Summary of evidence supporting COVID-19 public health measures
On this page
- Transmission of COVID-19
- Public health measures
- Adherence to public health measures
This evidence summary provides an overview of and supporting evidence for the public health measures (PHMs) that can help to reduce transmission of SARS-CoV-2, the virus that causes COVID-19, in community-based settings, places where people live, work, learn and play. It replaces the individual and community based measures guidance (last updated on August 11, 2021) with a summary of current scientific evidence that supports the Public Health Agency of Canada's (PHAC's) guidance for COVID-19 PHMs. It also identifies knowledge gaps in evidence related to PHMs for COVID-19 and where emerging evidence will continue to inform the pandemic response and transition planning. The information in this summary reflects current evidence and may be updated as new evidence becomes available. This summary does not provide practical recommendations and can be used to support decision-making for the implementation of PHMs.
There are evidence gaps on the effectiveness of PHMs as applied in First Nations, Inuit and Métis communities and remote and isolated communities. However, despite these gaps, it is important to consider various well-known social, environmental and economic factors when implementing PHMs in these communities, including housing, water quality or access, food security, pre-existing health conditions, precarious employment, education, income, and access to health care. Additional information can be found in the guidance for COVID-19 and Indigenous communities.
Information for health care settings, and long-term care, including workers in these settings, is out of scope for this document. For information regarding public health and infection prevention and control measures in these settings, readers are advised to refer to the relevant infection prevention and control guidelines.
The primary audiences for this document include federal/provincial/territorial (FPT) and regional/local public health authorities (PHAs), and other health professionals who provide advice to their clients related to PHMs. Operators of non-health care community-based settings accessible to the public (e.g. workplaces, businesses, schools, and recreational settings) may also benefit from information provided in this document, in addition to an available risk assessment and mitigation tool.
The responsibility and legislative authority for implementing PHMs belong to the relevant provinces and territories (PTs) and local PHAs, with the exception of international borders and travel-related advice or policies, for which the federal government is responsible. Consequently, when recommending or implementing PHMs, this document should be considered in conjunction with relevant PT and local legislation, regulations and policies, which should take into consideration existing treaties, agreements, relationships, and capacities within the First Nations, Inuit and Métis communities.
Where applicable and based on context, 'COVID-19' will be used generally throughout this document to refer to the SARS-CoV-2 virus or resulting disease.
The pandemic response goal is to minimize serious illness and overall deaths while minimizing societal disruption as a result of the COVID-19 pandemic. This goal and associated objectives are outlined in the Federal/Provincial/Territorial Public Health Response Plan for Ongoing Management of COVID-19.
Public Health Measures (PHMs) are interventions that can be implemented to help reduce the transmission of COVID-19 in communities. PHMs that can reduce COVID-19 transmission range from actions taken by individuals (e.g., wearing a mask, staying home when sick), to actions taken in community settings and workplaces (e.g., improving ventilation, policies promoting physical distancing), to even more restrictive population-based measures that can have considerable impact and require specific jurisdictional authority to implement (e.g., school closures, gathering size restrictions, business closures). For example, individuals can choose to wear masks to protect themselves and others, representing an individual-level PHM; while mask promotion policies in public settings would be considered a community-level PHM.
PHMs are usually implemented in combination, often referred to as "layering", as combinations of PHMs are more effective than single measures on their own, especially when implemented as early as possible.
The focus of this evidence summary is on non-pharmaceutical measures, rather than pharmaceutical interventions such as COVID-19 vaccines, antivirals, and therapeutics. While pharmaceutical interventions are not addressed here, more information can be found on the Vaccines for COVID-19 and COVID-19 treatments webpages on Canada.ca.
Although preventing all cases of COVID-19 is not the main public health goal at this point in the pandemic, evidence-informed PHMs are recommended for individuals and communities to help lower transmission of COVID-19, in order to:
- Reduce the number of people who experience severe illness and death due to COVID-19;
- Reduce the demand for health care services to a manageable level;
- Reduce the risk of post COVID-19 condition (i.e., long COVID) among those at risk in the population; and
- Avoid the need for other more restrictive population-based measures that may result in significant societal disruption (e.g., workplace/business closures, school closures, etc.).
The implementation of PHMs to reduce the risk of COVID-19 transmission should consider ethical principles of trust and solidarity, reciprocity, transparency, stewardship, equity and fairness. While PHMs are effective in reducing COVID-19 transmission, they can have important consequences beyond the scope of COVID-19 management. Some PHMs, such as school and business closures, are complex, costly to sustain, and can have unintended consequences such as extensive social, psychological, and economic impacts.Reference 1Reference 2Reference 3Reference 4 Due to social and health inequities, PHMs can have disproportionately negative consequences for at-risk populations and populations who are in vulnerable situations. This includes Indigenous people, new immigrants, refugees, persons who do not speak English or French, and persons with a mental or physical disability, among others.Reference 5Reference 6Reference 7Reference 8 These groups will need special consideration to support their adoption of recommended public health measures. However, these groups may also benefit from the implementation of PHMs that reduce COVID-19 transmission, as they may be at increased risk of exposure or severe outcomes from COVID-19.
Adjusting public health measures
As COVID-19 continues to circulate in Canadian communities, jurisdictions will adjust (maintain, ease, or re-instate) PHMs as required to manage resurgences of COVID-19 cases within their jurisdictions, based on the level of transmission and other indicators. Federal, provincial, and territorial guidance for PHMs will also be adjusted and updated based on new knowledge, expert scientific opinion, experiences to date, and risk assessments. Up-to-date and evolving information regarding COVID-19 can be found at COVID-19 epidemiology update.
