanitizers in food premises, and Nanomaterials as disinfectants, Disinfectant chemical exposures and health effects
Transmission via feces
SARS-CoV-2 is shed via feces. Patients with more severe COVID-19 have higher concentrations of SARS-CoV-2 in their stool and viral particles can be detected in stool long after respiratory samples test negative.120,121 Several studies have identified the presence of SARS-CoV-2 RNA in feces, but only a few have identified viable virus.121-125 Viral RNA has also been detected in the toilets of COVID-19 patients but to date viable virus has not been detected.75-77 There is little evidence to suggest that transmission via the fecal-oral pathway (e.g., passing in fecal particles from one person to the mouth, or fecal contamination of food) is significant in the current pandemic. Fecal aerosol transmission is implicated in a COVID-19 cluster in a high-rise in Guangzhou, China and exposure to sewage is implicated in an outbreak in an urban community with poor sanitation services, also in Guangzhou, China, but neither investigation could be definitive that fecal-oral transmission had occurred.126,127 Transmission through bathroom vents was also implied in a cluster of 10 cases in an apartment building in Seoul, South Korea, although it is not possible to conclude that transmission was associated with fecal aerosols.128
- See more from the NCCEH on Public washrooms in the time of COVID-19 and Wastewater based epidemiology
Like SARS and MERS, the SARS-CoV-2 virus is thought to have originated in bats, but may have had an intermediate mammalian host prior to transfer to humans, although the source of introduction into humans is still unknown.129,130 Experimental studies have shown that several mammal species including ferrets, cats, and dogs, can become infected with SARS-CoV-2, and the virus has been detected in some companion animals, zoo animals, and farmed mink.130-133 Evidence of transmission of SARS-CoV-2 from animals to humans is scarce. Transmission of SARS-CoV-2 from humans to animals and back to humans has been reported on mink farms in the Netherlands and Denmark, resulting in widespread culling of farmed mink.134 Mink farms in Spain, Sweden, Italy, Canada, and the US have also been affected by COVID-19 outbreaks.135 Between June and November 2020, 214 cases of COVID-19 in humans in Denmark were found to be associated with farmed mink. Twelve of these cases, identified on November 5, were found to have a unique variant with a decreased sensitivity to neutralizing antibodies in humans, resulting in the planned culling of the entire mink population of Denmark, to prevent further spread of the variant to humans.136 In December 2020, a COVID-19 outbreak was declared at a mink farm in B.C., with seventeen human cases associated with the outbreak. Subsequent genetic analysis indicated that the virus had been transmitted from humans to animals, but not from animals back to humans.137,138 Continued identification and surveillance of cases of zoonotic transmission is ongoing around the world to understand transmission pathways and the risk to humans.
- See more from the NCCEH on SARS-CoV-2 and mink
Other routes of transmission
To date there is no evidence to suggest that are there are primary routes of transmission other than those discussed above. Conjunctival transmission through the eyes or tears and vertical transmission (from a mother to a fetus) may occur but are likely to be uncommon.139-141 Food-borne transmission, sexual transmission, and transmission via other bodily fluids including blood, urine, breast milk, are unlikely to be occurring.130,142,143
While the precise dose of SARS-CoV-2 required to cause an infection is still unknown, findings from animals studies and modelling experiments have narrowed estimates of a median dose to between about 10 and 1000 viral particles.130 This suggests that the minimum infectious dose may be slightly higher than SARS-CoV-1 and lower than Middle East Respiratory Syndrome (MERS), e.g., approximately a few hundred viral particles.79,81,144-147 Human challenge trials are due to begin in the UK in 2021 to determine the minimum dose needed to cause infection.148 The efficiency of viral transmission during exposure can be affected by the number of infectious viral particles inhaled and the duration of exposure for a secondary case. Infection may occur due to a short but intense dose of infectious virus or following prolonged or repeated exposure to a smaller dose. Modeling by Goyal et al. reported that exposure to an infected person with a viral load of < 105 SARS-CoV-2 RNA copies is unlikely to result in transmission, compared to exposure to a person shedding > 107 SARS-CoV-2 RNA copies, which is much more likely to result in infection.149 Increasing the viral load and the number of people an infected person has contact with, is likely to result in greater secondary transmission. Exposure to a higher dose can result from both the intensity and duration of contact with an infected person.150 Animal studies have indicated that infectious dose and subsequent distribution of the virus in the host may vary by the route of infection.130,139 There is also some evidence to suggest that severity of disease may be influenced by the magnitude of the inoculum (e.g., the number of infectious particles a person is exposed to via respiratory droplets, aerosols, or contact with fomites).150-152
Timing of transmission
An infected person can transmit the virus to others both before they show any symptoms (pre-symptomatic) and when they are symptomatic. Peak infectiousness is thought to occur about one day before symptom onset.130,153 The mean incubation period (time between exposure to the virus and the appearance of symptoms) has been estimated to be around five days,154,155 with modelling indicating a range of about two to 11 days (2.5th and 97.5th percentiles).156,157
Pre-symptomatic and asymptomatic transmission
The occurrence of pre-symptomatic transmission (during the incubation phase of an infected person) and asymptomatic transmission (transmission via an infected person who never displays symptoms) has been recorded throughout the pandemic in various locations around the world.69,158-163 Pre-symptomatic persons can potentially infect others one to about three days before symptom onset.153,162 For asymptomatic spread, the period of transmission is still being investigated.164,165 The precise incidence of pre-symptomatic and asymptomatic transmission and overall importance to the spread of the virus is still unknown but could be significant, with recent modelling by Johannsen et al. (2020) estimating that at least 50% of transmission could be from infected persons without symptoms.166-168 Persons who are not symptomatic may be less likely to transmit the virus via large respiratory droplets due to the absence of coughing and sneezing.161 Other routes of transmission, such as via smaller respiratory aerosols released during breathing, speaking, laughing or singing, may be more important for pre-symptomatic or asymptomatic transmission.88 Current evidence suggests that asymptomatic transmission is more likely to occur following prolonged close contact, such as in family settings where there may be exposure during shared meals, talking, and contact with shared common objects and surfaces.69,163,169,170
Current evidence suggests that while peak infectiousness occurs slightly before symptom onset, most transmission occurs during the symptomatic phase.169 The viral load has been measured to be highest soon after symptom onset in the early stages of the disease, when level of transmission may also be highest, and decreases about one week following the peak.29,171,172 Symptomatic persons could be transmitting the virus to others for days to several weeks after symptom onset, although most cases are not infectious beyond eight to ten days after symptom onset.66,157,173-176 In a limited number of severe to critical cases, infectious virus has been detected for > 30 days.173 As infection progresses, the quantity of virus contained in droplets and aerosols expelled by an infected person will vary by the viral load in various parts of the respiratory tract and the stage of the disease. In the early stages of the disease viral load is found to be higher in sputum than in the throat.165,171 Median viral loads have been found to be between 104 and 106 copies per mL of respiratory fluid with an average emitter releasing about 106 copies per ml, but levels up to 1011 copies per mL have been detected in some cases.66,171,172,177,178 Infected super-emitters who release a greater number of respiratory droplets could present a greater risk for transmitting the virus to others, particularly if they also carry a high viral load.179 Genomic sequencing has helped to identify that SARS-CoV-2 tends to spread in clusters rather than in a steady manner, and increasing evidence indicates that a few people can infect many others.61,180,181
Persons who have been infected with COVID-19 may continue to shed virus beyond the period of infectiousness and after symptoms have resolved.66,157,174 Persistent shedding of viral RNA may be responsible for some patients testing positive again after an apparent negative RNA test.174,182 Reinfection with SARS-CoV-2 is possible, and genomic analysis has been used to distinguish between persistent shedding due to the original infection, and the presence of a new infection, which has occurred in a small number of cases.183,184
Sensitivity of SARS-CoV-2 to environmental factors
Research is ongoing to understand how environmental conditions affect the persistence of SARS-CoV-2, with various studies investigating the effect of different levels of temperature, humidity, and ultraviolet light and combinations of different conditions.
