COVID-19: Main modes of transmission

On this page

• How COVID-19 spreads
• Settings with higher risk of transmission
• Follow public health measures
• Ventilation

How COVID-19 spreads

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.  

Settings with higher risk of transmission

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.

Follow public health measures

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:

• maintain a physical distance of 2 metres from people outside of your household
• wear a non-medical mask when:
  • you are in public and you might come in close contact with others
  • you are in any indoor space with people from outside your immediate household
  • advised by your local public health authority
• wash your hands often and do not touch your face with unwashed hands
• keep the number of people you have prolonged contact with as small as possible

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:

• closed spaces
• crowded places
• close-contact settings and close-range conversations
• settings where there is singing, shouting or heavy breathing, for example, during exercise

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

Ventilation

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.