The adjustment of PHMs has been challenging during the COVID-19 pandemic, and jurisdictions have had to make decisions in the context of ongoing uncertainties due to:
- Gaps in available evidence and readily evolving scientific information;
- Emergence of more transmissible variants of concern (VOCs) that cause surges in cases globally;
- Potential for immune escape, particularly associated with VOCsFootnote a;
- Segments of the population that remain unvaccinated because they are either not eligible (e.g., medical contraindications, age limits) or choose not to be vaccinated; and
- The proliferation of misinformation and disinformation which impacts individual acceptance of various PHMs.
More information can be found in the Adjusting Public Health Measures in the Context of COVID-19 Vaccination document.
Transmission of COVID-19
The effectiveness of PHMs in reducing transmission of COVID-19 is directly linked to the ways that COVID-19 is transmitted from a person who is infected to others. This section provides some of the evidence about the modes of transmission of COVID-19, and the context for the PHMs that can effectively reduce the risk of transmission. The relative role of the different modes of transmission for COVID-19 remains unclear.
COVID-19 vaccines have had, and continue to have, an effect on transmission of COVID-19 in Canada and globally; directly by reducing the risk of COVID-19 transmission from those infected, and indirectly by reducing rates of infection and symptomatic disease. The evidence continues to emerge on these topics, including waning immunity over time, the effect of booster doses, and vaccine effectiveness for various VOCs; however, COVID-19 vaccines are not the focus of this evidence summary. For more information, refer to Vaccines for COVID-19 and COVID-19 vaccine: Canadian Immunization Guide.
Current evidence shows that SARS-CoV-2, the virus that causes COVID-19, is spread via inhalation of infectious respiratory particles of varying sizes, known as aerosols (smaller particles) and droplets (larger particles).Reference 9Reference 10Reference 11
People routinely emit respiratory particles in a wide range of sizes, from larger particles called droplets, to small aerosols that can remain suspended in the air for longer periods. Approximately 80% to 90% of emitted respiratory particles are aerosols, which are typically generated during more regular and frequent respiratory actions (e.g., talking, breathing), rather than through more forceful respiratory actions (e.g., singing, coughing, sneezing) that are usually associated with droplets.Reference 9Reference 12Reference 13 Aerosols containing the virus accumulate near the infected person. Droplets tend to be expelled in a projectile-like fashion, usually by more forceful respiratory activities such as coughing and sneezing, and typically fall more quickly onto nearby surfaces or the ground close to the person who is infected.
Transmission of COVID-19 is more likely to occur at close range where respiratory particles containing the SARS-CoV-2 virus may accumulate.Reference 13Reference 14 There is some evidence to show that aerosols containing the virus can accumulate in enclosed indoor spaces, especially when ventilation is poor, and that they are capable of travelling on ambient air currents for longer distances.Reference 9Reference 11Reference 13Reference 14Reference 15Reference 16Reference 17Reference 18 However, in well-ventilated areas, transmission over longer distances is less common, as aerosols tend to diffuse rapidly with distance and become diluted by the introduction of outdoor air, resulting in a decrease in concentration or quantity of virus. In rare instances, indirect long-range transmission of COVID-19 has been observed across adjacent rooms or separate floors between people with no previous direct contact.Reference 10Reference 19Reference 20
Real-world and laboratory-based experimental studies, as well as modelling and epidemiological analyses of COVID-19 outbreaks throughout the pandemic, indicate that transmission of COVID-19 can occur via inhalation of infectious aerosols. Studies have shown that the majority of infectious particles emitted from people with COVID-19 were aerosols produced during talking, breathing and singing.Reference 10Reference 21 Evidence of asymptomatic and pre-symptomatic COVID-19 transmission also suggests that transmission does not exclusively take place when symptoms are occurring (e.g., coughing, sneezing), and indicates an important role of respiratory actions that are more frequent and regular (e.g., talking, breathing, singing, exercising);Reference 10Reference 22 however, the relative contribution of these compared to symptomatic transmission is not known.Reference 22Reference 23Reference 24
Early in the pandemic, superspreading eventsFootnote b were seen to be major drivers of COVID-19 transmission, with modelling studies suggesting that a small number of COVID-19 cases were the source of many infections.Reference 10Reference 11Reference 25Reference 26Reference 27 However, with the emergence of Omicron, this may not be the case and more evidence is needed. Superspreading events and outbreaks have occurred in settings such as restaurants, offices, multi-unit residential buildings, nightclubs, recreational facilities, industrial workplaces, choir rehearsals, and venues used for religious services.Reference 19Reference 28Reference 29Reference 30Reference 31Reference 32Reference 33Reference 34Reference 35Reference 36 Common features or factors across these events and outbreaks have included: Reference 9Reference 11
- Poorly ventilated indoor areas that are often crowded and enclosed;
- Close-range interactions between individuals, often without properly worn well-fitting face masks;
- High-risk respiratory actions (e.g., coughing, sneezing, singing, shouting, and heavy breathing during exercise); and
- Extended duration of exposure to shared air with infected individuals.
Fomites are surfaces or objects, such as door handles, light switches, phones, tables or elevator buttons, that may be contaminated with infectious pathogens. Contact with fomites, followed by touching of the eyes, mouth or nose, is a possible mode of COVID-19 transmission.Reference 15 This mode of transmission is very difficult to study, and evidence is limited.
