Blood survey suggests COVID-19 was more prevalent in Manitoba during second wave than previously believed

A blood survey suggests COVID-19 was far more prevalent in Manitoba at the height of the pandemic's second wave than was previously believed.

A Canadian Blood Services survey conducted in mid-November found 8.56 per cent of Manitoba blood donors had COVID-19 antibodies in their system, which means they were exposed to the virus that causes COVID-19 and developed an immune response to it.

That rate is more than seven times the known COVID-19 infection rate for the entire provincial population at the time: 1.2 per cent, based on conventional PCR tests conducted on nasal swab samples.

Experts, however, warn against applying the results of the survey to Manitoba's entire population, even as they concede the disease likely was more prevalent than conventional testing revealed.

CBC News obtained a summary of the survey of 17,049 Canadian blood donors, including 380 from Manitoba, through a freedom-of-information request. The data was made public earlier this year and reported by other media outlets, including the Winnipeg Free Press.

The blood survey was conducted from Nov. 7 to 25, 2020, a period when COVID-19 cases were skyrocketing in Manitoba because of widespread community transmission.

The survey also found COVID-19 antibodies were four times more prevalent in Manitoba blood donors than the  average of just under two per cent for all Canadian blood donors.

The survey's authors warned against drawing too many conclusions from what their agency learned from drawing blood.

"Caution should be exercised in extrapolating the data to all healthy Canadian adults because donors self-select to be blood donors, in some areas access to donation clinic may be limited and because there are fewer elderly donors," the authors warn.

On the other hand, roughly 60 per cent of Manitoba's population lives in a single large metropolitan area, Winnipeg, where blood donation is easier and where the vast majority of the province's infections were reported during the survey period.

Manitoba public health has yet to concede COVID-19 was more prevalent in the province in November.

"At this point we are continuing to conduct our own testing and to review these numbers and what they mean for Manitoba," the province said in a statement, adding it remains to be seen whether provincial antibody surveys will yield similar results to the work conducted by the Canadian Blood Services.

"Blood donation tendencies of certain populations may be affecting these estimates disproportionately. For example, children and senior populations tend to be excluded from blood donation figures so would not be represented in these numbers."

Jason Kindrachuk, a University of Manitoba microbiologist and Canada Research Chair in emerging viruses, said he too would question how the Canadian Blood Services sample skewed the survey results.

At the same time, he said antibody surveys in the U.S. have made it clear COVID-19 is more prevalent than conventional testing reveals.

"This is not a virus that causes symptoms in everyone. We know that there are mild symptoms in a large portion of people and then, of course, those people have to recognize what those mild symptoms look like and then get tested," Kindrachuk said. 

"I think that we are always going to be kind of faced with this idea of how many people were truly infected or exposed versus how many actually ended up getting tested."