Experiments have found that high temperatures are more effective for deactivating the SARS-CoV-2 virus, and the virus is more persistent at colder temperatures. Experiments using viral suspension found minimal reduction over 14 days at 4°C, but detected no viable particles after four days at 22°C, within one day at 37° C, less than 30 minutes at 56°C and less than five minutes at 70°C.100,185,186 Studies of persistence of SARS-CoV-2 on various surfaces (skin, currency and clothing) also found that the virus remained stable for much longer at 4°C compared to experiments at 22°C and 37°C.104 A study of persistence of SARS-CoV-2 in milk found that pasteurization temperatures of 56°C and 63°C for 30 minutes resulted in no viable virus. At colder temperatures no reduction was detected after 48 hours stored at 4°C, and only a minimal reduction after 48 hours stored at -30°C.187
Humidity may influence viral transmission by affecting how droplets move and their rate of decay, and can influence susceptibility of individuals to infection.188 Humid conditions can reduce evaporation of liquid contained in respiratory droplets, reducing aerosolization and allowing droplets to fall to the ground or settle on surfaces more readily. This could potentially increase the risk of fomite transmission if virus in deposited droplets remains viable. In contrast, warm dry environments could enhance evaporation of droplets, resulting in a greater number of aerosols being dispersed.189 Aerosol transmission may be more likely in very dry environments compared to very humid ones.190 Humidity may also affect the persistence of the virus, as demonstrated with other coronaviruses, with decreased viability as temperature and humidity increases and potential to remain infectious for longer under cool, dry conditions.185,188 This has been demonstrated in experimental studies of SARS-CoV-2 in aerosols and on surfaces but the effect may also vary depending on the UV index, with the importance of temperature and humidity decreasing as the UV index increases.191,192 Humidity can affect the susceptibility of respiratory systems to viral infection, with dry conditions reducing the effectiveness of the mucosal lining of the respiratory tract to prevent infection.188
- See more from the NCCEH on humidity in High humidity environments and the risk of COVID-19 transmission
Light/Ultraviolet (UV) irradiation
UV irradiation has been shown to reduce viral loads for respiratory viruses, including SARS-CoV-1 in clinical and other controlled settings.193,194 Germicidal effects can occur between 200-320 nm, which covers the range of UV produced by natural sunlight (UV-B, 280-320 nm) and UV produced by lamps for specific applications (UV-C, below 280 nm) Solar UV-B has been shown to provide a disinfectant effect under a high UV-index over a sustained period.195 Disinfection using UV-C is more efficient than UV-B, and UV-C has been shown to be effective for inactivation of double-stranded, enveloped RNA viruses.196-199 UV irradiation has also been proposed as a decontamination method for personal protective equipment (PPE) contaminated by SARS-CoV-2.111,200,201 Initial results suggest that UV treatment may be more effective on smooth surfaces such as steel as compared to fabrics or porous materials.202 The use of UV-C for disinfection carries some risk, as exposure to UV-C can be harmful to human skin and eyes.203 Further study is needed to determine the optimum dose needed for inactivation of SARS-CoV-2, and how UV-C could be safely applied in public settings.
- See more from the NCCEH on UV disinfection in COVID-19 in indoor environments – Air and surface disinfection measures and Air cleaning technologies for indoor spaces
The information provided in this Basics of SARS-CoV-2 Transmission is based on current understanding and interpretations of the literature at the time of writing. There are still many knowledge gaps in understanding aspects of transmission and progression of the disease that continue to be researched, including the impact of emerging variants on transmission patterns. As new evidence and interpretations emerge, this document will be updated. Additional COVID-19 related resources to support environmental health can be found on our Environmental Health Resources for the COVID-19 Pandemic topic page.
- Day T, Gandon S, Lion S, Otto SP. On the evolutionary epidemiology of SARS-CoV-2. Curr Biol. 2020;30(15):R849-R57. Available from: https://doi.org/10.1016/j.cub.2020.06.031.
- Johns Hopkins University and Medicine. SARS-CoV-2 genetics. Baltimore, MD: Johns Hopkins University and Medicine; 2020 Apr 16. Available from: https://www.centerforhealthsecurity.org/resources/COVID-19/COVID-19-fact-sheets/200128-nCoV-whitepaper.pdf.
- Bedford T, Neher R, Hadfield J, Hodcroft E, Sibley T, Huddleston J, et al. Nextstrain SARS-CoV-2 resources. 2020; Available from: https://nextstrain.org/sars-cov-2.
- Korber B, Fischer WM, Gnanakaran S, Yoon H, Theiler J, Abfalterer W, et al. Tracking changes in SARS-CoV-2 spike: evidence that D614G increases infectivity of the COVID-19 virus. Cell. 2020 Jul 3. Available from: https://doi.org/10.1016/j.cell.2020.06.043.
- Grubaugh ND, Hanage WP, Rasmussen AL. Making sense of mutation: what D614G means for the COVID-19 pandemic remains unclear. Cell. 2020 Jul 3. Available from: https://doi.org/10.1016/j.cell.2020.06.040.
- Hemarajata P. SARS-CoV-2 sequencing data: The devil is in the genomic detail. Am Soc Microbiol. 2020 Oct 28. Available from: https://asm.org/Articles/2020/October/SARS-CoV-2-Sequencing-Data-The-Devil-Is-in-the-Gen.
- Page AJ, Mather AE, Le Viet T, Meader EJ, Alikhan N-FJ, Kay GL, et al. Large scale sequencing of SARS-CoV-2 genomes from one region allows detailed epidemiology and enables local outbreak management. medRxiv. 2020 Nov 16. Available from: https://doi.org/10.1101/2020.09.28.20201475.
- Worobey M, Pekar J, Larsen BB, Nelson MI, Hill V, Joy JB, et al. The emergence of SARS-CoV-2 in Europe and North America. Science. 2020;370(6516):564-70. Available from: https://science.sciencemag.org/content/sci/370/6516/564.full.pdf.
- US Centers for Disease Control and Prevention. SARS-CoV-2 variants. Atlanta, GA: Department of Health and Human Services; 2021 Mar 16. Available from: https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/variant-surveillance/variant-info.html.
- US Centers for Disease Control. Emerging SARS-CoV-2 variants. Atlanta GA: US Department of Health & Human Services; 2021 Jan 28. Available from: https://www.cdc.gov/coronavirus/2019-ncov/more/science-and-research/scientific-brief-emerging-variants.html.
- Public Health Agency of Canada. Coronavirus disease 2019 (COVID-19): Epidemiology update. Ottawa, ON: PHAC; 2021 March 11. Available from: https://health-infobase.canada.ca/covid-19/epidemiological-summary-covid-19-cases.html.
- Horby P, Huntley C, Davies N, Edmunds J, Ferguson N, Medley G, et al. NERVTAG note on B.1.1.7 severity. London, UK: SAGE; 2021 Jan 21. Available from: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/961037/NERVTAG_note_on_B.1.1.7_severity_for_SAGE_77__1_.pdf.
- Challen R, Brooks-Pollock E, Read JM, Dyson L, Tsaneva-Atanasova K, Danon L. Risk of mortality in patients infected with SARS-CoV-2 variant of concern 202012/1: matched cohort study. BMJ. 2021;372:n579. Available from: https://doi.org/10.1136/bmj.n579.
- Hou YJ, Chiba S, Halfmann P, Ehre C, Kuroda M, Dinnon KH, et al. SARS-CoV-2 D614G variant exhibits efficient replication ex vivo and transmission in vivo. Science. 2020;370(6523):1464-8. Available from: https://doi.org/10.1126/science.abe8499.
- Volz E, Hill V, McCrone JT, Price A, Jorgensen D, O’Toole Á, et al. Evaluating the effects of SARS-CoV-2 spike mutation D614G on transmissibility and pathogenicity. Cell. 2021;184(1):64-75.e11. Available from: https://doi.org/10.1016/j.cell.2020.11.020.
- Laffeber C, de Koning K, Kanaar R, Lebbink JH. Experimental evidence for enhanced receptor binding by rapidly spreading SARS-CoV-2 variants. bioRxiv. 2021 Feb 22. Available from: https://doi.org/10.1101/2021.02.22.432357.
- Kissler SM, Fauver JR, Mack C, Tai CG, Breban MI, Watkins AE, et al. Densely sampled viral trajectories suggest longer duration of acute infection with B.1.1.7 variant relative to non-B.1.1.7 SARS-CoV-2. medRxiv. 2021 Feb 19. Available from: https://doi.org/10.1101/2021.02.16.21251535.
- Kidd M, Richter A, Best A, Cumley N, Mirza J, Percival B, et al. S-variant SARS-CoV-2 lineage B1.1.7 is associated with significantly higher viral loads in samples tested by ThermoFisher TaqPath RT-qPCR. J Infect Dis. 2021. Available from: https://doi.org/10.1093/infdis/jiab082.
- Tegally H, Wilkinson E, Giovanetti M, Iranzadeh A, Fonseca V, Giandhari J, et al. Emergence and rapid spread of a new severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) lineage with multiple spike mutations in South Africa. medRxiv. 2020 Dec 22. Available from: https://doi.org/10.1101/2020.12.21.20248640.
- Weisblum Y, Schmidt F, Zhang F, DaSilva J, Poston D, Lorenzi JC, et al. Escape from neutralizing antibodies by SARS-CoV-2 spike protein variants. elife. 2020 Oct 28;9:e61312. Available from: https://doi.org/10.7554/eLife.61312.