Experimental studies have shown that SARS-CoV-2 has the potential to survive in the environment for anywhere from several hours to days depending on the surface type and relative temperature and humidity of the environment.Reference 15Reference 37Reference 38 There is also some evidence that the length of time the virus can survive on surfaces may differ between variants.Reference 39Reference 40 SARS-CoV-2 can remain viable longer on smooth surfaces such as plastic or steel, compared to porous surfaces such as cardboard or cotton.Reference 41Reference 42 However, experiments on SARS-CoV-2 persistence have been conducted in controlled laboratory settings, which are generally considered more favourable for viral survival and may not accurately mimic real-world conditions.Reference 43 Several studies have shown that fragments of SARS-CoV-2 can be detected on surfaces in community settings, but these studies measure viral genetic material, which will not distinguish whether the virus is viable or inactivated.Reference 44 The relative role of fomite transmission compared to respiratory transmission remains unclear.
Other factors affecting transmission
An important consideration for COVID-19 transmission is the infectious dose. Duration and concentration of exposure to the virus are key factors in transmission, where infection is more likely to occur when an individual is exposed to a higher dose of SARS-CoV-2 virus.Reference 13Reference 15 An infectious dose may result from exposure to high concentrations of virus for a short period of time, as well as prolonged or repeated exposures to smaller concentrations of virus.Reference 13Reference 15Reference 45 Concentration of exposure will be influenced by factors such as the viral load of the source, the type and setting of the exposure (e.g., close-range or long-range contact, indoor or outdoor exposure, etc.), and the number of infectious sources. Reference 13Reference 15Reference 45Reference 46 Currently, the infectious dose needed to transmit COVID-19 is not known or well-defined.
Variants of concern
As expected, mutations in the SARS-CoV-2 virus have resulted in genetic variants. Some of these mutations have led to variants of concern (VOCs) which are associated with increased transmission. More information on SARS-CoV-2 variants is available on Canada.ca.
The Alpha variant emerged in late 2020 and was considered to be up to 1.2 times (range: 0.3-1.2 times) more transmissible than the original strain.Reference 47 This was followed by the emergence of the Delta variant, with studies suggesting that Delta was up to 1.2 times (range: 0.4-1.2 times) more transmissible compared to Alpha.Reference 48 At the time of writing, the Omicron variant has become the dominant strain in Canada.Reference 49 When Omicron first emerged it was reported to be up to 5.6 times (range: 2.0-5.6) more transmissible than Delta.Reference 50Reference 51Reference 52 In recent months, Omicron subvariants have emerged with evidence of increased transmissibility compared to the original Omicron variant.Reference 52Reference 53Reference 54 Mutations identified in the Omicron variant have resulted in partial immune escape, where relatively more cases (compared to earlier variants) are occurring in vaccinated individuals and those who have recovered from previous infection, which has contributed to its rapid spread.Reference 53
Some population-level factors that can increase an individual's risk of being exposed to, contracting, and spreading COVID-19 may include:
- Local COVID-19 epidemiology and higher levels of community transmission;
- The emergence and spread of VOCs within the community, particularly VOCs with increased transmissibility and/or immune escape properties; and
- Easing of community-based PHMs (e.g., mask mandates, school and business closures, and gathering size restrictions) by governments, public health authorities (PHAs), and operators of public settings, when COVID-19 continues to spread in the population.
Some individual-level factors that can increase an individual's risk of being exposed to, contracting, and spreading COVID-19 may include:
- Not adhering to individual-level PHMs, such as properly wearing masks, staying home when sick, and practicing hand hygiene;
- Not adhering to community-based PHMs when they are implemented, such as avoiding large gatherings when restrictions are in effect; and
- COVID-19 vaccination status, particularly those who are unvaccinated, partially vaccinated, or requiring a booster dose.
More information on COVID-19 transmission and prevention can be found on Canada.ca.
Social and economic factors
Social and economic determinants of health also have an important impact on COVID-19 transmission, particularly among populations in vulnerable situations and those experiencing inequities.Reference 55Reference 56Reference 57 Some people may be at greater risk of exposure to COVID-19 due to their living and working conditions; for example, those living in overcrowded or substandard housing, and those who are working in service jobs or industry.Reference 55Reference 56Reference 58 Those facing socioeconomic inequities may also be at greater risk for serious complications from COVID-19 due to higher incidence of chronic health conditions in these populations, which may be related to personal health behaviours, food insecurity, low income, lack of access to adequate health care, and other factors that impact health.
Public health measures
Masks play an important role in reducing the risk of transmission of respiratory infections, such as COVID-19. Observational studies have shown a reduction in the incidence of COVID-19 infection in individuals wearing masks compared to those who did not wear masks.Reference 59Reference 60Reference 61Reference 62Reference 63
Medical masks (MMs) (such as surgical/procedure masks) and respirators, can reduce transmission of droplets and some aerosols from someone with COVID-19 to others (i.e., source control).Reference 59Reference 64 They can also provide protection to the wearer by reducing the risk that they will inhale infectious respiratory particles when exposed to someone who is infected (i.e., protection from infection).Reference 59Reference 63Reference 65 Results of observational studies and controlled trials comparing the effectiveness of MMs and respirators are mixed, with some studies suggesting no significant difference in protection when using a MM compared to a respirator, and others suggesting a greater protective effect from respirators.Reference 59Reference 66Reference 67 Respirators typically offer a better fit, giving them an advantage over medical masks which often have gaps between the mask and the face. MMs and respirators are regulated as Class I medical devices by Health Canada and must meet standards for characteristics such as particle filtration (i.e. filtration efficiency), pressure differential, flammability, and possible fluid resistance.
