This column is an opinion by James Downar, Maxwell Smith and Ross Upshur. Downar is head of the Division of Palliative Care at the University of Ottawa, and a critical care physician at The Ottawa Hospital. Smith is a bioethicist and co-director of the Health Ethics, Law, and Policy Lab at the University of Western Ontario. Upshur is a professor in the Department of Family and Community Medicine at the University of Toronto, and associate director of the Lunenfeld-Tanebaum Research Institute. For more information about CBC's Opinion section, please see the FAQ.

In a COVID-19 press briefing on Tuesday, Prime Minister Justin Trudeau announced that, "The measures we have taken so far are working. In fact, in many parts of the country, the curve has flattened." While this is welcome news, our job is far from finished — as he was also quick to point out.

To understand why, we can use one of the simplest and oldest approaches to modelling infectious diseases, the "SIR" model.

"S" represents the number of those susceptible to infection.

"I" represents the number of those who are currently infectious.

"R" represents the combined number of those who have recovered from the infection.

By understanding how many Canadians are in each of these different categories at any one time, we can learn where Canada is with respect to the spread of this virus, and use it to help us predict where we might be heading.

At the outset of the COVID-19 pandemic, all 37 million Canadians were in the "susceptible" category.

This high number of susceptible Canadians provided a forceful reason to take swift action to prevent Canadians from becoming infected — infections which would overwhelm our acute and critical care resources and likely result in numerous deaths. Strict measures were put in place, including quarantine, social distancing, school closures, and travel restrictions.

Part 1 of 3 of Rosemary Barton’s exclusive interview with Chief Public Health Officer Dr. Theresa Tam about Canada’s response to the COVID-19 pandemic. 9:44

Likewise, the decision to scale back these public health measures must take into account not only "the curve" and the degree to which it has been flattened, but also the number of Canadians who remain susceptible to the virus.

Assuming that recovery from the infection brings a measure of immunity, as more Canadians move from the susceptible category to infectious and ultimately to recovered, we can reasonably conclude there will be less need for such aggressive social distancing and restrictive public health measures.

Figures as of April 28 tell us that more than 49,000 Canadians have been confirmed as infected. Of that number, 2,766 Canadians have died and the rest either remain infectious or have recovered. While these figures very likely underestimate the total number of cases and deaths, they amount to just over one tenth of one per cent of our population.

Consequently, we must reasonably assume that upwards of 99 per cent of Canadians remain susceptible to the virus.

In other words, even accounting for an underestimation of cases, it is fair to assume that the vast majority of Canadians remain susceptible to COVID-19 infection. This means Canadians are in virtually the exact situation we were in at the beginning of March — a situation which warranted swift and aggressive public health measures.

However, things are different now in one important respect: we are no longer sitting on the same reserves of medications, personal protective equipment (PPE), and household financial savings. The economy has also taken a big hit, which puts even more pressure on Canadians and the health system.

This is a sobering reality which suggests that sustained vigilance and action is necessary.

The steps Canada has taken to curb the spread of this virus may be "flattening the curve," but we cannot forget that the vast majority of our population will continue to be susceptible to infection in the absence of measures that:

Reduce the proportion of Canadians who are susceptible (a vaccine)

Reliably tell us who is no longer susceptible (an antibody test)

Effectively treat illness caused by COVID-19 (drugs)

Interrupt transmission chains (widespread contact tracing)

Until an effective combination of these measures becomes possible, or the virus simply stops circulating in this part of the world, Canadians must resist drawing the conclusion that we can lower our guard.

Prime Minister Justin Trudeau says public health measures are working to slow the spread of COVID-19, but cautioned that easing restrictions too fast could reverse the progress made to date. 3:25

Despite the fact that the majority of Canadians remain susceptible to this virus, the good news is that we have bought ourselves time — time to prepare our health care system to handle a major surge in demand, and to start ramping up our supply chain in case a bigger wave of infections does hit. We have also learned a great deal from the experience of other nations.

"The curve has flattened, but we are not out of the woods yet," Trudeau said yesterday. "We are in the middle of the most serious public health emergency Canada has ever seen, and if we lift measures too quickly, we could lose the progress we've made."

In other words, we must ensure that we do not squander the time we have bought through social distancing and other public health measures by prematurely declaring victory, or neglecting important contingency planning for shortages of things like medications and ventilators.

And we must continue to use this time to strategically prevent widespread infection and to protect the most vulnerable segments of our society.