- Liu H, Zhang Q, Wei P, Chen Z, Aviszus K, Yang J, et al. The basis of a more contagious 501Y.V1 variant of SARS-COV-2. bioRxiv. 2021 Feb 2. Available from: https://doi.org/10.1101/2021.02.02.428884.
- Villoutreix BO, Calvez V, Marcelin A-G, Khatib A-M. In silico investigation of the new UK (B.1.1.7) and South African (501Y.V2) SARS-CoV-2 variants with a focus at the ACE2-Spike RBD interface. bioRxiv. 2021 Jan 24. Available from: https://doi.org/10.1101/2021.01.24.427939.
- Wang P, Nair M, Liu L, et al. Antibody resistance of SARS-CoV-2 variants B.1.351 and B.1.1.7. Nature. 2021 Mar 8. Available from: https://doi.org/10.1038/s41586-021-03398-2.
- Li Q, Nie J, Wu J, Zhang L, Ding R, Wang H, et al. No higher infectivity but immune escape of SARS-CoV-2 501Y.V2 variants. Cell. 2021 Feb 23. Available from: https://doi.org/10.1016/j.cell.2021.02.042.
- Public Health Ontario. COVID-19 P.1 variant of concern– what we know so far. Toronto, ON: Queen's Printer for Ontario; 2021 Feb 3. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/2021/02/wwksf-covid-19-p1-variant-of-concern.pdf?la=en.
- Public Health Agency of Canada. COVID-19 signs, symptoms and severity of disease: A clinician guide. Ottawa, ON: PHAC; 2020 Sep 18. Available from: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/guidance-documents/signs-symptoms-severity.html.
- Public Health Agency of Canada. Coronavirus disease (COVID-19): symptoms and treatment. Ottawa, ON: PHAC; 2021 [updated Jan 18]; Available from: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/symptoms.html.
- Harvard Health Publishing. COVID-19 basics. Symptoms, spread and other essential information about the new coronavirus and COVID-19. Boston, MA: Harvard Medical School; 2021 Jan 12. Available from: https://www.health.harvard.edu/diseases-and-conditions/covid-19-basics.
- Cevik M, Kuppalli K, Kindrachuk J, Peiris M. Virology, transmission, and pathogenesis of SARS-CoV-2. BMJ. 2020;371:m3862. Available from: https://doi.org/10.1136/bmj.m3862.
- Tang Y, Liu J, Zhang D, Xu Z, Ji J, Wen C. Cytokine storm in COVID-19: the current evidence and treatment strategies. Front Immunol. 2020;11:1708. Available from: https://doi.org/10.3389/fimmu.2020.01708.
- Engin AB, Engin ED, Engin A. Two important controversial risk factors in SARS-CoV-2 infection: obesity and smoking. Environ Toxicol Pharmacol. 2020 Aug;78:103411. Available from: https://doi.org/10.1016/j.etap.2020.103411.
- Jordan RE, Adab P. Who is most likely to be infected with SARS-CoV-2? Lancet Infect Dis. 2020 May 15. Available from: https://dx.doi.org/10.1016%2FS1473-3099(20)30395-9.
- Niedzwiedz CL, O’Donnell CA, Jani BD, Demou E, Ho FK, Celis-Morales C, et al. Ethnic and socioeconomic differences in SARS-CoV-2 infection: prospective cohort study using UK Biobank. BMC Med. 2020;18(1):160. Available from: https://doi.org/10.1186/s12916-020-01640-8.
- Clift A, Coupland C, Keogh R, Hemingway H, Hippisley-Cox J. COVID-19 mortality risk in down syndrome: results from a cohort study of 8 million adults. Ann Intern Med. 2020 Oct 21. Available from: https://doi.org/10.7326/M20-4986.
- Public Health Agency of Canada. From risk to resilience: An equity approach to COVID-19. Chief Public Health Officer of Canada's report on the state of public health in Canada 2020. Ottawa, ON: PHAC; 2020 Nov 3. Available from: https://www.canada.ca/en/public-health/corporate/publications/chief-public-health-officer-reports-state-public-health-canada/from-risk-resilience-equity-approach-covid-19.html#a2.
- Heneghan C, Brassey J, Jefferson T. SARS-CoV-2 viral load and the severity of COVID-19. Oxford, UK: University of Oxford, Centre for Evidence-Based Medicine, Nuffield Department of Primary Care Health Sciences; 2020 Mar 26. Available from: https://www.cebm.net/covid-19/sars-cov-2-viral-load-and-the-severity-of-covid-19/.
- Liu Y, Yan L-M, Wan L, Xiang T-X, Le A, Liu J-M, et al. Viral dynamics in mild and severe cases of COVID-19. Lancet Infect Dis. 2020 Mar 19. Available from: https://doi.org/10.1016/S1473-3099(20)30232-2.
- Pujadas E, Chaudhry F, McBride R, Richter F, Zhao S, Wajnberg A, et al. SARS-CoV-2 viral load predicts COVID-19 mortality. Lancet Respir Med. 2020;8(9):e70. Available from: https://doi.org/10.1016/S2213-2600(20)30354-4.
- Marshall M. The lasting misery of coronavirus long-haulers. Nature. 2020;585:339-41. Available from: https://doi.org/10.1038/d41586-020-02598-6.
- Tenforde M, Kim S, Lindsell C, Rose E, Shapiro N, Files D, et al. Symptom duration and risk factors for delayed return to usual health among outpatients with COVID-19 in a multistate health care systems network — United States, March–June 2020. Morb Mortal Wkly Rep. 2020;69(30):993-8. Available from: http://dx.doi.org/10.15585/mmwr.mm6930e1external.
- Alberta Health Services. COVID-19 Scientific Advisory Group rapid evidence report: chronic symptoms of COVID-19. Edmonton, AB: Government of Alberta; 2020 Nov 23. Available from: https://www.albertahealthservices.ca/assets/info/ppih/if-ppih-covid-19-sag-chronic-symptoms-of-covid-rapid-review.pdf.
- Davis HE, Assaf GS, McCorkell L, Wei H, Low RJ, Re’em Y, et al. Characterizing long COVID in an international cohort: 7 months of symptoms and their impact. medRxiv. 2020 Dec 27. Available from: https://doi.org/10.1101/2020.12.24.20248802.
- Public Health Ontario. Long-term sequelae and COVID-19 – what we know so far. Toronto, ON: Queen's Printer for Ontario; 2020 Oct 7. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/2020/07/what-we-know-covid-19-long-term-sequelae.pdf.
- Arnold DT, Hamilton FW, Milne A, Morley AJ, Viner J, Attwood M, et al. Patient outcomes after hospitalisation with COVID-19 and implications for follow-up: results from a prospective UK cohort. Thorax. 2020 Dec 3. Available from: https://doi.org/10.1136/thoraxjnl-2020-216086.
- Liguoro I, Pilotto C, Bonanni M, Ferrari ME, Pusiol A, Nocerino A, et al. SARS-COV-2 infection in children and newborns: a systematic review. Eur J Pediatr. 2020 Jul;179(7). Available from: https://doi.org/10.1007/s00431-020-03684-7.
- Ludvigsson JF. Systematic review of COVID-19 in children shows milder cases and a better prognosis than adults. Acta Paediatr. 2020;109(6):1088-95. Available from: https://doi.org/10.1111/apa.15270.
- Public Health Ontario. COVID-19 - what we know so far about... infection in children. Toronto, ON: Queen's Printer for Ontario; 2020 May 15. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/what-we-know-children-feb-21-2020.pdf?la=en.
- Shekerdemian LS, Mahmood NR, Wolfe KK, Riggs BJ, Ross CE, McKiernan CA, et al. Characteristics and outcomes of children with coronavirus disease 2019 (COVID-19) infection admitted to US and Canadian pediatric intensive care units. JAMA Pediatr. 2020 May 11. Available from: https://jamanetwork.com/journals/jamapediatrics/fullarticle/2766037.
- Stringhini S, Wisniak A, Piumatti G, Azman AS, Lauer SA, Baysson H, et al. Seroprevalence of anti-SARS-CoV-2 IgG antibodies in Geneva, Switzerland (SEROCoV-POP): a population-based study. The Lancet. 2020 Jun 11;396(10247):313-9. Available from: https://doi.org/10.1016/S0140-6736(20)31304-0.
- Jüni P, Maltsev A, Bobos P, Allen U, Choi Y, Connell J, et al. The role of children in SARS-CoV-2 Transmission. Toronto: Ontario COVID-19 Science Advisory Table; 2020 Aug 31. Available from: https://doi.org/10.47326/ocsat.2020.01.03.1.0.