Non-medical masks (NMMs) provide some level of source control and may also protect the wearer from exposure to infectious particles to varying degrees.Reference 59Reference 68 There are a wide range of NMMs available in Canada, from tightly woven fabric materials made with various numbers of layers, pockets, and filtration layers; to those that are made from synthetic polymers (e.g., melt-blown filtration materials) and look very much like MMs and respirators. Performance of NMMs is highly variable and dependent upon their construction and fit.Reference 59Reference 68 NMMs have generally been shown to be less effective than MMs or respirators, for both source control and protection for the wearer.Reference 59Reference 66 NMMs are not regulated in Canada and there are no national standards. Some NMMs commercially available in Canada may meet standards for NMMs established by the Bureau de normalisation du Québec, or those in other countries (e.g., ASTM F3502-19, CEN CWA 17553:2020, AFNOR Spec S76-001).
There are several factors that affect the effectiveness of masks, mainly fit, filtration efficiency and breathability.Reference 59Reference 66Reference 68 Of these factors, fit is considered to have the largest impact on the effectiveness of a mask.Reference 68Reference 69 Masks that are loose fitting or gaping away from the face have been found to have lower efficacy compared to tight-fitting masks with no gaps.Reference 70 Several methods have been identified for improving mask fit, and subsequently mask performance, including the modification of ear loops, tucking in the sides of a MM, and using a mask fitter over top of a mask.Reference 71Reference 72 Layering a cloth mask over a MM has also been identified as a method for improving mask fit and performance.Reference 72
Filtration efficiency refers to the ability of a mask to reduce particle penetration through the mask fabric and is another key driver of performance.Reference 59Reference 66 Laboratory studies indicate that respirators have the highest filtration efficiency, generally 95% or higher for certified respirators, followed by MMs and then NMMs.Reference 59Reference 66Reference 69 Although the filtration efficiency of NMMs is reported as being lower than MMs and respirators, it varies widely and is heavily dependent upon the materials and methods used to construct the mask.Reference 59Reference 66Reference 69 Studies have found that filtration efficiency is dependent on the fabric's quality (e.g., tightness of the weave, fibre or thread diameter) and inherent characteristics of the fabric (e.g., electrostatic charge and ability to withstand moisture).Reference 66Reference 68 Masks with multiple layers have a higher filtration efficiency compared to single layer masks, and the addition of a middle layer of filter-type fabric has been shown to increase filtration efficiency.Reference 68
There are few studies that discuss the impact of breathability on transmission risk; however the filtration efficiency of a mask must be balanced with its breathability. NMMs constructed of more than three layers or made with non-porous materials have been found to have very poor breathability, and are unlikely to be well tolerated.Reference 68Reference 73
PHAC provides recommendations for individuals on mask use, including when masks should be worn, how to select a mask, and how to wear a mask properly.
Face mask use at the community level
Ecological studies have suggested that the implementation of community-based masking policies at national and regional levels is associated with decreased COVID-19 incidence, hospitalization, and mortality.Reference 66Reference 74 These ecological studies, along with numerous modelling studies, estimate that universal mask wearing in a community can lead to a decrease in COVID-19 transmission, particularly when adherence is high.Reference 75 However, many ecological studies do not report on adherence to masking policies or on the types of masks used in the community, making it challenging to compare masking policies in different communities.
Masking policies can also be implemented within specific settings, such as schools and workplaces. Schools with mask requirements have been shown to have lower levels of COVID-19 transmission compared to schools without mask requirements; and some evidence suggests a reduction in COVID-19 transmission in work and community settings, such as meatpacking plants and hair salons, when masks are used and layered with other PHMs.Reference 76Reference 77Reference 78Reference 79Reference 80Reference 81 This evidence suggests that mask use may be beneficial in other environments where individuals are in close contact for extended periods of time, such as congregate living settings and on public transportation.
Information on mask use in workplaces can be accessed through the Canadian Centre for Occupational Health and Safety.
Current evidence suggests proper ventilation of indoor settings is a key PHM for limiting the transmission of COVID-19.Reference 12Reference 14Reference 18Reference 82Reference 83 Ventilating a room or indoor space involves diluting and replacing potentially contaminated indoor air with air from the outdoors, either through natural or mechanical means.Reference 82Reference 83 Results from epidemiological, experimental, and modelling studies have shown that factors such as ventilation, airflow, air filtration, and access to fresh air play an important role in reducing COVID-19 transmission in any indoor setting.Reference 82Reference 84Reference 85Reference 86Reference 87 During the COVID-19 pandemic, many outbreaks and superspreading events have taken place in indoor settings where ventilation and airflow were restricted or poor, highlighting the importance of this PHM.Reference 13Reference 19Reference 28Reference 29Reference 30Reference 31Reference 32Reference 33Reference 34Reference 35Reference 36Reference 86
Natural ventilation, which involves opening multiple exterior windows and doors of a space or a building to remove potentially contaminated indoor air, has been shown to be an effective, low-cost way to reduce the risk of COVID-19 transmission, especially in settings and older buildings where effective mechanical ventilation is not available.Reference 12Reference 82Reference 83 Opening windows and doors can help create a directional airflow from the outdoors that moves through a space and dilutes and exhausts indoor air, limiting the potential accumulation of infectious aerosols.Reference 12Reference 82Reference 83 It is important that the indoor air is expelled, and not just circulated within the space.Reference 88 Natural ventilation may be limited by settings that do not have opening windows, outdoor air quality, and climatic conditions (e.g., weather, outdoor air pollution, levels of pollen and other irritants).Reference 83
Mechanical ventilation via the use of heating, ventilation, and air conditioning (HVAC) systems, has been shown to reduce the risk for COVID-19 transmission in indoor spaces.Reference 83 Mechanical ventilation is most effective when HVAC systems are properly configured for:Reference 82Reference 85Reference 87
- Maximum fresh air input and air changes;
- Operation beyond times of occupancy or continuous use to help flush out potentially infectious indoor air;
- Filters with the highest Minimum Efficiency Reporting Value (MERV) rating that the system can handle and that are routinely changed as recommended by the manufacturer;
- The type of setting and activity; and
- The number of occupants.
