• Sebastian Leck

FACT CHECK: Could up to 70 percent of Canadians get COVID-19?

MOSTLY TRUE. But it requires some context.

On March 11, Patty Hajdu, Canada’s federal health minister, told the Globe and Mail and other national media that it was possible for up to 70 percent of Canadians to be infected with COVID-19. “There are a range of estimates, but I would say that it is safe to assume that it could be between 30 percent of the population that acquire COVID-19 and 70 percent of the population,” she said.

This may seem strange given that nowhere near 30 percent, let alone 70 percent, of the Chinese population has COVID-19. As of March 26, China had around 82,000 cases, with 67,000 in Hubei (Wuhan’s province), according to the World Health Organization (WHO). The population of Hubei is around 59 million, which means only 0.001 percent of that province’s population was infected. So why are the estimates of Canadian officials so much larger?

Understanding how viruses spread is key. In epidemiology, there’s a concept called the basic reproduction number, which is the average number of new infections that result from each case. If the number is less than 1, a virus will die out because the average person isn’t spreading it to anyone. If the number is 1 or higher, it spreads to more and more people.

In theory, the population will eventually reach herd immunity. At that point, there are enough people who have had the virus and are now immune that it’s unlikely for someone who isn’t immune to catch it. (There have been anecdotal reports of people being infected with COVID-19 a second time, but it’s not yet clear whether the virus is adapting or these cases resulted from faulty tests.) Assuming that herd immunity is possible, whether that’s at 30 percent or 70 percent depends on the basic reproduction number: if it’s large, then the number of immune people needed will be correspondingly large. (Scientific American has a great explanation here.)

In the early stages of COVID-19’s spread in Wuhan, several teams of researchers estimated its basic reproduction number. These estimates vary, but it’s likely somewhere between 2 and 3. The WHO, for its part, gave an estimate of 1.4 to 2.5 back in January. This meant the virus was spreading quickly: a team of Chinese researchers estimated that the number of cases was doubling about every seven days.

Given those numbers, it’s reasonable for Health Minister Patty Hajdu to say that between 30 and 70 percent of Canadians could be infected if no preventative measures are taken. At the lowest end, a reproduction number of 1.4 would mean around 30 percent of Canadians get the virus; at the high end of 3, close to 70 percent could get it (you can see the basic formula here). A disease-transmission model created by a team of University of Toronto researchers gave a similar estimate of 35 to 70 percent, with 35 percent reflecting the impact of “modest” control measures.

The range is so wide because minor variations in the reproduction number drastically change the spread, and models provide different pictures depending on their assumptions about the virus and the population. In Canada, a lack of reliable data also makes predictions difficult.

The basic reproduction number doesn’t tell us everything—it represents a virus’s potential, not reality—and how much the virus spreads depends on what a country does to counteract it. The Chinese government put more than 50 million people into mandatory quarantine in Hubei alone, closed borders and blocked roads, and tracked the phones of people who had visited infected areas to restrict their travel. These restrictions appear to have reduced the spread of COVID-19 (whether they are ethical or justified is another matter).

So, in summary, it’s possible that 30 to 70 percent of Canadians will get COVID-19, but it’s unlikely now that we are taking measures to prevent it. In Wuhan, the worst projections have not come to pass. Hopefully, the true number will be much lower in Canada as well.