TORONTO -- Health Canada has approved two more rapid tests that can indicate the prevalence of COVID-19 immunity, devices experts say should be used judiciously but can help ensure the success of vaccines.

While there's a more urgent push for rapid diagnostic tests to find current COVID-19 cases in the community, experts welcomed additional tools to reveal previous infections and antibodies that can shield populations from further outbreaks.

Occupational and public health expert Thomas Tenkate notes many questions still surround emerging COVID-19 vaccines, including what level of protection is required for immunity, how long vaccines guard against infection, and whether and how various populations may respond differently.

That's where strategic use of serology tests come in, such as the two point-of-care ones that got the green light Feb. 12, which join a list of others previously approved. While it's believed previous COVID-19 infections may offer some immunity, exactly how much and for how long is not clear, and experts still advise recovered patients get a vaccine.

"It really (provides) population-based research that will help in prevention, and better ways to fight the virus," says Tenkate, an associate professor at Ryerson University.

Unlike diagnostic tests that reveal active infections, serology tests detect the presence of antibodies sparked by previous infection or a successful response to a COVID-19 vaccine.

But the information they provide is limited and prone to misinterpretation, cautions Tenkate, who says the tests should be administered and interpreted by a health-care professional.

Such dangers are highlighted in in which two FDA officials regret the way emergency approvals arrived in the United States in March 2020.

"Knowing what we know now, we would not have permitted serology tests to be marketed without FDA review and authorization, even within the limits we initially imposed," Jeffrey Shuren and Timothy Stenzel say in the current print issue dated Thursday.

The co-authors say government officials over-promoted the role of serology tests in reopening the economy, while the market was flooded with questionable products, some which performed poorly and many marketed in a way that conflicted with FDA policy.

The FDA changed its policy May 4 to assess product claims, and by Feb. 1, 2021, had removed listings for 225 tests from its website, issued 15 warning letters, and placed 88 firms on import alert for violations.

Serology tests aren't available for at-home use in Canada but the curious can obtain a test privately -- LifeLabs advertises a $75 test on its website for residents in Ontario and British Columbia.

Susan Paish, co-chair of the federal COVID-19 Testing and Screening Expert Advisory Panel, finds it hard to see the benefit of a serology test for individuals, stressing the value they offer in evaluating broader testing and vaccine strategy.

Surveillance data can reveal things such as whether certain populations are more prone to asymptomatic infection, says Paish.

"That's important to know because asymptomatic individuals can certainly spread the virus," says Paish.

"You can make sure that you pay special attention to those individuals, make sure they're vaccinated."

National efforts are underway to better understand immunity through Statistics Canada, in partnership with Health Canada, the Public Health Agency of Canada and the COVID-19 Immunity Task Force.

The voluntary Canadian COVID-19 Antibody and Health Survey is mailing thousands of blood sample collection kits to measure the prevalence of COVID-19 antibodies among Canadians, including those who never had symptoms.

Health Canada's new approvals include one from China's Assure Tech and a point-of-care device by BTNX Inc. of Markham, Ont.

The Canadian test produces results in 15 minutes from a fingerstick blood sample, says chief financial officer Mitch Pittaway. It's meant to be delivered by a health-care professional in a clinic or health-care setting.

He says clients in Europe include people who suspect they had COVID-19 but never displayed symptoms.

"We see it used in dentists, doctor's office, any kind of clinics, pharmacies by the pharmacist on site," Pittaway says of overseas sales.

"And really people who are, let's say, pre-vaccine. People who have been sick and are looking ... to confirm if they have in fact contracted COVID."

Paish says comprehensive testing and screening programs will likely be needed in various environments such as long-term care facilities and workplaces, "until such time as we are really clear on the impact and accuracy of the vaccine programs."

"It would be dangerous for society to think that once vaccines start to roll out there is no place for testing and screening strategies," she says.

This report by The Canadian Press was first published Feb. 18, 2021.