Evidence supports the effectiveness of ventilation in reducing the risk of COVID-19 transmission beyond residential settings. The same measures and principles when applied to non-residential settings (e.g., schools, workplaces, commercial spaces, retail areas) have been shown to decrease the risk for COVID-19 transmission, especially when used in conjunction with other effective public health measures (e.g., universal masking).Reference 11
Natural or mechanical ventilation may not always be possible within all settings due to structural or economic constraints. For example, older buildings may not have a functioning HVAC system, or windows that can open. The significant cost of implementing and operating a mechanical ventilation system in buildings may present a financial barrier for some facility operators. In these situations, where natural or mechanical ventilation is not possible or cannot be improved, evidence indicates that air filtration, via the use of portable air purifiers, can help to reduce the concentration of COVID-19 virus in the air.Reference 89Reference 90Reference 91Reference 92Reference 93 Portable air purifiers are most effective when equipped with a high-efficiency particulate air (HEPA) filter and appropriately sized for the room in which it will be used.Reference 87Reference 88Reference 89Reference 90Reference 91 Although using air purifiers indoors can add an additional layer of protection, they should be used alongside other individual PHMs.
Ultraviolet germicidal irradiation (UVGI) has been proposed as a method to inactivate the SARS-CoV-2 virus in the air or on surfaces and objects. While UVGI has been shown to be effective against SARS-CoV-2 in laboratory settings and simulation studies, there is little evidence available to support claims that this is an effective tool to reduce the risk for COVID-19 transmission in real-world applications.Reference 94Reference 95Reference 96Reference 97
Measurement of carbon dioxide (CO2) levels has been proposed as a surrogate for determining how well a room is ventilated, given that high CO2 levels may be an indicator of low air changes within a space.Reference 98 However, a low level of CO2 in an indoor space does not necessarily mean that the risk of COVID-19 transmission is low, and CO2 levels alone do not reflect all transmission risks. Evidence is still limited on the effectiveness of monitoring CO2 levels for reducing the risk of COVID-19 transmission.
PHAC provides recommendations on how to improve ventilation in the context of COVID-19, and guidance on indoor ventilation during the COVID-19 pandemic.
Physical distancing and reducing contacts
Physical distancing, where some specified distance between individuals is maximized or maintained, is another PHM that can reduce the risk of transmission of COVID-19.Reference 15Reference 99 Physical distancing recommendations for COVID-19 were originally developed and implemented based on observations that close-range transmission is more common than long-range transmission for influenza, COVID-19, and certain other respiratory infectious diseases. Transmission across longer distances is likely less efficient than at shorter distances, as there is a greater likelihood for a person to be directly exposed to a larger dose of virus at close range.Reference 9Reference 13Reference 15 Close contact exposures increase risk of transmission as respiratory particles containing the SARS-CoV-2 virus tend to be more concentrated closer to someone who is infected.Reference 13
Evidence shows that physical distancing of at least one metre lowers the risk of COVID-19 transmission compared to distances of less than one metre, but distances of two metres could be more effective, suggesting that protection against COVID-19 transmission is increased as distance is increased.Reference 99Reference 100Reference 101 Generally, the risk of COVID-19 transmission is lower when the number of interactions between people, the number of individuals involved in each interaction, and duration of each interaction are minimized, and when interactions occur at the greatest distance possible.Reference 9Reference 11Reference 13Reference 15 Clusters and outbreaks in indoor spaces have been reported widely throughout the pandemic, with a lack of physical distancing being one of several factors contributing to COVID-19 transmission.Reference 15
Community-level measures: Physical distancing and reducing contacts
Physical distancing at a community level can help reduce COVID-19 transmission. Outbreaks have been a significant source of COVID-19 spread in Canada and point to increased risk in closed and crowded settings, such as long-term care and retirement residences, and congregate living settings (e.g., farmworker residences, homeless shelters, student residences, detention centres, correctional facilities), where physical distancing can be more difficult.Reference 102Reference 103Reference 104Reference 105 This also highlights the risk for Indigenous communities, particularly those that are remote and isolated, which typically experience higher rates of overcrowding and poor housing conditions.
Physical distancing can be implemented at the community level when businesses, restaurants, and schools implement policies and provide tools (e.g., signage and floor markers) to ensure that individuals are able to safely distance. In schools, physical distancing is a supplementary measure that can be used in combination with other PHMs to reduce COVID-19 transmission, especially during outbreaks or periods of increasing or high transmission.
Measures that limit the number of contacts in the community have also been effective at reducing COVID-19 transmission. Transmission events in high-risk community settings, such as sporting events, restaurants, and nightclubs, have been attributed to a high number of close and sustained contacts, further highlighting the importance of reducing crowding in community settings.Reference 106 Observational and modelling studies have shown that when COVID-19 cases rise, gathering restrictions at national and regional levels (e.g., gathering size restrictions, school and workplace closures, occupancy limits in businesses) have played an important role in reducing transmission of COVID-19.Reference 107Reference 108Reference 109Reference 110Reference 111Reference 112
Internal PHAC modelling has shown that restrictive measures that limit contacts, like school and business closures, and gathering size restrictions, are associated with reductions in the number of COVID-19 cases, hospitalizations, and deaths. The Oxford Stringency Index, developed by the University of Oxford and captured in the COVID-19 Government Response Tracker, shows similar patterns in other countries when these types of restrictive measures are implemented.
