Nearly all Canadian adults had COVID antibodies for about half of 2022, most through previous infection: survey

A newly released survey finds nearly all Canadian adults had antibodies against COVID-19 for about half of 2022, with most acquiring them through a previous infection.

The results, released Tuesday by Statistics Canada in partnership with the Public Health Agency of Canada and COVID-19 Immunity Task Force, show that 98.1 per cent of Canadian adults living in the country's 10 provinces had antibodies against SARS-CoV-2, the virus that causes COVID-19, between April and August 2022.

These individuals acquired antibodies through either vaccination, previous infection or both.

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Over that five-month period, 53.9 per cent developed antibodies through a past infection.

This is in comparison to the results of a previous version of the survey done between November 2020 and April 2021 that found 2.6 per cent of Canadians had COVID-19 antibodies due to a previous infection.

A report detailing the findings says while almost 54 per cent of Canadians surveyed had antibodies from an infection, the actual proportion of those infected since the start of the pandemic is likely higher.

"This is due, in part, to the fact that an antibody response to an infection may not always be detectable, especially among those who are vaccinated who may generate fewer antibodies following a subsequent infection. In addition, antibody levels can decrease over time," the report says.

ANTIBODIES

The survey used a technique known as dried blood spot testing to detect COVID-19 antibodies from vaccination or a past infection, as well as saliva samples to determine a current or recent infection.

A total of 105,998 people were chosen to participate in the survey, of which 30.7 per cent completed a questionnaire asking about general health and exposure to COVID-19. Of those who completed the questionnaire, nearly 54 per cent provided a blood sample and 54.5 per cent provided a saliva sample.

The body produces antibodies to defend against diseases and in response to vaccination.

However, they do decrease over time and the report says some people previously infected with COVID-19 or who have been vaccinated may no longer have detectable levels of antibodies.

But while antibodies may decrease with time, the report says it does not mean a person's immunity has waned.

Rather, an individual may have developed cell-mediated immunity, which activates specialized white-blood cells called T cells.

The report says dried blood spot testing also does not measure cell-mediated immunity.

Even without detectable antibodies, the report adds that cell-mediated immunity could be enough to protect someone against severe COVID-19 symptoms. The mere presence of antibodies alone also isn't enough to suggest a person could fend off a new infection or any serious symptoms.

MANY UNAWARE THEY WERE INFECTED

The survey found as many as two out of five Canadians with antibodies from a previous infection were unaware they had been infected, either because they never tested positive or did not suspect an infection.

Among those who previously tested positive for COVID-19, antibody levels were higher between 14 and 90 days post-infection before gradually decreasing over time.

"While this is indicative of decreasing antibodies, there are other factors which could impact this analysis," the report says.

"For example, since vaccines may lead to a lesser antibody response following an infection, the percentage of Canadians with antibodies from infection may be affected by changing vaccination rates over time. Further, since the survey focuses on first infections, re-infections would also impact this analysis as these could lead to a rebound in antibody levels."

Canadians with a disability or underlying health conditions were less likely to have antibodies from a previous infection compared to those who did not report a disability or health condition.

Those 18-34 and 35-49 were also more likely to have antibodies from a previous infection than older Canadians were.

Antibody levels from infection varied between population groups, ranging from as low as 53 per cent for Chinese Canadians to as high as 80.7 per cent for Black Canadians.

Across provinces, Nova Scotia had the lowest proportion of residents who developed antibodies from infection at 46.5 per cent, while Quebec (56.5 per cent), Alberta (57.4 per cent) and Saskatchewan (57.4 per cent) had the highest rates.

The survey did not include data from the three territories, those under 18 or people living on reserve, in institutions or Canadian Forces Bases.