Serosurveillance survey shows SARS-CoV-2 infections growing much faster in poorer neighbourhoods and amongst racialized populations

Results from the latest Canadian Blood Services/COVID-19 Immunity Task Force (CITF) collaborative nine-province seroprevalence study confirm that the pandemic is increasingly affecting poorer neighbourhoods and racialized populations.

The inequalities in risk in getting COVID-19 were identified early in the pandemic and were the focus of the Chief Public Health Officer’s annual report back in October 2020. The inequalities, however, are not only persistent but appear to be growing.

“The dire concentration of infections in poorer neighbourhoods and amongst racialized communities has become more obvious in the current variant-fuelled third wave, and was foreshadowed in the seroprevalence data from January 2021,” says CITF Executive Director Dr. Tim Evans. “Current efforts to protect these communities through public health surveillance, safer workplaces, and focused, tailored vaccine distribution need to be scaled up urgently.”


Increasing risk of infection in poorer neighbourhoods

In May-June 2020, blood donors in the most materially deprived neighbourhoods (as measured by the Material Deprivation Index, based on postal codes) were 1.2 times more likely to test positive for antibodies to SARS-CoV-2, the virus that causes COVID-19, compared to those living in affluent neighbourhoods. This inequality between most- and least-deprived neighbourhoods had grown by January 2021. Residents of lower income neighbourhoods were 3.45 times more likely to have had a SARS-CoV-2 infection by early this year.

This reflects growing inequality in the risk of infection over time between most- and least-deprived neighbourhoods. In more affluent neighbourhoods, the risk of contracting SARS-CoV-2 increased by 77 per cent between May-June 2020 and January 2021. But in the most deprived neighbourhoods, this risk increased by over 400 per cent.


Increasing racial inequality in infection

In May-June 2020, racialized blood donors (Black, Indigenous, and People of Colour) in the nine provinces were about 1.6 times more likely to have antibodies to SARS-CoV-2, which indicates a previous infection, than self-identified white donors. In January 2021, they were twice as likely to have the antibodies. This suggests that over this period, risk of infection in racialized populations increased faster compared to white populations.


Overall seroprevalence in Canada remains very low

The overall seroprevalence in Canada (the percentage of people with antibodies to SARS-CoV-2, suggesting a previous infection) as measured in blood donors, remains very low in all nine provinces. Although the numbers in this report are no doubt an underestimate of the true seroprevalence rate in Canada as most blood donors are healthy, the reality is that a very large majority of Canadians remain susceptible to infection, making clear the importance of rapid, accelerated vaccination.


Blood donors as a measure of seroprevalence

The report is based on the analysis of 34,921 blood samples collected in January at blood donor centres across Canada (excluding Quebec and the Territories).

Serosurveys using blood donor samples have been important in this pandemic.

“Canadian Blood Services is proud to help inform decision-makers as they forge ahead through the next phases of this pandemic,” says Dr. Graham Sher, CEO of Canadian Blood Services. “We are uniquely positioned to perform this seroprevalence work thanks to our existing infrastructure, rapid access to blood samples, experience, and expertise.”

“Our serosurveys offer important insight into the effects of the pandemic in Canada,” says Dr. Sheila O’Brien, the study’s primary investigator and Associate Director of Epidemiology and Surveillance at Canadian Blood Services. “But it’s important to acknowledge that serosurveys based on blood donors do have limitations. Blood donors self-select to be blood donors and donation selection criteria ensure blood donors are healthy, so caution should be exercised in extrapolating findings to all adult Canadians.”

Other factors include that in some areas access to a donation clinic or time to donate blood may be limited. There are also fewer young or elderly blood donors compared to the general population.

These gaps are why the CITF is currently funding 54 other studies, several of which are performing serosurveillance research on a wide variety of general and priority populations in Canada.


More about the report

The full CBS report is available on the CITF website here. Canadian Blood Services is moving towards using a new assay to test blood samples that will allow them to identify a higher number of positive cases and will differentiate between natural and vaccine-acquired immunity.