What We Know So Far: COVID-19 Transmission


3 June 2021

What does it mean to evolve? It’s a term we hear a lot, but what does it mean? To evolve means to gain through experiences; to learn and to grow. Evolution is a natural state of our everyday reality. Just like people evolve over time, so does our understanding of viruses.
At Public Health Ontario (PHO), we are continually monitoring, reviewing and assessing important research on COVID 19 – including how SARS-CoV-2 (the virus that causes COVID 19) passes from one person to another (also called transmission). Most recently, we looked at COVID 19 transmission through large respiratory droplets and smaller droplets known as aerosols (droplets that can be produced when you cough, sneeze, sing, shout and talk).

Here is what we know so far…

Our understanding of COVID 19 transmission has evolved, but the key prevention measures have not changed. 

No single public health measure on its own is perfect at preventing the spread of COVID 19, but layers of measures will help provide the best protection. These include: 
  • getting vaccinated
  • staying home when sick and when exposed
  • physical distancing and avoiding crowded spaces
  • wearing a well-fitting mask in public when physical distancing is not possible
  • being outdoors or in well-ventilated indoor spaces
  • washing your hands, covering coughs and sneezes, and proper disinfection
Check out our Multilingual Resources page for helpful resources on masking, physical distancing, reducing your risk from COVID 19 and other public health measures.

Transmission of COVID 19 is complex and depends on many factors

COVID 19 can be transmitted at short-range through large respiratory droplets and smaller respiratory droplets known as aerosols (remember – those droplets from coughing, sneezing, singing, shouting and talking). Long-range transmission through aerosols can occur under specific circumstances, particularly when it is crowded and you are in a closed space with poor ventilation. 
The highest risk for transmission is close contact (generally less than 2 metres) over an extended period of time (more time=more exposure) with a person who has COVID 19. However, transmission does depend on many factors, including:
  • singing, shouting, coughing and sneezing with a lot of force (that pushes the droplets out)
  • type of contact with a person who has COVID 19 – the longer and closer contact with an infected person, the higher risk you have
  • personal protective measures such as physical distancing, wearing a well-fitted mask and being vaccinated 
  • the environment – being outdoors or in indoor spaces with good ventilation means lower risk
  • amount of virus an infected person has – more virus is present early in an individual’s infection
Many aerosol droplets are blocked from release by wearing a mask. Blocking their release means they don’t get a chance to remain and collect in the air. 

The bottom line: the best way to prevent COVID 19 is to avoid the “3 C’s”: closed spaces, crowded places and close contact.
PHO will continue to monitor, review and assess COVID 19 as the pandemic evolves and we continue to learn more. The best way to protect yourself is to follow the recommended public health measures and get tested if you think you have or have been exposed to COVID 19. Visit our Prevention and Management page for more information.

If you have specific questions about COVID 19 or think you have been exposed, please contact your local public health unit.
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Updated 3 June 2021