SARS-CoV-2 is transmitted by exposure to infectious respiratory fluids, and the mechanisms by which this occurs are now better understood. Irrespective of improved understanding of transmission, and despite ongoing knowledge gaps, the available evidence continues to demonstrate that existing recommendations to prevent SARS-CoV-2 transmission remain effective. These include physical distancing, community use of well-fitting masks, adequate ventilation, avoidance of crowded indoor spaces, hand hygiene, respiratory etiquette and environmental cleaning.
Exposure to SARS-CoV-2 occurs in three principal ways: (1) inhalation of very fine respiratory droplets and aerosol particles, (2) deposition of respiratory droplets and particles on exposed mucous membranes in the mouth, nose, or eye by direct splashes and sprays, and (3) touching mucous membranes with hands that have been soiled either directly by virus-containing respiratory fluids or indirectly by touching surfaces with virus on them.
People release respiratory fluids during exhalation (e.g., quiet breathing, speaking, shouting, singing, exercise, coughing, sneezing) in the form of droplets across a spectrum of sizes. These droplets carry virus and transmit infection.

  • The largest droplets settle out of the air rapidly, within seconds to minutes.
  • The smallest very fine droplets, and aerosol particles formed when these fine droplets rapidly dry, are small enough that they can remain suspended in the air for minutes to hours.

Infectious exposures to respiratory fluids carrying SARS-CoV-2 occur in three principal ways (not mutually exclusive):

  1. Inhalation of air carrying very small fine droplets and aerosol particles that contain infectious virus. Risk of transmission is greatest within three to six feet of an infectious source where the concentration of these very fine droplets and particles is greatest.
  2. Deposition of virus carried in exhaled droplets and particles onto exposed mucous membranes (i.e., “splashes and sprays”, such as being coughed on). Risk of transmission is likewise greatest close to an infectious source where the concentration of these exhaled droplets and particles is greatest.
  3. Touching mucous membranes with hands soiled by exhaled respiratory fluids containing virus or from touching inanimate surfaces contaminated with virus.

There are documented occurrences in which SARS-CoV-2 appears to have been transmitted over long distances or times. These transmission events have typically involved the presence of an infectious person producing respiratory droplets in an enclosed space with poor ventilation. Factors that increase the risk of SARS-CoV-2 infection under these circumstances include:

  • Enclosed spaces with inadequate ventilation or air handling within which the concentration of exhaled respiratory fluids, especially very fine droplets and aerosol particles, can build-up in the air space.
  • Increased exhalation of respiratory fluids if the infectious person is engaged in physical exertion or raises their voice (e.g., exercising, shouting, singing).
  • Prolonged exposure to these conditions, typically more than 15 minutes.

 The risk of infection is known to be much lower outdoors where ventilation is better.

 Multiple SARS-CoV-2 variants are circulating globally. Potential consequences of emerging variants are the following:

  • Ability to spread more quickly in people. There is already evidence that one mutation, D614G, confers increased ability to spread more quickly than the wild-type SARS-CoV-2.
  • Ability to cause either milder or more severe disease in people. 
  • Ability to evade natural or vaccine-induced immunity. 

Fully vaccinated people have been found to have a lower likelihood of severe illness and death, even if breakthrough infection occurs. They can still spread infection to others, although for a shorter time.

  • Window for Transmission
  • Face Masks
  • General measures to limit COVID-19 transmission in the workplace
  • Infection control and prevention
  • PPE recommendations for Emergency Departments
  • General recommendations
  • Rational use of PPE
  • PPE breach risk assessment
  • Alignment with regional guidance
  • References
  • Resources
  • Section disclaimer