Virus test-positivity rate rockets to 7.1% in province Number a concern but doesn't tell complete COVID story, city epidemiologist says
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This article was published 26/10/2020 (769 days ago), so information in it may no longer be current.
Manitoba’s COVID-19 test positivity rate reached a record high above seven per cent Monday after steadily rising since Thanksgiving weekend, pointing toward increasingly higher rates of community spread.
While the high number is cause for significant concern, there are several more moving parts at play to accurately determine community spread, Winnipeg-based epidemiologist Cynthia Carr said.
“Test positivity rate is literally just the number of tests that are taken, and of those how many come back from the lab as being positive for a virus,” Carr said.
Chief public health officer Dr. Brent Roussin announced a provincewide test positivity rate of 7.1 per cent Monday. The number in Winnipeg was 8.3 per cent and 4.6 per cent in the Northern health region.
Jurisdiction-by-jurisdiction numbers are not always available in the province, though the rolling five-day positivity rate has tracked higher in Winnipeg than other parts of the province in recent days.
The positivity rate is calculated by dividing the number of positive tests by the total number of tests completed in the same period. Manitoba uses a five-day period, though many other jurisdictions use seven day.
Factors such as weekends, holidays and testing delays can impact test positivity rates, Carr said, explaining the reasons why officials use rolling averages over periods of five to seven days.
“There’s two things that we want to watch for,” she said. “Is the trend tending to kind of go up or down? And are there differences in trends by different geographies?”
Manitoba’s positivity rate has been fluctuating, but generally increasing since the beginning of September. On Sept. 1 the province recorded a five-day test positivity rate of 1.7 per cent, peaking at 2.5 per cent midway through the month. On Oct. 1 it was 2.2 per cent, leaping to 5.3 per cent by Oct. 15 and climbing steadily to 7.1 Monday.
Globally, the benchmark for high community spread is considered to be five per cent. In May, the World Health Organization recommended that governments hold lockdowns in place until two weeks have passed with positivity rates below that threshold.
In Toronto, seven-day test positivity rates reached peaks in March and April as high as 17 per cent, but have hovered near four per cent in recent weeks. In Boston, a positivity rate above four per cent was enough to trigger jurisdiction-wide school shutdowns last week.
“It appears to me that we are in an area of concern, and it appears to me that unfortunately, the concern appears to be related to our choices that we’re making as community residents,” Carr said.
“We know that the increase number of cases, when there are outbreaks there can be a really large number that happens really fast because people are in close quarters… but the virus doesn’t originate within that setting, it gets in from community-based spread.”
Test positivity rate is generally considered as an indicator of the severity of community-based spread, but Carr noted it isn’t the only factor to look out for, and may not accurately convey how widespread the virus is among the population
“It depends on the rules, too. in a jurisdiction; if only symptomatic people are allowed to get tested, chances are that a higher portion of those people with symptoms may actually have the virus compared to another area where anybody could get tested,” she said.
How much weight to give the number is dependent on the rules for testing, she said. In Manitoba, where symptomatic testing is the priority, the positive test rate indicates how many people with symptoms are, in fact, COVID-19 carriers, and could help predict hospital and ICU needs.
“But do we know what the actual burden or spread of the virus is? We do not, because we are not, for the most part, testing asymptomatic people,” she added.
As an epidemiologist, Carr said she’s always interested in “sentinel surveillance,” or looking for risks that are unseen. Testing asymptomatic people could help detect those risks early and result in more accurate test positivity rates, but requires significant resources in test capacity, lab capacity and public health turnaround times, she said.
“We have to think about the purpose of testing from a risk-reduction, a harm-reduction and opportunity to treat lens,” she said. “That test positivity rate is telling us a part of the story, but we don’t know still what the actual rate of spread is in the population.”
Manitoba public health officials did not respond to Free Press requests for comment Monday.
Julia-Simone Rutgers is a climate reporter with a focus on environmental issues in Manitoba. Her position is part of a three-year partnership between the Winnipeg Free Press and The Narwhal, funded by the Winnipeg Foundation.