- Public Health Ontario. Enhanced epidemiological summary. COVID-19 infection in children: January 15, 2020 to July 13, 2020. Toronto, ON: Queen's Printer for Ontario; 2020 Jul 26. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/epi/2020/05/covid-19-epi-infection-children.pdf?la=en.
- British Columbia Centre for Disease Control, Provincial Health Services Authority. British Columbia COVID-19 dashboard. Vancouver, BC: BCCDC; 2020; Available from: https://experience.arcgis.com/experience/a6f23959a8b14bfa989e3cda29297ded.
- Government of Quebec. Situation of the coronavirus (COVID-19) in Québec. Quebec: Government of Quebec; 2021 [updated Jan 20]; Available from: https://www.quebec.ca/en/health/health-issues/a-z/2019-coronavirus/situation-coronavirus-in-quebec/#c63039.
- Pierce CA, Preston-Hurlburt P, Dai Y, Aschner CB, Cheshenko N, Galen B, et al. Immune responses to SARS-CoV-2 infection in hospitalized pediatric and adult patients. Sci Transl Med. 2020;12(564). Available from: https://stm.sciencemag.org/content/scitransmed/12/564/eabd5487.full.pdf.
- World Health Organization. Multisystem inflammatory syndrome in children and adolescents temporally related to COVID-19. Geneva, Switzerland: WHO; 2020 May 15. Available from: https://www.who.int/news-room/commentaries/detail/multisystem-inflammatory-syndrome-in-children-and-adolescents-with-covid-19.
- Licciardi F, Pruccoli G, Denina M, Parodi E, Taglietto M, Rosati S, et al. SARS-CoV-2-induced Kawasaki-like hyperinflammatory syndrome: a novel COVID phenotype in children. Pediatrics. 2020 Jul;146(1). Available from: https://pediatrics.aappublications.org/content/early/2020/07/24/peds.2020-1711.
- Public Health Ontario. COVID-19 – what we know so far about...Kawasaki disease-like illness. Toronto, ON: Queen's Printer for Ontario; 2020 May 30. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/2020/05/what-we-know-kawasaki-disease-like-illness.pdf?la=en.
- Inglesby TV. Public health measures and the reproduction number of SARS-CoV-2. JAMA. 2020;323(21):2186-7. Available from: https://jamanetwork.com/journals/jama/fullarticle/2765665.
- World Health Organization. Statement on the meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus (2019-nCoV). Geneva, Switzerland: WHO; 2020 Jan 23. Available from: https://www.who.int/news/item/23-01-2020-statement-on-the-meeting-of-the-international-health-regulations-(2005)-emergency-committee-regarding-the-outbreak-of-novel-coronavirus-(2019-ncov).
- Liu Y, Gayle AA, Wilder-Smith A, Rocklöv J. The reproductive number of COVID-19 is higher compared to SARS coronavirus. J Travel Med. 2020;27(2). Available from: https://doi.org/10.1093/jtm/taaa021.
- Endo A, Abbott S, Kucharski A, Funk S. Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China. Wellcome Open Res. 2020;5(67). Available from: https://doi.org/10.12688/wellcomeopenres.15842.3.
- Yan Y, Shin W, Pang Y, Meng Y, Lai J, You C, et al. The first 75 days of novel coronavirus (SARS-CoV-2) outbreak: recent advances, prevention, and treatment. Int J Environ Res Public Health. 2020;17(7):2323. Available from: https://doi.org/10.3390/ijerph17072323.
- Chisholm RH, Campbell PT, Wu Y, Tong SYC, McVernon J, Geard N. Implications of asymptomatic carriers for infectious disease transmission and control. Royal Soc Open Sci. 2018;5(2). Available from: https://doi.org/10.1098/rsos.172341.
- Public Health Agency of Canada. Update on COVID-19 in Canada: epidemiology and modelling. Ottawa, ON: PHAC; 2021 Jan 15. Available from: https://www.canada.ca/content/dam/phac-aspc/documents/services/diseases-maladies/coronavirus-disease-covid-19/epidemiological-economic-research-data/update-covid-19-canada-epidemiology-modelling-20210115-en.pdf.
- Centre for Mathematical Modelling of Infectious Diseases. National and subnational estimates for Canada. London, UK: London School of Hygiene & Tropical Medicine; 2021 Mar 16. Available from: https://epiforecasts.io/covid/posts/national/canada/.
- Wölfel R, Corman VM, Guggemos W, Seilmaier M, Zange S, Müller MA, et al. Virological assessment of hospitalized patients with COVID-2019. Nature. 2020 May;581(7809):465-9. Available from: https://doi.org/10.1038/s41586-020-2196-x.
- Dietz L, Horve PF, Coil DA, Fretz M, Eisen JA, Van Den Wymelenberg K. 2019 Novel Coronavirus (COVID-19) pandemic: built environment considerations to reduce transmission. Msystems. 2020;5(2). Available from: https://msystems.asm.org/content/5/2/e00245-20.
- Furuse Y, Sando E, Tsuchiya N, Miyahara R, Yasuda I, Ko YK, et al. Clusters of coronavirus disease in communities, Japan, January-April 2020. Emerg Infect Dis. 2020 Jun 10;26(9). Available from: https://doi.org/10.3201/eid2609.202272.
- Qian G, Yang N, Ma AHY, Wang L, Li G, Chen X, et al. COVID-19 transmission within a family cluster by presymptomatic carriers in China. Clin Infect Dis. 2020 Aug;71(15). Available from: https://doi.org/10.1093/cid/ciaa316.
- Leclerc QJ, Fuller NM, Knight LE, Group CC-W, Funk S, Knight GM. What settings have been linked to SARS-CoV-2 transmission clusters? Wellcome Open Res. 2020 Jun 5;5:83. Available from: https://wellcomeopenresearch.org/articles/5-83.
- Bourouiba L. Turbulent gas clouds and respiratory pathogen emissions: potential implications for reducing transmission of COVID-19. JAMA. 2020;323(18):1837-8. Available from: https://doi.org/10.1001/jama.2020.4756.
- Public Health Agency of Canada. COVID-19: main modes of transmission. Ottawa, ON: PHAC; 2020 Nov 5. Available from: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/health-professionals/main-modes-transmission.html.
- Fears AC, Klimstra WB, Duprex P, Hartman A, Weaver SC, Plante KS, et al. Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions. Emerg Infect Dis. 2020 Sep;29(9). Available from: https://doi.org/10.3201/eid2609.201806.
- Gorbunov B. Aerosol particles laden with viruses that cause COVID-19 travel over 30m distance. Preprints. 2020 May 21. Available from: https://www.preprints.org/manuscript/202004.0546/v2.
- Ong SWX, Tan YK, Chia PY, Lee TH, Ng OT, Wong MSY, et al. Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. JAMA. 2020 Mar 4;323(16):1610-2. Available from: https://doi.org/10.1001/jama.2020.3227.
- Santarpia JL, Rivera DN, Herrera VL, Morwitzer MJ, Creager HM, Santarpia GW, et al. Aerosol and surface contamination of SARS-CoV-2 observed in quarantine and isolation care. Sci Rep. 2020;10(1):12732-. Available from: https://doi.org/10.1038/s41598-020-69286-3.
- Liu Y, Ning Z, Chen Y, Guo M, Liu Y, Gali NK, et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature. 2020;582(7813):557-60. Available from: https://www.nature.com/articles/s41586-020-2271-3.
- World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention precautions. Geneva, Switzerland: WHO; 2020 Jul 9. Available from: https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions.
- Sia SF, Yan L-M, Chin AWH, Fung K, Choy K-T, Wong AYL, et al. Pathogenesis and transmission of SARS-CoV-2 in golden hamsters. Nature. 2020;583(7818):834-8. Available from: https://www.nature.com/articles/s41586-020-2342-5.
- Bae S, Kim H, Jung T-Y, Lim J-A, Jo D-H, Kang G-S, et al. Epidemiological characteristics of COVID-19 outbreak at fitness centers in Cheonan, Korea. J Korean Med Sci. 2020;35(31):e288. Available from: https://doi.org/10.3346/jkms.2020.35.e288.
- Kim Y-I, Kim S-G, Kim S-M, Kim E-H, Park S-J, Yu K-M, et al. Infection and rapid transmission of SARS-CoV-2 in ferrets. Cell Host Microbe. 2020;27(5):704-9.e2. Available from: https://doi.org/10.1016/j.chom.2020.03.023.
- Banik RK, Ulrich AK. Evidence of short-range aerosol transmission of SARS-CoV-2 and call for universal airborne precautions for anesthesiologists during the COVID-19 pandemic. Anesth Analg. 2020 Aug;131(2):e102-e4. Available from: https://dx.doi.org/10.1213%2FANE.0000000000004933.
