On average, how many people does one person with the novel coronavirus infect?

Reduced to its most basic level, the answer to the question can tell whether the virus’s spread is unmanageable, overwhelming hospitals or becoming a scary but manageable burden.

Epidemiologists call it the “R number,” or reproduction number, and want it to be as low as possible.

“The usefulness is basically for monitoring epidemic control,” says University of Toronto epidemiologist Ashleigh Tuite. “It provides, effectively, a snapshot of current transmission.”

Think about it this way:

If one person, on average, infects one other person, R is one.

If on average one person infects two, R is two.

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An R of two is catastrophic, if the cycle keeps going, out of control: one person infects two, who infect four, who infect eight, who infect 16, who infect 32 and so forth. Each doubling would happen roughly every five days, and in theory the process only stops when the virus has infected the whole population, which in Canada’s case would happen at about the four-month point.

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