Physical screens and barriers
There is limited evidence related to the effectiveness of screens and barriers, which are often made of plexiglass, in reducing the risk of COVID-19 transmission.Reference 113 Screens and barriers have been used to prevent the exchange of potentially infectious respiratory particles between two individuals interacting in close proximity.Reference 113 They are more likely to be effective if individuals are face-to-face or very close together; interactions between individuals are short, so that aerosols do not accumulate; and when the barriers are appropriately sized and designed to work with the airflow of a space.Reference 113Reference 114
If not designed to work with the airflow of a space, accumulating evidence suggests that screens and barriers can have a negative impact on ventilation by potentially blocking or re-directing airflow and creating pockets of poor air circulation.Reference 114 Some evidence suggests that screens and barriers are a complementary measure to ventilation, as barriers can re-direct respiratory particles away from other people, and allow enough time for any ventilation measures in place to dilute and replace potentially contaminated air.Reference 113 There is also evidence to suggest that screens and barriers can reduce surface contamination in some settings, which could reduce the risk for transmission by fomites, although evidence for this effect is currently limited.Reference 114Reference 115
Stay home when ill
Many people with COVID-19 will develop symptoms of disease; however, the exact percentage of asymptomatic versus symptomatic cases is unclear and will vary by variant. These symptoms could include sore throat, runny nose, sneezing, new or worsening cough, shortness of breath or difficulty breathing, temperature of 38°C or higher, feeling feverish, chills, fatigue, myalgia, new loss of smell or taste, headache, gastrointestinal symptoms, and feeling very unwell.Reference 116
Evidence from early in the pandemic, prior to the emergence of VOCs, showed that the median incubation period (time from infection to symptom onset) for COVID-19 was five to six days, with 95% of symptomatic cases displaying symptoms by day 14.Reference 117 Limited evidence, suggests that the incubation period for the Omicron variant may be slightly shorter (i.e. 3-4 days).Reference 118Reference 119Reference 120Reference 121Reference 122Reference 123 A higher viral load is associated with increased infectiousness, which tends to peak just before or at symptom onset and is significantly reduced 7-10 days later, in the majority of cases.Reference 124Reference 125Reference 126
The infectious period for COVID-19 is generally considered to start two days before symptom onset and can last until ten days following symptom onset, and evidence suggests that the infectious period for Omicron is similar to other variants.Reference 127Reference 128Reference 129Reference 130Reference 131 People who experience severe disease or who are immunocompromised tend to have longer infectious periods.Reference 126 Vaccination may reduce the infectious period for breakthrough infections; however, further research is needed on this topic.Reference 132
Separating individuals who are infected from susceptible individuals is one way to reduce transmission of an infectious disease and to break the chain of transmission. One way to accomplish this is for individuals to self-monitor for symptoms compatible with COVID-19 and to stay at home and away from others if symptoms occur. Although this PHM will not be effective against asymptomatic and pre-symptomatic transmission, modelling suggests that the isolation of individuals with symptoms compatible with COVID-19 can help to reduce transmission.Reference 133 Because infectiousness peaks just prior to symptom onset, isolating cases as soon as possible is helpful in breaking the chain of transmission.Reference 126
Many schools, workplaces, and other community-based settings have implemented policies for COVID-19 symptom screening prior to attending various activities, events, or community settings. The purpose of these policies and requirements has been to ensure that people with symptoms of COVID-19 do not enter community settings and risk spreading COVID-19 within the setting.
Screening policies can be passive or active. Passive screening refers to self-monitoring and self-reporting possible symptoms of infection or exposure (with reminders communicated through signage, voicemail, and websites) and active screening refers to asking individuals questions about possible symptoms of infection or exposure.Reference 134
Evidence from the United States suggests that workplace policies supported by state-level emergency sick leave provisions, which allow sick individuals to stay at home, may be associated with decreased COVID-19 cases.Reference 135 Additional research is warranted that investigates the effect on transmission of community-based policies that support individuals with COVID-19 symptoms to stay at home.
Guidance on how to quarantine or isolate at home if you have or may have COVID-19 is available on Canada.ca.
Additional resources for operators of non-health care community settings related to reducing COVID-19 risk in community settings and on the public health management of cases and contacts associated with COVID-19 are also available.
Hand hygiene and respiratory etiquette
Evidence shows that hand hygiene is effective at reducing transmission of many respiratory infections.Reference 136Reference 137Reference 138 Hand hygiene refers to washing hands with soap and water for at least 20 seconds or using an alcohol-based hand sanitizer containing at least 60% alcohol.Reference 139Reference 140Reference 141 Both methods are considered reliable for cleaning hands contaminated with enveloped viruses, such as SARS-CoV-2.Reference 139 While direct evidence is limited related to transmission of COVID-19 via direct contact, hand hygiene can reduce the risk of infection from touching surfaces contaminated with viruses and bacteria by breaking the chain of transmission between contaminated hands and susceptible mucous membranes in the eyes, nose, and mouth.
Respiratory etiquette is a long-standing public health recommendation, where individuals are advised to cough or sneeze into a tissue or the bend of their arm, instead of their hand. They are also advised to perform hand hygiene immediately afterwards, and to dispose of the used tissue as soon as possible. Although there is no direct evidence about the effectiveness of this measure, particularly in the context of COVID-19, this PHM may be effective in reducing transmission of COVID-19 via source control, by reducing the dispersion of infectious respiratory droplets from an infected individual. Evidence suggests that the dispersion of respiratory droplets can be reduced when individuals cough or sneeze into a handkerchief, elbow covered with a sleeve, or a mask.Reference 142Reference 143Reference 144 However, use of N95 respirators is the most effective method for reducing the dispersion of respiratory droplets expelled by coughing.Reference 143
There is limited evidence on the specific role of hand hygiene (e.g., hand sanitizer stations in businesses, workplaces not involved with providing health care, and retail settings) and promotion of proper respiratory etiquette as community-based measures for reducing the transmission of COVID-19. However, the evidence available on the role of these measures in reducing COVID-19 transmission at the individual level suggests that their promotion in community settings may be beneficial.
