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COVID-19: Main modes of transmission

COVID-19: Main modes of transmission

SARS-CoV-2, the virus that causes COVID-19, spreads from an infected person to others through respiratory droplets and aerosols created when an infected person coughs, sneezes, sings, shouts, or talks. The droplets vary in size from large droplets that fall to the ground rapidly (within seconds or minutes) near the infected person, to smaller droplets, sometimes called aerosols, which linger in the air under some circumstances.

The relative infectiousness of droplets of different sizes is not clear. Infectious droplets or aerosols may come into direct contact with the mucous membranes of another person's nose, mouth or eyes, or they may be inhaled into their nose, mouth, airways and lungs. The virus may also spread when a person touches another person (i.e., a handshake) or a surface or an object (also referred to as a fomite) that has the virus on it, and then touches their mouth, nose or eyes with unwashed hands.  

Outbreak investigations and scientific studies are revealing more about COVID-19 and this new knowledge is being applied to reduce its spread. We know that the virus is most frequently transmitted when people are in close contact with others who are infected with the virus (either with or without symptoms). We also know that most transmission occurs indoors.

Reports of outbreaks in settings with poor ventilation suggest that infectious aerosols were suspended in the air and that people inhaled the virus. These settings have included a choir practice, fitness classes, and restaurants. Transmission in these settings may have been facilitated by certain environmental conditions, such as re-circulated air.

There is no evidence at this time that the virus is able to transmit over long distances through the air, for example, from room to room through air ducts. It is still unclear how easily the virus spreads through contact with surfaces or objects.

While we do not yet fully understand all modes of transmission and their relative importance, and it is likely that multiple modes of transmission occur, we do know that several actions can be taken to help prevent transmission:

Using all of these layers of protection will help to reduce the risk of transmission.

The following settings are particularly risky for transmission of the virus. Avoid or take additional measures and keep exposure very brief in:

It is particularly important to avoid settings where these risks overlap, e.g., closed, crowded spaces where close-range conversations occur.

As winter approaches in Canada, it is more difficult to socialize outdoors. Maximize ventilation by ensuring that heating, ventilation and air conditioning (HVAC) systems are in good working order. Drawing as much fresh air as possible from outside will decrease the concentration of aerosols that may be suspended in the air, and reduce the chances of SARS-CoV-2 spread if those aerosols happen to contain the virus. If the weather permits, open a window. Reduce the noise level in public spaces, for example turn off the music, so people can speak as quietly as possible.

The public health measures we have been practicing continue to be effective in preventing the spread of the virus that causes COVID-19.