- Liu L, Li Y, Nielsen PV, Wei J, Jensen RL. Short-range airborne transmission of expiratory droplets between two people. Indoor Air. 2017;27(2):452-62. Available from: https://doi.org/10.1111/ina.12314.
- Atkinson J, Chartier Y, Pessoa-Silva CL, Jensen P, Li Y, Seto W-H. Annex C. Respiratory droplets. Geneva, Switzerland: World Health Organization; 2009. Available from: https://www.ncbi.nlm.nih.gov/books/NBK143281/.
- National Academies of Sciences Engineering and Medicine. Rapid expert consultation on social distancing for the COVID-19 pandemic. Washington, DC: The National Academies Press; 2020 Mar 19. Available from: https://www.nap.edu/read/25753/chapter/1.
- Public Health Agency of Canada. Physical distancing: how to slow the spread of COVID-19. Ottawa, ON: PHAC; 2020 Jun 26. Available from: https://www.canada.ca/en/public-health/services/publications/diseases-conditions/social-distancing.html.
- Chu DK, Akl EA, Duda S, Solo K, Yaacoub S, Schünemann HJ, et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. The Lancet. 2020;395(10242):1973-87. Available from: https://doi.org/10.1016/S0140-6736(20)31142-9.
- Arav Y, Klausner Z, Fattal E. Theoretical investigation of pre-symptomatic SARS-CoV-2 person-to-person transmission in households. medRxiv. 2020 Sep 24. Available from: https://doi.org/10.1101/2020.05.12.20099085.
- Anderson EL, Turnham P, Griffin JR, Clarke CC. Consideration of the aerosol transmission for COVID-19 and public health. Risk Anal. 2020;40(5):902-7. Available from: https://doi.org/10.1111/risa.13500.
- Buonanno G, Stabile L, Morawska L. Estimation of airborne viral emission: quanta emission rate of SARS-CoV-2 for infection risk assessment. Environ Int. 2020;141:105794. Available from: https://doi.org/10.1016/j.envint.2020.105794.
- Miller SL, Nazaroff WW, Jimenez JL, Boerstra A, Buonanno G, Dancer SJ, et al. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event. Indoor Air. 2020;00:1-10. Available from: https://doi.org/10.1111/ina.12751.
- Morawska L, Cao J. Airborne transmission of SARS-CoV-2: the world should face the reality. Environ Int. 2020 Apr 10;139:105730. Available from: https://doi.org/10.1016/j.envint.2020.105730.
- Stadnytskyi V, Bax CE, Bax A, Anfinrud P. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proc Nat Acad Sci USA. 2020 Jun;117(22):11875-7. Available from: https://www.pnas.org/content/117/22/11875.
- Borak J. Airborne transmission of COVID-19. Occup Med. 2020 Jul 17;70(5):297-9. Available from: https://doi.org/10.1093/occmed/kqaa080.
- van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. 2020;382:1564-7. Available from: https://doi.org/10.1056/NEJMc2004973.
- Lednicky JA, Lauzardo M, Fan H, Jutla AS, Tilly TB, Gangwar M, et al. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. Int J Infect Dis. 2020 Aug 4;100:476-82. Available from: https://doi.org/10.1016/j.ijid.2020.09.025.
- Qian H, Miao T, Liu L, Zheng X, Luo D, Li Y. Indoor transmission of SARS-CoV-2. Indoor Air. 2020 Oct 31. Available from: https://doi.org/10.1111/ina.12766.
- British Columbia Centre for Disease Control, British Columbia Ministry of Health. Tools and strategies for safer operations during the COVID-19 pandemic. Vancouver, BC: BC Centre for Disease Control and the BC Ministry of Health; 2020 Jul. Available from: http://www.bccdc.ca/Health-Info-Site/Documents/COVID19_ToolsStrategiesSaferOperations.pdf.
- Institut national de santé publique. COVID-19 : Modes de transmission et mesures de prévention et de protection contre les risques, incluant le rôle de la ventilation. Montreal, QC: INSPQ; 2021 Jan 13. Available from: https://inspq.qc.ca/covid-19/environnement/modes-transmission.
- Chin AWH, Chu JTS, Perera MRA, Hui KPY, Yen H-L, Chan MCW, et al. Stability of SARS-CoV-2 in different environmental conditions. Lancet Microbe. 2020;1(1):e10. Available from: https://doi.org/10.1016/S2666-5247(20)30003-3.
- Kampf G, Todt D, Pfaender S, Steinmann E. Persistence of coronaviruses on inanimate surfaces and their inactivation with biocidal agents. J Hosp Infect. 2020;104(3):246-51. Available from: https://doi.org/10.1016/j.jhin.2020.01.022.
- Pastorino B, Touret F, Gilles M, Lamballerie Xd, Charrel R. Prolonged infectivity of SARS-CoV-2 in fomites. Emerg Infect Dis. 2020 Sep;26(9). Available from: https://wwwnc.cdc.gov/eid/article/26/9/20-1788_article.
- Corpet D. Why does Sars-CoV-2 survive longer on plastic than on paper? Med Hypotheses. 2020 Nov 28. Available from: https://doi.org/10.1016/j.mehy.2020.110429.
- Harbourt D, Haddow A, Piper A, Bloomfield H, Kearney B, Fetterer D, et al. Modeling the stability of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on skin, currency, and clothing. PLoS Negl Trop Dis. 2020 Jul 3. Available from: https://doi.org/10.1371/journal.pntd.0008831.
- Jones C. Environmental surface contamination with SARS-CoV-2 - a short review J Hum Virol Retrovirolog. 2020;8(1):15-9. Available from: https://doi.org/10.15406/jhvrv.2020.08.00215.
- Kasloff SB, Strong JE, Funk D, Cutts TA. Stability of SARS-CoV-2 on critical personal protective equipment. Sci Rep. 2021 Jan 13;11(984). Available from: https://doi.org/10.1038/s41598-020-80098-3.
- Liu Y, Li T, Deng Y, Liu S, Zhang D, Li H, et al. Stability of SARS-CoV-2 on environmental surfaces and in human excreta. J Hosp Infect. 2020;107:105-7. Available from: https://doi.org/10.1016/j.jhin.2020.10.021.
- Hirose R, Ikegaya H, Naito Y, Watanabe N, Yoshida T, Bandou R, et al. Survival of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus on human skin: importance of hand hygiene in coronavirus disease 2019 (COVID-19). Clin Infect Dis. 2020 Oct 3. Available from: https://doi.org/10.1093/cid/ciaa1517.
- Ratnesar-Shumate S, Williams G, Green B, Krause M, Holland B, Wood S, et al. Simulated sunlight rapidly inactivates SARS-CoV-2 on surfaces. J Infect Dis. 2020;222(2):214-22. Available from: https://doi.org/10.1093/infdis/jiaa274.
- Riddell S, Goldie S, Hill A, Eagles D, Drew TW. The effect of temperature on persistence of SARS-CoV-2 on common surfaces. Virol J. 2020;17(1):145. Available from: https://doi.org/10.1186/s12985-020-01418-7.
- Simmons SE, Carrion R, Alfson KJ, Staples HM, Jinadatha C, Jarvis WR, et al. Deactivation of SARS-CoV-2 with pulsed-xenon ultraviolet light: implications for environmental COVID-19 control. Infect Control Hosp Epidemiol. 2020:1-4. Available from: https://doi.org/10.1017/ice.2020.399.
- Zhou J, Otter JA, Price JR, Cimpeanu C, Garcia DM, Kinross J, et al. Investigating SARS-CoV-2 surface and air contamination in an acute healthcare setting during the peak of the COVID-19 pandemic in London. Clin Infect Dis. 2020 Jun 2. Available from: https://doi.org/10.1093/cid/ciaa905.
- Boyce JM, Pittet D. Guideline for hand hygiene in health-care settings. Recommendations of the Healthcare Infection Control Practices Advisory Committee and the HICPAC/SHEA/APIC/IDSA Hand Hygiene Task Force. MMWR Morb Mortal Wkly Rep. 2002 Oct 25(51(RR16)):1-44. Available from: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5116a1.htm.
- Weber TP, Stilianakis NI. Fomites, hands, and the transmission of respiratory viruses. J Occup Environ Hyg. 2020 Dec 7:1-4. Available from: https://doi.org/10.1080/15459624.2020.1845343.
- US Centers for Disease Control. Handwashing: clean hands save lives. How to wash your hands. Atlanta, GA: US Department of Health and Human Services; 2020 [updated Dec 7]; Available from: https://www.cdc.gov/handwashing/show-me-the-science-handwashing.html.
