6 Ways to Prevent COVID-19 Transmission this Winter

November 23, 2021

Note: this post was written before we became aware of the Omicron variant. While we await the results of the investigation by the scientific community, it is prudent to continue with all prevention measures that were previously recommended, including those referred to in this post. If, as suspected, the Omicron variant is even more transmissible than Delta, these measures will be even more important.


High vaccination rates in Ontario and vaccine mandates have reduced the chances of COVID-19 transmission; however, the Delta variant of the virus is at least twice as transmissible as the original strain. Breakthrough infections (infections in people who are fully vaccinated) have been occurring, although at much lower rates than infections among unvaccinated people.

Because of breakthrough infections, the Ontario COVID Science Advisory Table has said that even in settings where everyone is vaccinated, we must continue to observe familiar preventive measures like masks and distancing if we are going to prevent exponentially increasing transmission rates.

Almost no activities that involve people outside your own household are without risk – the goal is to reduce risk as much as possible. To do so, we should adhere to the “Swiss cheese” model, introduced by safety expert James Reason in the 1990s. This has been adapted for COVID-19 to show that, while no preventive measure is 100% percent effective by itself, we can minimize risk by using a variety of strategies. If the virus figuratively gets through a hole in one slice of Swiss cheese, it can be blocked by subsequent layers.


Multiple layers Improve success

As we return to former activities, we can ensure that we maintain these barriers as much as possible through the following measures:

Vaccination: the single most important thing we can do to avoid COVID-19 is to get vaccinated, and to limit contact as much as possible with unvaccinated people. Vaccine mandates are now legally required in many venues, and many employers are requiring on-site workers to be vaccinated. On November 19, Health Canada authorized vaccines for children ages 5 to 11, and Ontario parents can now book vaccine appointments for their kids. Because vaccine effectiveness wanes with time, the Ontario government has also started a program for booster shots for eligible fully vaccinated adults.

Occupancy limits and physical distancing: The Ontario government has lifted or reduced capacity limits in many settings where proof of vaccination is required. Regardless of legal requirements, physical distancing from anyone not in your own household remains an important prevention measure in any setting. Distancing is important because it reduces the likelihood that infectious droplets and aerosols emitted by an infected person will land on or be inhaled by another person who can become infected.

Ventilation:  It is now widely accepted that SARS-CoV-2, the virus that causes COVID-19, can be transmitted through infectious aerosols that are emitted when an infected person breathes, talks, sings, coughs, or sneezes. The risk of infection increases greatly in indoor settings where infectious aerosols can accumulate in the air to a critical concentration. Where infectious aerosols reach this concentration, transmission may occur through inhalation, even at a distance greater than 2 metres from the source.  That is why indoor transmission of COVID-19 has been found to be about 17 times higher than outdoor transmission.

Good ventilation can help prevent infectious aerosols from reaching a critical concentration in indoor environments, either through dilution with cleaner air, or through capture of harmful aerosols through filtration. Several authorities have issued useful guidance on ventilation measures to prevent COVID-19 transmission, including:

Increasing outdoor air intake through Heating, Ventilation and Air-Conditioning (HVAC) systems as much as possible.

Where possible, installing MERV13 or higher-rated filters in the HVAC system. (MERV stands for Minimum Efficiency Reporting Value and is a measure of how well a filter captures particles of varied sizes.)

Providing at least 6 air changes per hour in most settings, achieved through a combination of fresh air plus adequately filtered air.

Using portable air cleaners with HEPA filters where other ventilation sources  do not provide sufficient air exchange.

Several calculators like this one from the Occupational Health Clinics for Ontario Workers (OHCOW), are available to help you assess ventilation in your building and choose a portable air cleaner if you need one.

Face coverings: face coverings like masks or respirators are another important way of limiting the chance of COVID-19 transmission.  Face coverings are still required in public spaces like stores, and in workplaces where 2 m physical distancing is not maintained. Legally acceptable face coverings include cloth masks, medical or surgical procedural masks, and filtering facepiece respirators (FFRs), the most familiar of which are N95’s.

In the early days of the pandemic, the public was discouraged from using FFRs, so they could be reserved for high-risk workers like those in health care. Recently, however, there is much greater availability of FFRs that provide good fit and filtration, including some that are manufactured in Canada. Where we return to settings that could otherwise be high risk, like crowded indoor spaces, we can improve protection by wearing a well-fitted FFR. Health Canada maintains a list of devices approved for COVID-19 use, including some FFRs. Generally, headband-style FFRs are better fitting than ear loop styles. When legally required under the Occupational Health and Safety Act, respirators must be fit-tested. However, this would not apply to their discretionary use as a face covering. There is evidence that when the user performs seal-checking procedures, even non-fit-tested FFRs provide better protection than other types of face coverings. FFRs with exhalation valves should not be used as a COVID-19 face covering, because potentially infectious aerosols can be emitted through the valve.

If you are using a cloth or medical mask, you can try some of the tips recommended by the US CDC for improving fit and effectiveness.

Good hygiene practices: Transmission by fomites (contaminated surfaces or objects) is not believed to be as important as was thought when the pandemic started, but washing your hands or using hand sanitizer remain essential. Cleaning of high touch points and surfaces also remain important, although care must be taken not to use hazardous cleaning materials or methods.

Screening, testing and tracing: Public health units continue to perform contact tracing for people exposed to COVID-19, so it is important to maintain records of who has been present in your facility so public health can be informed in the event of a positive case. Workplaces and public venues continue to be expected to screen workers and occupants for COVID-19 symptoms before entry into a facility. Some employers are enrolled in a rapid antigen testing program, which provides rapid test kits for free. These tests, which provide results in 15 minutes, can be used for routine, regular screening for COVID-19-19. They are not to be used for diagnosis where COVID-19 is suspected. Information about the provincial program is available here. The province has also announced a rapid testing program for children in high transmission areas. Individuals may choose to purchase rapid test kits for personal use. All other required or recommended prevention measures should continue even if rapid testing is conducted.

Ontario has also expanded the availability of PCR tests, used to diagnose COVID-19, for eligible people like those with symptoms or who have been exposed to someone with the disease.

By using some or all of these multiple barriers to COVID-19 transmission, we increase our chances of enjoying a healthy, happy season.

ECOH can help you with the assessment and planning of your workplace’s COVID-19 prevention measures, including program review, ventilation assessments and respirator fit testing. Contact us at info@ecoh.ca

Access recordings of our past COVID-19 webinars via the links below:

Covid-19: 2020 Hindsight; 2021 Foresight Webinar

Mitigating Airborne Transmission with Effective Ventilation Strategies Webinar

Marianne Levitsky, MES, CIH, ROH, FAIHA

Senior Industrial Hygiene Associate

Marianne Levitsky is a Certified Industrial Hygienist/Registered Occupational Hygienist and a Senior Industrial Hygiene Associate at ECOH, where she has been involved in developing ECOH’s COVID-19 response and advising clients on re-opening plans. She was founding President of Workplace Health Without Borders, a non-profit organization that engages volunteers in promoting occupational health for workers everywhere. She was previously Director, Best Practices, Prevention for the WSIB and an occupational hygienist with the Ontario Ministry of Labour. She is adjunct faculty at the University of Toronto and was a member of the Toronto Board of Health. Marianne has received the OHAO Hugh Nelson award and the Yant Award from AIHA, both of which recognize excellence in occupational hygiene. She has served as chair of the AIHA International Affairs Committee and is an AIHA Fellow.

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