COVID-19 is highly contagious. Government restrictions to impose physical distancing such as business and school closures have curbed its contagion and new cases are down from their peak across the country.
Provinces across Canada have started easing those restrictions, although each in its own way and its own pace. Some have also already said restrictions may need to be reimposed if the outbreak worsens.
But when is it really safe to ease those restrictions? How does that relate to the “peak” of the epidemic? And when might restrictions need to be reimposed?
Tracking the disease’s changing contagion is a key, researchers say. Here’s how it’s done and what that means.
What is the reproduction number?
The contagiousness of diseases is represented by a seemingly simple number: the number of other people a single infected person infects. This is known as the reproduction number, commonly abbreviated with the letter “R.”
The basic reproduction number, R0, pronounced “R-naught,” where “naught” means “subscript zero,” is the fundamental infectiousness of a new disease, when no one has any immunity and no interventions have been imposed to curb its spread.
What R0 means
This diagram illustrates a disease with an R0 of two as it spreads from an initial infection through four “generations.” Each dot represents an infected person.
How contagious is COVID-19?
For COVID-19, the R0 averages around 2.6 to 2.7 based on data from China and South Korea, researchers from the Centre for Evidence-Based Medicine at the University of Oxford report. That means in the absence of interventions, the average infected person gave the disease to between two and three people. That makes COVID-19 about twice as contagious as the flu, more contagious than Ebola, only half as contagious as smallpox and a lot less contagious than measles.
R0 and contagiousness
These diagrams show how quickly each of five viruses spreads over four generations, depending on its R0, which represents how contagious it is.
COVID-19
R0 = 2.6
1 total infection
Influenza
(Spanish flu)
R0 = 1.8
1 total infection
Ebola
(high)
R0 = 2
1 total infection
Smallpox
(high)
R0 = 6
1 total infection
Measles
(high)
R0 = 15
1 total infection
