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If COVID-19 cases are falling in Ontario, why are so many tests still coming back positive?

If COVID-19 cases are falling in Ontario, why are so many tests still coming back positive?
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In the last 25 days, the province’s seven-day average for reported cases has fallen 40 per cent, from about 4,350 cases a day to roughly 2,600. Taken by itself, this drop in reported cases seems to suggest the third wave of COVID-19 is receding very quickly in this province, that things are looking up.

But in that time period, another key indicator of the virus’s spread has remained stubbornly high; on Sunday, 9.1 per cent of all COVID-19 tests processed in the province’s testing labs came back positive, not far off the province’s one-day record from two weeks earlier.

And the current seven-day average for test positivity, 7.1 per cent, is still elevated too, down only about 20 per cent from its highest at the peak of the third wave.

What’s more, the province is testing far fewer people than it was a month ago, down from averaging about 55,000 completed tests a day in early April to about 40,000, as of Thursday.

The sum of all this might create a confusing picture; how can we be sure the falling daily case average is truly good news, when the province is both doing far fewer tests, and still seeing an alarmingly high number of positives?

One way to know is to look at hospitalization and death rates, says Raywat Deonandan, an associate professor and epidemiologist at the University of Ottawa.

“You can’t fake those. If those are either stable or falling, it tracks,” said Deonandan, who noted the caveat that ICU admissions are a lagging indicator.

What are those numbers doing? They’re also trending down; hospitalizations have fallen about a third from the worst of Wave 3. ICU cases aren’t down quite that much; the drop is about 18 per cent, but they are also trending better.

What’s more, Deonandan says, it makes sense that positivity rates would stay higher, even as cases fall, as those people still going to get tested are the ones more likely actually have the disease.

“It also suggests that there are a fair number of cases in the community who might be asymptomatic. They wouldn’t get tested, so they don’t enter our data stream. So there’s going to be some fuzziness in the data,” he said.

Ultimately, though, the degree of any undercounting of cases is unlikely to be large enough to change the direction of Ontario’s trends, Deonandan says.

“I’d be really surprised if it wasn’t good news,” he said.

If positivity is still high, surely Ontario would find more cases if it simply did more tests, you might ask.

Ashleigh Tuite, an infectious disease epidemiologist at the Dalla Lana School of Public Health, says that’s undoubtedly true to an extent, but the fact positivity rates, while still high, are nonetheless also declining is more evidence the drop in cases is real.

“Which is not to say that we shouldn’t be testing more, because we should be. We don’t want positivity to be as high as it is now,” she noted. But, she said, the drop in testing is “not driving the trend in cases that we’re seeing.”

For a year, experts have explained that test positivity is one of the best indicators of the true spread of the virus. If the pandemic is growing quickly, but lab capacity is scarce — if, for example, the province can only process a limited number of tests in a day — a rising positivity could tell you the real number of infections is growing faster than our ability to count cases.
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