COVID-19 for health professionals: Transmission

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Modes of transmission

Learn about the modes of transmission of COVID-19.

Infectious period

The time period in which an individual with COVID-19 is infectious remains uncertain. A person may be infectious for up to 3 days before showing symptoms (pre-symptomatic infectiousness).

Viral RNA levels appear to be highest just before or soon after symptom onset.

Incubation period

The incubation period ranges from 1 to 14 days. The median is 5 to 6 days between exposure and symptom onset. Most people (97.5%) develop symptoms within 11.5 days of exposure.


There is emerging evidence of human re-infection with SARS-CoV-2. This has been documented by individuals confirmed to have been infected by different strains of the virus. Further research is required to fully comprehend the relationship between positive antibody tests and any protection against re-infection.

Currently, we do not know:

Early research appears to indicate that neutralizing antibody levels can decrease following recovery from COVID-19. As serological testing for antibodies expands, the results may provide insight into re-infection and duration of immunity.

The Government of Canada launched the COVID-19 Immunity Task Force in April 2020. The task force is focused on tracking the spread of the virus in the general population and shedding light on immune responses to SARS-CoV-2 in a diversity of:

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