- US Centers for Disease Control. Handwashing: clean hands save lives. When & how to use hand sanitizer in community settings. Atlanta, GA: US Department of Health and Human Services; 2020 [updated Sep 10]; Available from: https://www.cdc.gov/handwashing/show-me-the-science-hand-sanitizer.html.
- Health Canada. Hard surface disinfectants and hand sanitizers: list of hard-surface disinfectants for use against coronavirus (COVID-19). Ottawa, ON: Health Canada; 2021 [updated Jan 8]; Available from: https://www.canada.ca/en/health-canada/services/drugs-health-products/disinfectants/covid-19/list.html.
- US Environmental Protection Agency. List N: disinfectants for use against SARS-CoV-2 | Pesticide registration. Washington, DC: US EPA; 2020 [updated Dec 15]; Available from: https://www.epa.gov/pesticide-registration/list-n-disinfectants-use-against-sars-cov-2.
- Public Health Agency of Canada. Cleaning and disinfecting public spaces during COVID-19. guidance. Ottawa, ON: PHAC; 2020 Oct 5. Available from: https://www.canada.ca/en/public-health/services/publications/diseases-conditions/cleaning-disinfecting-public-spaces.html
- Gupta S, Parker J, Smits S, Underwood J, Dolwani S. Persistent viral shedding of SARS-CoV-2 in faeces - a rapid review. Colorectal Dis. 2020;22(6):611-20. Available from: https://doi.org/10.1111/codi.15138.
- Xiao F, Tang M, Zheng X, Liu Y, Li X, Shan H. Evidence for gastrointestinal infection of SARS-CoV-2. Gastroenterology. 2020;158(6):1831-3. Available from: https://doi.org/10.1053/j.gastro.2020.02.055.
- Chen Y, Chen L, Deng Q, Zhang G, Wu K, Ni L, et al. The presence of SARS-CoV-2 RNA in the feces of COVID-19 patients. J Med Virol. 2020;92(7):833-40. Available from: https://doi.org/10.1002/jmv.25825.
- Heneghan C, Spencer E, Brassey J, Jefferson T. SARS-CoV-2 and the role of orofecal transmission: systematic review. medRxiv. 2020 Aug 10. Available from: https://doi.org/10.1101/2020.08.04.20168054.
- Hindson J. COVID-19: faecal–oral transmission? Nat Rev Gastroenterol Hepatol. 2020 Mar 25;17(5):259. Available from: https://doi.org/10.1038/s41575-020-0295-7.
- Wang W, Xu Y, Gao R, Lu R, Han K, Wu G, et al. Detection of SARS-CoV-2 in different types of clinical specimens. JAMA. 2020;323(18):1843-4. Available from: https://doi.org/10.1001/jama.2020.3786.
- Kang M, Wei J, Yuan J, Guo J, Zhang Y, Hang J. Probable evidence of fecal aerosol transmission of SARS-CoV-2 in a high-rise building. Ann Intern Med. 2020. Available from: https://doi.org/10.7326/M20-0928.
- Yuan J, Chen Z, Gong C, Liu H, Li B, Li K, et al. Coronavirus Disease 2019 outbreak likely caused by sewage exposure in a low-income community: Guangzhou, China, April 2020. SSRN. 2020 May. Available from: http://dx.doi.org/10.2139/ssrn.3618204.
- Hwang SE, Chang JH, Bumjo O, Heo J. Possible aerosol transmission of COVID-19 associated with an outbreak in an apartment in Seoul, South Korea, 2020. Int J Infect Dis. 2020 Dec 16. Available from: https://doi.org/10.1016/j.ijid.2020.12.035.
- World Health Organization. How WHO is working to track down the animal reservoir of the SARS-CoV-2 virus. Geneva, Switzerland: WHO; 2020 Nov 6. Available from: https://www.who.int/news-room/feature-stories/detail/how-who-is-working-to-track-down-the-animal-reservoir-of-the-sars-cov-2-virus.
- US Department of Homeland Security. Master question list for COVID-19 (caused by SARS-CoV-2). Washington, DC: US Department of Homeland Security Science and Technology Directorate; 2021 Jan 12. Available from: https://www.dhs.gov/publication/st-master-question-list-covid-19.
- Shi J, Wen Z, Zhong G, Yang H, Wang C, Huang B, et al. Susceptibility of ferrets, cats, dogs, and other domesticated animals to SARS–coronavirus 2. Science. 2020;368(6494):1016-20. Available from: https://science.sciencemag.org/content/sci/368/6494/1016.full.pdf.
- Animal and Plant Health Inspection Service. Confirmed cases of SARS-CoV-2 in animals in the United States. Washington, DC: US Department of Agriculture; 2021 Jan 19. Available from: https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/sa_one_health/sars-cov-2-animals-us.
- El Masry I, von Dobschuetz S, Plee L, Larfaoui F, Yang Z, Song J, et al. Exposure of humans or animals to SARS-CoV-2 from wild, livestock, companion and aquatic animals. Rome, Italy: Food and Agriculture Organization of the United Nations; 2020 Jul 28. Available from: http://www.fao.org/documents/card/en/c/ca9959en.
- Oude Munnink BB, Sikkema RS, Nieuwenhuijse DF, Molenaar RJ, Munger E, Molenkamp R, et al. Jumping back and forth: anthropozoonotic and zoonotic transmission of SARS-CoV-2 on mink farms. bioRxiv. 2020 Sep 1. Available from: https://doi.org/10.1101/2020.09.01.277152.
- US Department of Agriculture. USDA confirms SARS-CoV-2 in mink in Utah. Washington, DC: USDA; 2020 Aug 25. Available from: https://www.aphis.usda.gov/aphis/newsroom/stakeholder-info/sa_by_date/sa-2020/sa-08/sare-cov-2-mink.
- World Health Organization. SARS-CoV-2 mink-associated variant strain – Denmark. Geneva, Switzerland: WHO; 2020 Nov 6. Available from: https://www.who.int/csr/don/06-november-2020-mink-associated-sars-cov2-denmark/en/.
- Fraser Health. Community: Fraser Health has declared a COVID-19 outbreak at a mink farm in the Fraser Valley. Surrey BC: Fraser Health Authority; 2020 Dec 6. Available from: https://www.fraserhealth.ca/news/2020/Dec/fh-declares-covid-19-ltc-and-al-outbreaks-over-and-declares-outbreak-at-a-mink-farm#.YAdukmhKhPZ.
- British Columbia Centre for Disease Control. Genetic sequencing results completed for mink farm outbreak. Vancouver BC: BCCDC; 2020 Dec 23. Available from: http://www.bccdc.ca/about/news-stories/news-releases/2020/genetic-sequencing-results-completed-for-mink-farm-outbreak.
- Deng W, Bao L, Gao H, Xiang Z, Qu Y, Song Z, et al. Ocular conjunctival inoculation of SARS-CoV-2 can cause mild COVID-19 in rhesus macaques. Nat Commun. 2020;11(1):4400. Available from: https://doi.org/10.1038/s41467-020-18149-6.
- Qing H, Yang Z, Shi M, Zhang Z. New evidence of SARS-CoV-2 transmission through the ocular surface. Graefes Arch Clin Exp Ophthalmol. 2020. Available from: https://doi.org/10.1007/s00417-020-04726-4.
- Schwartz DA, Morotti D, Beigi B, Moshfegh F, Zafaranloo N, Patanè L. Confirming vertical fetal infection with coronavirus disease 2019: neonatal and pathology criteria for early onset and transplacental transmission of Severe Acute Respiratory Syndrome Coronavirus 2 from infected pregnant mothers. Arch Pathol Lab Med. 2020;144(12):1451-6. Available from: https://doi.org/10.5858/arpa.2020-0442-SA.
- Public Health Ontario. COVID-19 routes of transmission – what we know so far Toronto, ON: Queen's Printer for Ontario; 2020 Dec 1. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/2020/12/routes-transmission-covid-19.pdf?la=en.
- Leblanc J-F, Germain M, Delage G, OʼBrien S, Drews SJ, Lewin A. Risk of transmission of severe acute respiratory syndrome coronavirus 2 by transfusion: A literature review. Transfusion. 2020;60(12):3046-54. Available from: https://doi.org/10.1111/trf.16056.
- Bao L, Gao H, Deng W, Lv Q, Yu H, Liu M, et al. Transmission of severe acute respiratory syndrome coronavirus 2 via close contact and respiratory droplets among human angiotensin-converting enzyme 2 mice. J Infect Dis. 2020;222(4):551-5. Available from: https://doi.org/10.1093/infdis/jiaa281.
- Karimzadeh S, Bhopal R, Tien HN. Review of viral dynamics, exposure, infective dose, and outcome of COVID-19 caused by the SARS-CoV-2 virus: comparison with other respiratory viruses. PrePrint. 2020 Dec 7. Available from: https://doi.org/10.20944/preprints202007.0613.v3.
