Brooks Fallis is a critical care physician and the former director of critical care at the William Osler Health System. David Fisman is an epidemiologist and professor at the Dalla Lana School of Public Health.
Despite overwhelming evidence of aerosol-based transmission of SARS-CoV-2, Canada’s infection control strategies have not adequately evolved. Lack of public recognition of airborne spread by hospital-based infection control experts is holding back effective transmission mitigation in schools, indoor workplaces and homes. SARS-CoV-2 is primarily a community virus, not a hospital virus. We are taking a backwards approach to the problem.
It is time to leave the debates in the academic sphere and ask a more practical question: Would publicly declaring COVID-19 airborne and implementing strategies to prevent airborne transmission reduce the spread of the virus and the resulting devastating impacts on essential workers and their families? The answer quite clearly is yes.
Aerosols are tiny particles that float in the air like smoke. Aerosols are constantly produced, even with breathing, but increase with talking, singing or coughing. Without adequate ventilation, infectious aerosols will concentrate indoors, allowing infection despite distancing. Droplets are larger particles produced when sneezing or coughing but drop quickly due to gravity within about two meters of the source.
Much SARS-CoV-2 transmission comes from people who have no symptoms or very mild symptoms but have high respiratory tract viral loads. High viral loads emerge one day before symptoms start and persist for about five days after symptom onset, longer in people who are sicker. This is significant because the highest risk of transmitting the infection is usually before patients require hospitalization.
The uncoupling of onwards transmission from severe symptoms might be the most significant reason the global pandemic has been so difficult to control – people spread the virus without yet realizing they are sick.
COVID-19 contact tracing shows transmission follows the 80/20 rule – 80 per cent of future infections are spread by just 20 per cent of current cases. Super spreader events occur in closed, close, crowded places where aerosols can accumulate, and where individuals engage in vocal activities that generate a lot of aerosol: singing karaoke, shouting over the noise of machinery, breathing heavily during a workout. In many of these settings, the spark is an infectious individual who has no symptoms yet or has symptoms but dares not stay home from work.
Current strategies to mitigate transmission in the community and in particular indoor workplaces fail to recognize these key epidemiologic characteristics of SARS-CoV-2. Community mitigation is modelled around hospital infection prevention and control principles, and heavily impacted by experts whose focus is on preventing transmission in healthcare. Hospital infection prevention and control teams feel confident in droplet precautions – emphasis on hand washing, surgical masks and face shields. But hospitals are not the same as workplaces and schools.
Hospitals are generally very well ventilated. Hospital personal protective equipment is of high standard, so even if staff are not wearing N95 respirator masks, a well fitted surgical mask can provide up to 80 per cent equivalent protection and serves as good source control by limiting aerosol from the wearer. Severe symptoms requiring hospitalization tend to begin about a week into the illness, which is typically past the peak viral load, meaning hospitalized patients may be less infectious than individuals in the community. Health care workers in Canada are highly vaccinated, providing further protection. Despite all these advantages compared to other indoor workplaces, hospital outbreaks do still occur.
Current workplace guidelines follow hospital principles. Continuous masking is not required if distancing is observed, and masks are removed in lunchrooms. The current approach is clearly inadequate based on the indoor workplace transmission we are witnessing.
A transition to an airborne approach revolves around a simple and powerful concept: COVID spreads by breathing shared air, and infectious air accumulates indoors.
Primary mitigation strategies shift from two metres distancing and handwashing to ventilation and high-quality masks. Monitor ventilation using portable carbon dioxide monitors, open windows and emphasize improvements in ventilation or air filtration when ventilation systems are poor. Distancing remains important but high-quality respirators should always be worn while indoors. All breaks and meals where masks will be removed must be moved outdoors or, when outdoors is not possible, in large well-ventilated rooms with maximal distancing and small numbers of people.
While airborne transmission is now acknowledged, changes to infection prevention strategies and efforts to increase public awareness are lacking. These changes will reduce onward transmission in high-risk settings and provide opportunities for a return to normalcy in some spheres: outdoor activities (where aerosols are rapidly dispersed) and even some indoor spaces with guidance on how to eliminate aerosol combined with proper masking.
Shifting to an airborne prevention approach will be life-saving for people working in high-risk essential jobs, and for the families to whom they bring home the infection. It will also impact overall epidemic control and help guide a safer more sustainable economic reopening.
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