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Less than 1 per cent of British Columbians were infected with COVID-19 during the first wave of the coronavirus pandemic, according to a study by researchers at the B.C. Centre for Disease Control.

The study findings highlight the effectiveness of the province’s measures to combat the pandemic, and underscore that most B.C. residents remain susceptible to infection.

“We estimate [less than] 1 per cent of British Columbians were infected with SARS-CoV-2 by the time first-wave restrictions were relaxed in May,” says the report, released on Tuesday in a preliminary form that has not yet been peer reviewed.

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Those findings reinforce other data that show the province kept a lid on transmission rates in the early part of the year, the report said, adding, “This success, however, constitutes a double-edged sword, further highlighting substantial residual susceptibility.”

The study also suggests the province’s true infection rate is about eight times the rate based on reported cases, providing a glimpse of how much virus was in the community and potentially spreading undetected as public health strategies were put in place.

The study is the first in Canada to report infection rates based on seroprevalence, which is a measure of the presence in blood samples of antibodies produced to resist the virus.

Determining exactly how many people in Canada have been exposed to COVID-19 is a key goal of the immunity task force the federal government set up in April.

Timothy Evans, a member of the task force and director of McGill University’s school of population and global health in Montreal, said the B.C. survey indicates the province’s deft management of the first wave of the pandemic resulted in very low exposure across its population.

“The low prevalence of population immunity suggests that continued vigilance and adherence to best practices to reduce risk of infection will be critical, especially in the context of the second wave of the pandemic,” Dr. Evans said.

He added that the eight-to-one ratio of actual to reported cases is consistent with international studies and that he expected a similar result across Canada.

The survey was based on blood samples from more than 1,700 people in two periods, one in mid-March and a second in late May. The data were gathered anonymously from residual blood drawn from individuals at diagnostic clinics in B.C.’s Lower Mainland. The subjects were males and females of varying ages, including children.

Prabhat Jha, director of the Centre for Global Health Research at St. Michael’s Hospital in Toronto, said the sample size of the B.C. survey was relatively small, with only 20 individuals showing exposure to COVID-19.

“Small sample sizes for any study – including well-done ones like this – make the interpretation difficult,” he said.

Dr. Jha, who is leading a seroprevalence study that aims to sample as many as 10,000 Canadians, also said the individuals in the B.C. study may not be representative of the province’s population. For example, the study may be skewed toward healthy people who were having their blood tested as a precaution, or by those who were already ill.

It also captured the presence of antibodies in blood samples before and after the first wave but not during the peak in April.

Another key piece of information the B.C. study does not provide – and was not designed to – is whether the individuals found to have antibodies for COVID-19 are now immune to the coronavirus and, if so, for how long.

Answering that question will require a more detailed form of antibody testing being developed by other groups.

B.C. Health Minister Adrian Dix said on Tuesday that the study results are “a dual-edged question – a low level of transmission but also very few people with antibodies to deal with potential future spikes of COVID-19.”

With a report from The Canadian Press

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