- Chan JF-W, Yuan S, Zhang AJ, Poon VK-M, Chan CC-S, Lee AC-Y, et al. Surgical mask partition reduces the risk of noncontact transmission in a golden syrian hamster model for coronavirus disease 2019 (COVID-19). Clin Infect Dis. 2020. Available from: https://doi.org/10.1093/cid/ciaa644.
- Jayaweera M, Perera H, Gunawardana B, Manatunge J. Transmission of COVID-19 virus by droplets and aerosols: a critical review on the unresolved dichotomy. Environ Res. 2020 Sep;188:109819. Available from: https://dx.doi.org/10.1016%2Fj.envres.2020.109819.
- Callaway E. Dozens to be deliberately infected with coronavirus in UK ‘human challenge’ trials. Nature. 2020 Oct 20;586:651-2. Available from: https://doi.org/10.1038/d41586-020-02821-4.
- Goyal A, Reeves DB, Cardozo-Ojeda EF, Schiffer JT, Mayer BT. Viral load and contact heterogeneity predict SARS-CoV-2 transmission and super-spreading events. eLife. 2021 Feb 23;10:e63537. Available from: https://doi.org/10.7554/eLife.63537.
- Calisti R. SARS-CoV-2: exposure to high external doses as determinants of higher viral loads and of increased risk for COVID-19. A systematic review of the literature. Epidemiol Prev. 2020;44((5-6)Suppl 2):152-9. Available from: https://www.epiprev.it/materiali/suppl/2020_EP5-6S2/152-159_INT-Calisti.pdf.
- Fain B, Dobrovolny HM. Initial inoculum and the severity of COVID-19: a mathematical modeling study of the dose-response of SARS-CoV-2 infections. Epidemiologia. 2020;1(1):5-15. Available from: https://doi.org/10.3390/epidemiologia1010003.
- Guallar MP, Meiriño R, Donat-Vargas C, Corral O, Jouvé N, Soriano V. Inoculum at the time of SARS-CoV-2 exposure and risk of disease severity. Int J Infect Dis. 2020 Aug;97:290-2. Available from: https://doi.org/10.1016/j.ijid.2020.06.035.
- He X, Lau EHY, Wu P, Deng X, Wang J, Hao X, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med. 2020 May;26(5):672-5. Available from: https://doi.org/10.1038/s41591-020-0869-5.
- Lauer SA, Grantz KH, Bi Q, Jones FK, Zheng Q, Meredith HR, et al. The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application. Ann Intern Med. 2020 Mar 10;172(9):577-82. Available from: https://pubmed.ncbi.nlm.nih.gov/32150748/.
- Public Health Ontario. COVID-19 incubation period and considerations for travellers’ quarantine duration Toronto, ON: Queen's Printer for Ontario; 2020 Dec 3. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/main/2020/12/covid-19-incubation-travellers-quarantine-duration.pdf?la=en.
- Backer JA, Klinkenberg D, Wallinga J. Incubation period of 2019 novel coronavirus (2019-nCoV) infections among travellers from Wuhan, China, 20–28 January 2020. Euro Surveill. 2020;25(5). Available from: https://doi.org/10.2807/1560-7917.ES.2020.25.5.2000062.
- Public Health Ontario. COVID-19 – what we know so far about… the period of communicability Toronto, ON: Queen's Printer for Ontario; 2020 Mar 30. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/covid-wwksf/what-we-know-communicable-period-mar-27-2020.pdf?la=en.
- European Centre for Disease Prevention and Control. Transmission of COVID-19. Stockholm, Sweden: ECDC; 2020 Aug 10. Available from: https://www.ecdc.europa.eu/en/covid-19/latest-evidence/transmission.
- Furukawa NW, Brooks JT, Sobel J. Evidence supporting transmission of Severe Acute Respiratory Syndrome Coronavirus 2 while presymptomatic or asymptomatic. Emerg Infect Dis. 2020;26(7). Available from: https://wwwnc.cdc.gov/eid/article/26/7/20-1595_article.
- Health Information and Quality Authority. Evidence summary for asymptomatic transmission of COVID-19. Dublin, Ireland: HIQA; 2020 Apr 21. Available from: https://www.hiqa.ie/sites/default/files/2020-04/Evidence-summary-for-asymptomatic-transmission-of-COVID-19.pd.
- Kimball A, Hatfield K, Arons M, James A, Taylor J, Spicer K, et al. Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility — King County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 3;69:377-81. Available from: https://www.cdc.gov/mmwr/volumes/69/wr/mm6913e1.htm?s_cid=mm6913e1_w.
- Public Health Ontario. COVID-19 – what we know so far about… asymptomatic infection and asymptomatic transmission. Toronto, ON: Queen's Printer for Ontario; 2020 May 22. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/what-we-know-jan-30-2020.pdf?la=en.
- Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic transmission of SARS-CoV-2 - Singapore, January 23-March 16, 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 10;69(14):411-5. Available from: https://doi.org/10.15585/mmwr.mm6914e1.
- Li W, Su Y-Y, Zhi S-S, Huang J, Zhuang C-L, Bai W-Z, et al. Viral shedding dynamics in asymptomatic and mildly symptomatic patients infected with SARS-CoV-2. Clin Microbiol Infect. 2020. Available from: https://doi.org/10.1016/j.cmi.2020.07.008.
- Zou L, Ruan F, Huang M, Liang L, Huang H, Hong Z, et al. SARS-CoV-2 viral load in upper respiratory specimens of infected patients. N Engl J Med. 2020;382(12):1177-9. Available from: https://www.nejm.org/doi/full/10.1056/NEJMc2001737.
- Heneghan C, Brassey J, Jefferson T. COVID-19: What proportion are asymptomatic? Oxford, UK: University of Oxford, Centre for Evidence-Based Medicine 2020 Apr 6. Available from: https://www.cebm.net/covid-19/covid-19-what-proportion-are-asymptomatic/.
- Kamiya H, Fujikura H, Doi K, Kakimoto K, Suzuki M, Matsui T, et al. Epidemiology of COVID-19 outbreak on cruise ship quarantined at Yokohama, Japan, February 2020. Emerg Infect Dis. 2020;26(11):2591-7. Available from: https://dx.doi.org/10.3201/eid2611.201165.
- Johansson MA, Quandelacy TM, Kada S, Prasad PV, Steele M, Brooks JT, et al. SARS-CoV-2 transmission from people without COVID-19 symptoms. JAMA Network Open. 2021;4(1):e2035057-e. Available from: https://doi.org/10.1001/jamanetworkopen.2020.35057.
- Alberta Health Services. COVID-19 Scientific Advisory Group rapid response report. Key research question: what is the evidence supporting the possibility of asymptomatic transmission of SARS-CoV-2? Edmonton, AB: Government of Alberta; 2020 Apr 13. Available from: https://www.albertahealthservices.ca/assets/info/ppih/if-ppih-covid-19-rapid-response-asymptomatic-transmission.pdf.
- Hu Z, Song C, Xu C, Jin G, Chen Y, Xu X, et al. Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Sci China Life Sci. 2020 May 1;63(5):706-11. Available from: https://doi.org/10.1007/s11427-020-1661-4.
- Pan Y, Zhang D, Yang P, Poon LLM, Wang Q. Viral load of SARS-CoV-2 in clinical samples. Lancet Infect Dis. 2020;20(4):411-2. Available from: https://doi.org/10.1016/s1473-3099(20)30113-4.
- To KK-W, Tsang OT-Y, Leung W-S, Tam AR, Wu T-C, Lung DC, et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study. Lancet Infect Dis. 2020;20(5):565-74. Available from: https://doi.org/10.1016/S1473-3099(20)30196-1.
- Walsh KA, Spillane S, Comber L, Cardwell K, Harrington P, Connell J, et al. The duration of infectiousness of individuals infected with SARS-CoV-2. J Infect. 2020;81(6):847-56. Available from: https://doi.org/10.1016/j.jinf.2020.10.009.
- Jefferson T, Spencer E, Brassey J, Heneghan C. Viral cultures for COVID-19 infectious potential assessment - a systematic review. Clin Infect Dis. 2020 Dec 3;ciaa1764. Available from: https://doi.org/10.1093/cid/ciaa1764.
- Cevik M, Tate M, Lloyd O, Maraolo AE, Schafers J, Ho A. SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis. Lancet Microbe. 2021;2(1):e13-e22. Available from: https://doi.org/10.1016/S2666-5247(20)30172-5.
