Question Period Note: Aerosol Transmission

What does the best available evidence say about the role of aerosol transmission?

• COVID-19 spreads from an infected person to others when respiratory droplets and aerosols are expelled during coughing, sneezing, heavy breathing (e.g. during aerobic exercise), talking or singing. Susceptible individuals may be exposed to the virus by inhaling infectious droplets and/or aerosols or when these infectious droplets come into direct contact with mucous membranes of the eyes, nose or mouth.

• The current evidence indicates aerosols can linger in the air and move a distance of several meters from the infected person. However, there is no evidence that transmission has occurred at longer distances, from room to room or via air ducts.

• Mitigation measures, such as avoiding closed spaces without adequate ventilation, crowded places and close contact situations, together with the use of well-constructed/well-fitting masks decreases the risk of aerosol transmission.

Aerosol transmission refers to the spread of small infectious particles , also known as “aerosols”, from an infected person to others. Droplets vary in size from large droplets that fall to the ground rapidly (within seconds or minutes) near the infected person, to smaller droplets (aerosols), which linger in the air under some circumstances. Large and small respiratory droplets can be inhaled; however, larger droplets fall to the ground faster and may not travel as far as small aerosols. Emerging epidemiological and experimental evidence indicates that there is some risk of aerosol transmission of SARS-CoV-2.

Scientific information on COVID-19 continues to evolve rapidly and PHAC continues to evaluate new evidence as it becomes available to inform intervention and mitigation strategies.

Results from simulation experiments and biological monitoring studies among COVID-19 patients in hospitals and animal transmission experiments support aerosol transmission of SARS-CoV-2. However, it remains unclear to what extent and under what conditions COVID-19 spreads through aerosol transmission and the dose of SARS-CoV-2 that will cause an infection.

Investigators have demonstrated that infectious aerosols can be expelled from COVID-19 cases when breathing, talking, singing and coughing, with some studies identifying the virus in air samples at distances greater than 2 meters from the original source.

Experimental evidence has demonstrated that it is feasible for SARS-CoV-2 within aerosols to remain suspended in the air and viable for hours.

Epidemiological investigations of COVID-19 clusters in different settings (e.g. meat processing plants, indoor choir practice, restaurant, fitness and dance facilities) found aerosol transmission as the most likely source for SARS-CoV-2 infection. Common characteristics included closed indoor environments, the infectious index case was pre-symptomatic or had just started exhibiting symptoms, and there was an extended duration of exposure. Additional possible risks included sub-optimal ventilation and a lack of air circulation/air currents.

SYNOPSIS
Current evidence suggests that there is a risk of aerosol transmission of COVID-19, particularly in poorly ventilated spaces, and when people engage in activities that are likely to generate these aerosols (singing, aerobic activities, loud speech). The Public Health Agency of Canada (PHAC) continues to evaluate new evidence regarding aerosol transmission to inform our intervention and mitigation strategies. PHAC updated its guidance on aerosol transmission on its website on November 3, 2020.
IF PRESSED…

• Aerosol transmission means that very small droplets containing the virus – released when an infected person breathes, talks, sings, coughs or speaks – remain infectious even while suspended in the air for a period of time.

• The amount of exposure and circumstances under which infection can occur with aerosols is not well defined. Known clusters of cases attributed to aerosol transmission have been traced to closed poorly ventilated indoor settings.