Question Period Note: AEROSOL TRANSMISSION OF COVID-19

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

KEY MESSAGES
• COVID-19 spreads from an infected person to others through respiratory droplets and aerosols that are created when an infected person is breathing and talking, and may be amplified when coughing, sneezing, singing or breathing heavily. Individuals may be exposed to the virus when they inhale infectious respiratory droplets or aerosols, or when these droplets come in contact with mucous membranes of their eyes, nose or mouth.
• Current evidence indicates aerosols can linger in the air for several minutes to hours and move a distance of several meters from an infected person. However, there is no conclusive evidence that transmission occurs at longer distances, such as from room to room or via air ducts.
• Risk of aerosol transmission can be reduced by avoiding closed spaces with inadequate ventilation, crowded places, close contact settings, and close-range conversations; as well as use of well-designed and well-fitting non-medical masks and practicing good hand hygiene.

IF PRESSED…
• Aerosol transmission refers to infections caused by exposure to very small respiratory droplets, sometimes called aerosols, that contain the virus and that can linger in the air, especially in poorly ventilated spaces
• The amount of exposure needed, and the circumstances under which infection can occur with aerosols, are not well established. However, clusters of COVID-19 cases have been attributed to aerosol transmission in poorly ventilated and crowded indoor settings, often when masks and physical distancing were not in place. Preventative measures reduce transmission by this route.

BACKGROUND
Aerosol transmission refers to the spread of small infectious particles, also known as “aerosols”, from an infected person to others. Respiratory particles vary in size from large droplets that fall to the ground rapidly (within seconds or minutes), to smaller respiratory particles (aerosols), which linger in the air under some circumstances. Any size of respiratory particles can be inhaled; however, larger particles 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 causes an infection is not yet established.

Investigators have demonstrated that infectious aerosols can be expelled from COVID-19 cases when coughing, sneezing, singing, shouting or talking, 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 possible for SARS-CoV-2 in artificially suspended aerosols to remain infectious for up to 16 hours.

Epidemiological investigations of COVID-19 clusters in different settings (e.g. meat processing plants, indoor choir practices, restaurants, fitness and dance facilities) found aerosol transmission to be the most likely mode of SARS-CoV-2 transmission. Common characteristics in these clusters included closed indoor environments, a pre-symptomatic or newly symptomatic infectious index case(s) and extended duration of exposure. Additional possible risks included sub-optimal ventilation and a lack of air circulation or presence of air currents.

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