- Owusu D, Pomeroy MA, Lewis NM, Wadhwa A, Yousaf AR, Whitaker B, et al. Persistent SARS-CoV-2 RNA shedding without evidence of infectiousness: a cohort study of individuals with COVID-19. J Infect Dis. 2021. Available from: https://doi.org/10.1093/infdis/jiab107.
- Riediker M, Tsai D-H. Estimation of viral aerosol emissions from simulated individuals with aysmptomatic to moderate Coronavirus Disease 2019. JAMA. 2020;3(7):e2013807. Available from: https://doi.org/10.1001/jamanetworkopen.2020.13807.
- Anand S, Mayya YS. Size distribution of virus laden droplets from expiratory ejecta of infected subjects. Sci Rep. 2020;10(1):21174. Available from: https://doi.org/10.1038/s41598-020-78110-x.
- Asadi S, Wexler AS, Cappa CD, Barreda S, Bouvier NM, Ristenpart WD. Aerosol emission and superemission during human speech increase with voice loudness. Sci Rep. 2019 Feb;9(1):2348. Available from: https://doi.org/10.1038/s41598-019-38808-z.
- Lau MSY, Grenfell B, Thomas M, Bryan M, Nelson K, Lopman B. Characterizing superspreading events and age-specific infectiousness of SARS-CoV-2 transmission in Georgia, USA. Proc Nat Acad Sci USA. 2020;117(36):22430-5. Available from: https://doi.org/10.1073/pnas.2011802117.
- Adam DC, Wu P, Wong JY, Lau EHY, Tsang TK, Cauchemez S, et al. Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong. Nat Med. 2020 Sep 17. Available from: https://doi.org/10.1038/s41591-020-1092-0.
- National Collaborating Centre for Methods and Tools. Rapid evidence review: what is known on the potential for COVID-19 re-infection, including new transmission after recovery? Hamilton, ON: NCCMT; 2020 Sep 28. Available from: https://www.nccmt.ca/uploads/media/media/0001/02/cd34d373c03e481993d06980892c0081ff0e3edd.pdf.
- Iwasaki A. What reinfections mean for COVID-19. Lancet Infect Dis. 2020 Oct 12. Available from: https://doi.org/10.1016/S1473-3099(20)30783-0.
- Alberta Health Services. COVID-19 Scientific Advisory Group rapid evidence report: COVID-19 risk of reinfection. Edmonton, AB: Government of Alberta; 2020 Nov 6. Available from: https://www.albertahealthservices.ca/assets/info/ppih/if-ppih-covid-19-sag-reinfection-rapid-review.pdf.
- National Academies of Sciences Engineering and Medicine. Rapid expert consultation on SARS-CoV-2 survival in relation to temperature and humidity and potential for seasonality for the COVID-19 pandemic (April 7, 2020). Washington, DC: The National Academies Press; 2020. Available from: https://www.nap.edu/catalog/25771/rapid-expert-consultation-on-sars-cov-2-survival-in-relation-to-temperature-and-humidity-and-potential-for-seasonality-for-the-covid-19-pandemic-april-7-2020.
- Wang T, Lien C, Liu S, Selveraj P. Effective heat inactivation of SARS-CoV-2. medRxiv. 2020 May 5. Available from: https://doi.org/10.1101/2020.04.29.20085498.
- Walker GJ, Clifford V, Bansal N, Stella AO, Turville S, Stelzer-Braid S, et al. SARS-CoV-2 in human milk is inactivated by Holder pasteurization but not cold storage. J Paediatr Child Health. 2020;56(12):1872-4. Available from: https://doi.org/10.1111/jpc.15065.
- Moriyama M, Hugentobler WJ, Iwasaki A. Seasonality of respiratory viral infections. Annu Rev Virol. 2020;7(1). Available from: https://www.annualreviews.org/doi/abs/10.1146/annurev-virology-012420-022445.
- Zhao L, Qi Y, Luzzatto-Fegiz P, Cui Y, Zhu Y. COVID-19: effects of environmental conditions on the propagation of respiratory droplets. Nano Lett. 2020;20(10):7744-50. Available from: https://doi.org/10.1021/acs.nanolett.0c03331.
- Ahlawat A, Wiedensohler A, Mishra S. An overview on the role of relative humidity in airborne transmission of SARS-CoV-2 in indoor environments. Aerosol Air Qual Res. 2020;20:1856-61. Available from: https://doi.org/10.4209/aaqr.2020.06.0302.
- Biryukov J, Boydston JA, Dunning RA, Yeager JJ, Wood S, Reese AL, et al. Increasing temperature and relative humidity accelerates inactivation of SARS-CoV-2 on surfaces. mSphere. 2020;5(4):e00441-20. Available from: https://doi.org/10.1128/mSphere.00441-20.
- Dabisch P, Schuit M, Herzog A, Beck K, Wood S, Krause M, et al. The influence of temperature, humidity, and simulated sunlight on the infectivity of SARS-CoV-2 in aerosols. Aerosol Sci Technol. 2020 Nov:1-12. Available from: https://doi.org/10.1080/02786826.2020.1829536.
- Duan SM, Zhao XS, Wen RF, Huang JJ, Pi GH, Zhang SX, et al. Stability of SARS coronavirus in human specimens and environment and its sensitivity to heating and UV irradiation. Biomed Environ Sci. 2003 Sep;16(3):246-55. Available from: https://www.ncbi.nlm.nih.gov/pubmed/14631830.
- International Ultraviolet Association. IUVA Fact sheet on UV disinfection for COVID-19. Chevy Chase, MD: IUVA; 2020 Mar. Available from: https://www.iuva.org/IUVA-Fact-Sheet-on-UV-Disinfection-for-COVID-19.
- Seyer A, Sanlidag T. Solar ultraviolet radiation sensitivity of SARS-CoV-2. Lancet Microbe. 2020;1(1):e8-e9. Available from: https://dx.doi.org/10.1016%2FS2666-5247(20)30013-6.
- Kowalski W. Ultraviolet germicidal irradiation handbook. New York, NY: Springer; 2009. Available from: https://link.springer.com/book/10.1007/978-3-642-01999-9.
- Blázquez E, Rodríguez C, Ródenas J, Navarro N, Riquelme C, Rosell R, et al. Evaluation of the effectiveness of the SurePure Turbulator ultraviolet-C irradiation equipment on inactivation of different enveloped and non-enveloped viruses inoculated in commercially collected liquid animal plasma. PLoS ONE. 2019;14(2). Available from: https://doi.org/10.1371/journal.pone.0212332.
- Heßling M, Hönes K, Vatter P, Lingenfelder C. Ultraviolet irradiation doses for coronavirus inactivation – review and analysis of coronavirus photoinactivation studies. GMS Hyg Infect Control. 2020 May 14;15. Available from: https://dx.doi.org/10.3205%2Fdgkh000343.
- Houser KW. Ten facts about UV radiation and COVID-19. LEUKOS. 2020;16(3):177-8. Available from: https://doi.org/10.1080/15502724.2020.1760654.
- Bianco A, Biasin M, Pareschi G, Cavalleri A, Cavatorta C, Fenizia C, et al. UV-C irradiation is highly effective in inactivating and inhibiting SARS-CoV-2 replication. medRxiv. 2020 Jun 23. Available from: https://doi.org/10.1101/2020.06.05.20123463.
- US Centers for Disease Control and Prevention. Decontamination and reuse of filtering facepiece respirators. Atlanta, GA: US Department of Health and Human Services; 2020 [updated Apr 30]; Available from: https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontamination-reuse-respirators.html.
- Fischer R, Morris DH, van Doremalen N, Sarchette S, Matson J, Bushmaker T, et al. Assessment of N95 respirator decontamination and re-use for SARS-CoV-2. Emerg Infect Dis. 2020 Sep;26(9). Available from: https://wwwnc.cdc.gov/eid/article/26/9/20-1524_article.
- Raeiszadeh M, Adeli B. A critical review on ultraviolet disinfection systems against COVID-19 outbreak: applicability, validation, and safety considerations. ACS Photonics. 2020 2020/11/18;7(11):2941-51. Available from: https://doi.org/10.1021/acsphotonics.0c01245.
To provide feedback on this document, please visit www.ncceh.ca/en/document_feedback
Permission is granted to reproduce this document in whole, but not in part. Production of this document has been made possible through a financial contribution from the Public Health Agency of Canada through the National Collaborating Centre for Environmental Health
|Citation||O’Keeffe J, Freeman S, Nicol A. National Collaborating Centre for Environmental Health (NCCEH). The Basics of SARS-CoV-2 Transmission. Vancouver, BC: NCCEH. 2021 Mar.|
|Publication Date||Mar 24, 2021|
|Author||Juliette O'Keeffe, Shirra Freeman, Anne-Marie Nicol|
|Posted by NCCEH||Mar 24, 2021|