How to make sense of the number of COVID-19 cases in Quebec

There are a lot of numbers flying around as Quebec hunkers down to fend off the outbreak of the novel coronavirus. 

But some are more useful than others for trying to understand how the province is faring in the pandemic.  

The total number of confirmed cases jumped from 219 on March 22 to more than 3,000 this week.

The province has the highest number of confirmed cases in the country. So, cause for alarm?

Not necessarily. 

First, other provinces count cases differently, aka, the apples-to-oranges problem.

Before March 23, Quebec was keeping a separate tally of probable and confirmed cases.

In the early days of the outbreak, swabs were tested at the hospital where they were taken (a probable result) and then sent to a provincial laboratory for confirmation. 

But public health officials are now confident in the quality of the results available in hospital labs, making that second step unnecessary.

A whole bunch of cases that were previously considered 'probable' were then moved into the confirmed column. Result, a big spike in numbers on Monday. 

Other provinces — including Saskatchewan and Manitoba — still keep the probable and confirmed numbers separate. 

But bigger provinces, like British Columbia and Ontario, have reporting procedures roughly similar to Quebec's. 

When looking at Ontario's overall number, though, keep in mind the province has had difficulty processing results quickly. As of last week, nearly 11,000 people were still waiting to find out whether they have COVID-19. 

The fish-in-a-barrel problem

Another hazard that arises when comparing figures between provinces is differences in who is getting a COVID-19 test.

Health authorities in Quebec have spent the initial days of the outbreak focusing their testing capacity on people who travelled and had symptoms of the virus, as well as anyone who came into contact with them.

Given the roughly two week lag between infection and symptoms, the current cohort of test results in Quebec is thought to include thousands who travelled abroad during the first week of March, the province's spring break.

That was just before the province introduced isolation guidelines for travellers, and as many have pointed out, popular holiday locations for Quebecers — such as Europe, Caribbean cruises or Florida — have since become disease hot spots. 

Alberta, on the other hand, cancelled classes before spring break, likely reducing the number of people who travelled abroad.

These two factors — narrowing testing to travellers and travellers in at-risk locations — likely influence the current number of confirmed cases in Quebec. 

That, though, was the point. 

"In the first phase of an epidemic, when you're still trying to limit its spread, you go where people are the most susceptible to testing positive," said Cécile Tremblay, an infectious disease specialist at the Centre hospitalier de l'Université de Montréal.

This more targeted approach allows public health officials to identify and research cases of transmission, said Tremblay, who is also a scientific director at Quebec's public health laboratory.

But as the disease begins to spread more widely, in what's known as the community transmission phase, priorities for testing change.

As Quebec enters that phase, test criteria is widening. It is no longer necessary to both have symptoms and have travelled — the first suffices to qualify for a test, the province's public health director, Horacio Arruda, said last week.

Other provinces have also shifted their testing criteria based on available resources, demographics and the disease's progression within their borders.

British Columbia, for instance, recently stopped testing travellers with mild symptoms, and opted to focus on health-care workers, hospital patients and employees in long-term care facilities. 

The priority there is protecting the health-care system's ability to respond to the more severe cases. "We are focusing on where it makes the most difference," said Dr. Bonnie Henry, B.C. chief public health officer.

With provinces prioritizing different populations to test, the varying infection rates are in effect measuring different things.

How to compare with ourselves

It is possible that testing practices between provinces will eventually be similar enough to make meaningful comparisons possible. 

But until then, said Alex Carignan, an epidemiologist and infectious disease specialist at the Université de Sherbrooke, there's not much point worrying about the numbers elsewhere in Canada. 

"We have to compare with ourselves," he told Radio-Canada. 

But to do so, a few extra calculations are necessary when you're looking at the overall number of confirmed cases in Quebec.

That overall number is likely to continue rising as more testing is conducted. What's important to consider, though, is the speed, or rate, at which it is rising. 

Quebec only began implementing social distancing measures two weeks ago. It takes about that time for someone with COVID-19 to seek out a test, so it's still too early to see a reduction in the overall number of cases.

The first signs of success will come in the form of the day-over-day rate of increase (new case divided by existing cases x 100). When that number trends downward, we'll know social distancing is working.

"The very first thing you'll see is called an inflection point, which is just where the rate of increase starts to slow down," said Matthew Oughton, an infectious disease specialist at the Jewish General Hospital.

"It's as if you're in a car and you'll be pushing a little less hard on the accelerator. You're still speeding up, you're just not speeding up as fast as you were before."

How are we doing on that front? There is not enough data yet to draw a meaningful conclusion, thanks to the recent change in counting method.

But, for the record, here are numbers since that change went into place:

Hospitalization rates

The other important figure to consider is hospitalizations, and with it, the percentage of cases that require hospitalizations. 

In even the more severe cases of COVID-19, it remains treatable for most people. But treatment requires significant hospital resources — stays of between 10 days and three weeks, by some accounts.

The rapid rise in deaths in places like Italy are a factor not simply of the potency of the virus, but also health care systems being completely overrun and unable to treat the number of patients on hand.

In early March, Italian hospitals were left coping with hospitalization rates of close to 50 per cent. Elsewhere in the world, the range has been between 10 and 20 per cent. 

According to several media reports, Quebec's effort to have 10,000 hospital beds ready to treat COVID-19 patients is based on an estimate the hospitalization rate could reach 15 per cent. 

For the past week, the province's hospitalization rate was hovered between six and seven per cent.

"[The hospitalization rate] may give us a better sense of the underlying number of people who have the virus, even if not everybody is getting tested," said Gaston De Serres, an epidemiologist at Quebec's public health institute.

"So we will certainly be watching very closely the number of hospitalized cases."