If you’ve been watching the news, you’ve probably seen it: an exponential increase in COVID-19 cases, as well as a recent spike in deaths.
But a less-reported number is the positivity rate, which the B.C. Centre for Disease Control “has increased steadily and steeply” in recent weeks. In the week of Nov. 8-14, it rose to above six per cent according to a situation report released by the organization, hitting a level that is above anything seen during the first wave of COVID-19 this spring.
The maximum positivity rate during the first wave hit five per cent in April, but occurred when testing was much less widespread and targeted at high-risk individuals. The number of tests at the time was about nine times lower than it was during November, at only 7,500 tests compared to about 70,000 currently. For context, the positivity rate in the first half of October was under two per cent.
“That rate is the fraction of the number of positive tests that come back divided by the total number of tests,” said Daniel Coombs, a mathematics professor at the University of B.C. Positivity rates are typically more accurate when more people are being tested.
B.C.’s increasing positivity rate in recent weeks is doubly concerning because the number of weekly tests has stayed between 60,000 and 70,000 even as the positivity rate rose from 1.82 per cent to six per cent.
|B.C.'s positivity rate has reached record highs in November 2020 as the province struggles with the second wave of the COVID-19 pandemic. (B.C. Centre for Disease Control)
“If you suddenly have more positive people and the number of tests stayed the same, then [the rate] would go up,” Coombs told Black Press Media by phone.
“You don’t want this number to test rapidly unless your testing policy is also changing rapidly, and that’s not the case, so having it go up is probably a bad sign. It’s indicating we are finding more.”
The positivity rate backs up what the daily case count and rising hospitalizations are saying: the number of people with COVID-19 in B.C. is on the rise, with health officials attributing much of the spread to private social gatherings and high-risk exercise classes. B.C hit a record 762 cases cases Wednesday (Nov. 18) and recorded its deadliest day of the pandemic on Tuesday with 11 deaths.
To put B.C.’s six per cent positivity rate into context, Coombs points to Stockholm in Sweden which has a positivity rate of about 20 per cent, or one in five people testing positive. Sweden, which has refused to bring in lockdowns, has an overall positivity rate of above 10 per cent.
Canada’s overall rate has about 6.6 per cent of tests coming back positive.
But what B.C.’s rising positivity rate means is hard to tell. B.C. imposed region-specific restrictions for the Lower Mainland on Nov. 7, which heavily limited gatherings of any size in public or private, and expanded those restrictions – and brought in a mask mandate – province-wide on Thursday (Nov. 19). However, the preliminary positivity rate reported on the B.C. CDC COVID dashboard for Thursday (Nov. 19), the last day public data was released prior to an expected drop this Monday afternoon, was even higher than it was prior at 6.7 per cent. The Fraser Health positivity rate reported Thursday is at 9.6 per cent, in line with the region having a disproportionally high number of new cases. But positivity rates are rising in other regions too, with Vancouver Coastal Health at 5.1 per cent and Northern Health at 5.3 per cent as of Thursday. Interior Health was at a 3.5 per cent positivity rate, while Island Health is at 1.4 per cent.
B.C. has not publicly linked any particular positivity rate to levels of COVID-19 restrictions like other regions. In Ontario, positivity rates are tied into the COVID-19 response framework, the plan that sets out when certain restrictions will come into place. At above 2.5 per cent, Ontario would enter the most restrictive level of COVID-19 measures short of a full lockdown. In New York City, schools shut down last week because the city’s positivity rate reached its three per cent threshold. In May, the World Health Organization recommended that the rate remain below five per cent for at least two weeks before governments consider reopening.
Provincial health officer Dr. Bonnie Henry said that rate is being monitored, but not what higher rates could lead to.
“Per cent positive is something we watch,” said Henry. “It’s one of those metrics that if it’s above five per cent… that tells us there’s transmission in the community that is concerning. We’d like to see it lower.”
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