Guidance is available on hand washing and staying healthy, as well as on how to protect your health. A list of hand sanitizers that are authorized for sale in Canada is also available.
Cleaning and disinfection
Cleaning and disinfecting surfaces that may be contaminated with the SARS-CoV-2 virus is an intervention that may reduce the risk of transmission by fomites, should individuals touch contaminated surfaces and then touch their mucous membranes (e.g., eyes, nose, or mouth) prior to practicing hand hygiene.Reference 37 Cleaning and disinfecting of surfaces and objects with soap and water, cleaning products, and/or disinfectant products can inactivate or remove SARS-CoV-2 if it is present on a surface.Reference 37Reference 38Reference 41Reference 43 A list of disinfectants with evidence for use against COVID-19 is available on Canada.ca.
Routine cleaning and disinfection of common high touch surfaces (e.g., electronic devices, toilets, light switches, door handles, and children's toys) has been recommended as a PHM to reduce the risk of COVID-19 transmission throughout the pandemic.Reference 37 Evidence in community settings, including schools, workplaces, and social venues, suggests that cleaning and disinfecting high-touch surfaces may help to reduce the presence of SARS-CoV-2 on environmental surfaces and the possibility of transmission.Reference 43
The misuse and overuse of cleaning and disinfection products may cause adverse acute and chronic health impacts, such as allergic reactions, poisoning, asthma, rhinitis, and dermatitis.Reference 145 While cleaning and disinfecting practices should be conducted regularly or as needed to reduce potential SARS-CoV-2 contamination, increased frequency of cleaning and disinfection may be beneficial only in certain instances depending on various factors, such as high community transmission, lack of other layered PHMs, and the possibility that a space has been occupied by a suspected or confirmed case of COVID-19.Reference 37
Implementation of public health measures
The choice and manner of implementation of the PHMs described above are determined by the settings in which they are being used. Characteristics of a setting, such as the intensity of interactions between people in the setting (e.g., number, duration, and distance of interactions) and environmental factors (e.g., size, layout, and ability to ventilate the space) will influence the effectiveness and feasibility of using different PHMs. For example, physical distancing may be a challenge in crowded settings (e.g., public transportation) which, along with aerosol transmission of COVID-19, highlights the importance of other PHMs such as ventilation and use of masks in these settings.
Other settings where individuals are in close contact for extended periods of time, such as workplaces and congregate living settings, require special consideration in terms of how PHMs are applied and the need for involvement of appropriate experts (e.g., occupational health and safety, HVAC professionals, local PHAs). Settings such as First Nations, Inuit and Métis communities and remote and isolated communities have additional considerations and require tailored approaches based on culture, social determinants of health, the history of colonization, language, and geography.
Adherence to public health measures
When PHMs are implemented, they will be less effective at reducing COVID-19 transmission if people are not adhering to recommended or required measures, either by choice or because they are not able to due to life circumstances.
Data from the most recently completed wave of Canada's COVID-19 Snapshot Monitoring Study (COSMO), collected in April 2022, suggest that adherence to PHMs in Canada is moderate to high.Reference 146 Among respondents surveyed, 91% report wearing a face mask when it is mandatory; 57% report wearing a face mask when it is not mandatory; 77% report frequent hand washing with soap or hand sanitizer; 59% report usually practicing physical distancing; and 87% report usually staying home when sick.Reference 146 Data from previous waves of the survey show that the primary motivators for adhering to PHMs include protecting loved ones and the community, protecting oneself, and following recommendations of health experts, scientists and government. Furthermore, anxiety related to family's health, trust in government sources of information, and trust in medical experts were the strongest predictors of adherence to PHMs.Reference 146
Research shows that adherence to PHMs over time may be influenced by a variety of factors, such as housing, working and community conditions; financial and social circumstances (e.g., caregiving responsibilities); attitudes and beliefs (e.g., trust in government, science, and health care; ideologies; social norms); demographic factors; and, cultural and spiritual factors.Reference 147Reference 148Reference 149Reference 150Reference 151 The length of the pandemic and the resulting pandemic fatigue may also affect adherence to PHMs.Reference 152
Research also suggests that the following factors are associated with increased adherence to COVID-19 PHMs: older age, female gender, higher trust in government and science, higher perceived risk of COVID-19, greater use of traditional news media, higher COVID-19 knowledge, increased anxiety, and perceived self-efficacy to adopt public health measures.Reference 153Reference 154 In contrast, non-adherence to PHMs has been shown to be associated with living in a rural area, suffering from depression, belief in conspiracy theories, psychological reactance (i.e., response to the perceived or real threat or loss of a behavioural freedom), narcissism, strong support for personal freedom, and smoking.Reference 154Reference 155Reference 156Reference 157Reference 158Reference 159
Some populations have a higher incidence of negative consequences associated with restrictive PHMs, which may reduce adherence to such measures and consequently lead to ongoing transmission.Reference 55Reference 56Reference 57Reference 58 For instance, lockdowns and business closures can interfere with work, income and access to care; gathering restrictions can reduce access to social supports and networks; and school closures can affect work and education.Reference 1Reference 2Reference 3Reference 4 These measures can lead to greater substance use, mental health issues, and exposure to domestic violence.Reference 1Reference 2Reference 3Reference 4Reference 160 People with physical or mental disabilities, racialized populations, and Indigenous people may be particularly affected as they face greater inequities.Reference 5Reference 6Reference 7Reference 8Reference 55Reference 58Reference 116 Average life satisfaction and the prevalence of high self-rated mental health and high community belonging were all lower among racialized individuals during the pandemic compared to before the pandemic.Reference 162 Some populations may also have reduced capacity to adhere to PHMs due to socioeconomic barriers, such as sub-standard and overcrowded housing, lack of access to potable water, and food insecurity.
