Seroprevalence: How lifesaving blood donations can also help provide valuable public health insights during a pandemic

October 5, 2023
Abby Wolfe

In May 2020, Canadian Blood Services formed a research partnership with the COVID-19 Immunity Task Force (CITF) to analyze the seroprevalence of COVID (SARS-CoV-2) in the blood donor population. Their goal: to generate insights about the level and type of immunity arising from both infection and vaccination to better understanding the virus’ movement through the Canadian population.

In August, Dr. Sheila O’Brien, Canadian Blood Services’ associate director of epidemiology and surveillance, joined the Public Health Ontario (PHO) Rounds webinar series to describe Canadian Blood Services’ efforts to collect and analyze data on the rates of infection in the blood donor population.

Dr. Sheila O’Brien
Dr. Sheila O’Brien, Canadian Blood Services’ associate director of epidemiology and surveillance

Seroprevalence: how blood provides clues about public health 

Seroprevalence is a type of health information surveillance that indicates infection history in a population, based on a measure of blood proteins (known as antibodies). Antibodies are powerful indicators of past infection because, when someone gets sick with an infection, antibodies are produced as part of a normal response to the offending agent (known as the antigen) and remain in the blood even after the individual is no longer experiencing symptoms of illness. Testing blood samples for the presence of specific antibody proteins can help scientists and health-care professionals understand more about an infectious agent such as a virus or bacteria, like how many people have been infected by a certain type of virus and how this number changes over time. By comparing the number of people with antibodies for a specific virus to the total number of people in the population of interest, the prevalence of that infection can be reported as a percentage.

So why do blood donors and seroprevalence provide such great opportunity for public health insights? Says Dr. O’Brien: “Canadian Blood Services’ blood donors are reasonably representative of the healthy adult population. Blood donations are collected by Canadian Blood Services nearly every day of the year in all major cities and many smaller cities and towns in all provinces except Quebec. This is very important for on-going monitoring.” 

Gathering the data 

Dr. O’Brien’s presentation also described the immunoassays used to test the blood samples: “We specifically looked for antibodies to proteins on the SARS-CoV-2 virus called nucleocapsid protein and spike protein. If a test indicated that nucleocapsid protein was present, it meant the individual developed antibodies as part of the body’s natural response to getting sick from COVID-19. If a test indicated the presence of spike protein, but not nucleocapsid, it could be determined that the individual’s antibodies were developed as part of a normal response after receiving a COVID-19 vaccination, at least earlier in the pandemic.”

Canadian Blood Services collects approximately 850,000 donations per year from about 400,000 donors in all provinces except Quebec. It’s important to note that these seroprevalence tests were completed on small vials of blood called retention tubes that are collected during donations, meaning this seroprevalence testing did not affect the total volume of donated blood available to reach recipients.

Retention tubes, as Dr. O’Brien explains, are routinely collected during any whole blood donation to ensure that a small amount of blood—separate from the volume gathered in the blood bag— may be available, if needed, for additional kinds of testing. Approximately 80 per cent of the time, these retention tubes are discarded as extra testing is not needed to qualify the donation. For the purposes of this study, these “leftover” retention tubes were de-identified and chosen at random to generate a selection of samples for testing. These steps helped to create a sample set that would be representative of the broad donor base throughout Canada.

SARS-CoV-2 seroprevalence data from 2021-2023

By the end of May 2023, over 800,000 samples had been tested. Once tests were completed and the results were adjusted for age and gender, monthly reports were prepared by the Canadian Blood Services’ team to present information to the CITF secretariat, the Public Health Agency of Canada (PHAC), as well as provincial and territorial public health ministries. These reports helped illustrate changing trends over time, such as the percentage of donors in different age groups with COVID-19 antibodies due to vaccination. Due to varying dates of vaccine eligibility according to age, rates of COVID-19 vaccine antibodies increased among Ontario donors 60 years of age or older first, followed by progressively younger age groups.

Results had international impact, too, as they helped inform initiatives like SeroTracker, whose partnership with the CITF helped to visualize SARS-CoV-2 seroprevalence estimates across geographic areas.

What’s next: seroprevalence data in 2023 and beyond

Canadian Blood Services is actively developing a post-pandemic role in public health research and health surveillance. As Dr. O’Brien shared in the final slides of her presentation, “Blood donor sero-surveillance over three years of the pandemic demonstrates the value that donors can bring to public health surveillance. The role of blood donors in public health surveillance is a rapidly developing area, with lots of enthusiasm around the world among blood operators and public health to see how to best work together to leverage synergies and make better use of the resources blood operators have.”

Canadian Blood Services has received $6 million in funding from the Public Health Agency of Canada to continue its seroprevalence research in the blood donor population throughout 2023-2024. Canadian Blood Services is grateful to blood donors for making this research possible.

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The opinions reflected in this post are those of the author and do not necessarily reflect the opinions of Canadian Blood Services nor do they reflect the views of Health Canada or any other funding agency.