In a cross-sectional survey administered to Canadians in May 2020, 90% of respondents reported confidence in their ability to adhere to a variety of PHMs.Reference 149 The survey suggested that those who identified as male, were younger, and in the paid workforce were less likely to consider recommended PHMs as effective, and had less confidence in their ability to adhere to recommended measures.
Although self-reported mask use and adherence to other PHMs in Canada is moderate to high, strategies for promoting public adherence to PHMs, including effective risk communication strategies, continue to be important. This is particularly relevant as the duration of the pandemic increases and the likelihood of pandemic fatigue grows.
Communication of information and advice during a public health emergency is a critical public health role. It is an essential activity for keeping the public informed about evolving public health risks, as well as appropriate evidence-based actions that can be taken to protect health at the population and individual level. Communication and public education strategies should complement other regulatory measures and efforts to remove barriers to following PHMs so that these behaviours are as easy as possible to enact.
A vast body of literature exists regarding effective strategies for risk communication and promoting adherence to PHMs. A review of the evidence identified the following effective, evidence-based communication strategies:Reference 163Reference 164
- Clear and action-oriented messaging that is repeated often;
- Leveraging and collaborating with trusted messengers (e.g., physician, community leader, public health professional);
- Trust in the message and in the person delivering the message can be built by addressing uncertainty, acknowledging previous errors, and being transparent about the fact that evidence-based recommendations and information change as evidence evolves;Reference 164
- Having messages translated into official languages, languages of Indigenous communities, and other languages as appropriate;Reference 165Reference 166
- Use of mainstream media, including peer health communication and intensive multimedia interventions;
- Use of frequencies over percentages and use of relative risk over absolute risk or number needed to treat when reporting statistics; and
- Tailoring messages and the communication medium to target audiences (e.g., with respect to age, sex, gender, race, ethnicity, ability status, and parental and caregiving responsibilities, among other demographic and identity factors).Reference 167Reference 168Reference 169Reference 170
The engagement of stakeholders is important for identifying the appropriate message framing and medium that is culturally competent, accessible, and meaningful to the lived experiences of the target audience.Reference 163Reference 167Reference 168Reference 169Reference 170Reference 171Reference 172 Consideration should be given to those who may not be able to use or access standard resources, and those for whom standard messaging is not suited or tailored to their circumstances (e.g. persons who do not speak English or French, persons with disabilities, First Nations, Inuit and Métis communities and remote and isolated communities).Reference 163
Evidence from infectious disease outbreaks has shown that misinformation represents a significant threat to public health, largely because adults who are more susceptible to believing health misinformation are less likely to adhere to PHMs.Reference 173Reference 174 It has been shown that public health messaging should aim to improve general knowledge of COVID-19, communicating what is known, what remains uncertain and what evidence is informing recommendations.Reference 148Reference 175Reference 176 Qualitative research suggests that targeting groups of individuals at higher risk of non-adherence to PHMs or those with negative attitudes about the guidelines can be an effective strategy to promote adherence to PHMs.Reference 148
In the April 2022 wave of the Canadian COSMO survey 65% of respondents reported trusting Canadian government sources of information (e.g., briefings and/or websites) related to COVID-19.Reference 146 Furthermore, data suggest that the most frequently trusted sources of information for COVID-19 among respondents are healthcare workers, such as doctors and nurses (82%); scientific experts (82%); international health authorities, such as the World Health Organization (68%); and provincial/territorial government briefings and/or websites (60%).Reference 146 However, around 40% of respondents reported that it was difficult to find true and reliable information to make health-related decisions. In April 2022, about one third of respondents indicated that they approved of the Government of Canada's handling of the pandemic so far.Reference 146
Currently, important knowledge gaps exist surrounding communication with respect to adherence to COVID-19-specific PHMs. These include:
- The ideal delivery of messaging to change behaviour and to increase adherence to PHMs;
- Determining the ideal person to deliver messages to specific target audiences (including characteristics of a trusted leader);
- Addressing and mitigating adherence to PHMs in the presence of pandemic fatigue; and
- Effective social marketing campaign designs, aimed at communicating COVID-19-specific knowledge, attitudes and behaviour change.Reference 163
This summary provides an overview of the evidence available to inform appropriate actions that may help to reduce the risk for COVID-19 transmission among individuals and communities across Canada.
As the evidence is continually evolving, conclusions and approaches may change. For this reason, this summary has focused more on reviewing the current state of evidence, emphasizing those strategies that have a fairly well-established evidence base, and less on the practical application of specific PHMs. Given the jurisdictional role of the federal government in public health, implementation of the PHMs, at local, regional, provincial and territorial levels, is up to local or regional public health authorities and governments. Individuals should always follow the latest recommendations and guidance from their local or regional public health authorities and provincial or territorial governments.
For practical examples and recommendations to prevent and reduce the spread of COVID-19, readers are encouraged to refer to PHAC's Prevention and Risks page. For individuals interested in assessing their risk of COVID-19 transmission when visiting or gathering with others, refer to the National Institute on Ageing's COVID-19 Visit Risk Calculator.
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