SARS‐CoV‐2 seroprevalence among blood donors after the first COVID‐19 wave in Canada

Canadian Blood Services (CBS) has been conducting a monthly nationwide study since May 2020 to detect antibodies that recognize SARS-CoV-2, the virus that causes COVID-19.  By doing seroprevalence surveys of blood donors, they aim to understand the percentage of people in Canada who have antibodies against SARS-CoV-2, and to track how this percentage changes over time. This work was funded by the Government of Canada through the COVID-19 Immunity Task Force (CITF).

Information on antibody tests using blood donation samples:
When blood is donated in Canada, small samples are taken to find out the donor’s blood type and to screen for certain infectious diseases to make sure it is safe for use. Individual blood donors are asked to consent to also have their blood tested for the presence of antibodies to SARS-CoV-2. These serological tests cannot determine if a person is currently infected with SARS-CoV-2, rather they inform if a person has antibodies, from a past infection or vaccination.

Overall low SARS-CoV-2 seroprevalence at the early stages of the pandemic in Canada

Results from CBS’s initial survey, conducted between May 9th and July 21st, 2020 and published in the journal Transfusion, were based on almost 75,000 blood samples collected at all Canadian Blood Services locations (excluding Quebec and the Territories). Results were stratified by region, age, gender, race/ethnicity, and blood type. Overall, 552 of the 74,642 donors tested had detectable antibodies, which gave an adjusted seroprevalence of 7.7 per 1000 donors. Prevalence varied by geographical regions but remained stable over the 10-week period studied. Ontario had the highest rate, with 8.8 per 1000 donors, almost double that of the Atlantic region with a rate of 4.5 per 1000 donors. Samples from donors that self-identified as an ethnic minority were more likely positive than samples from Caucasian donors.

Continuous seroprevalence surveys are necessary to identify dynamic trends and monitor overall infection rates, including asymptomatic cases.

Ideally, random sampling of the general population should yield the best results. But those types of surveys take time to organize and can be expensive. Blood services worldwide were deployed to conduct seroprevalence studies from blood donors with the intent of informing public health policies. When the COVID-19 pandemic hit, Canada’s blood system became the first place to seek evidence of infection and immunity in the Canadian population: blood donation centres are a rapid and reliable resource for generating insights into the patterns of illness as they have an existing infrastructure, experience, expertise, and a donor base which is reasonably representative of healthy Canadians between the ages of 17 and about 60.

Using blood services does have certain limitations. Primarily, people who donate blood may not be an accurate representation of the general population. They are often healthy, non-pregnant adults. Some groups may be underrepresented, including younger adults, and people from certain racial and ethnic groups. Anyone under legal age to donate blood or living in institutionalized settings are excluded. However, the results reported by Canadian Blood Services were consistent with seroprevalence estimates from anonymized residual sera obtained from outpatient laboratory networks. For example, a small study from the Greater Vancouver Area, including children and adults collected in May 2020, found a seroprevalence of 6 per 1000 samples.

Interpreting antibody results from seroprevalence surveys

Seroprevalence surveys have certain limitations that should be considered when interpreting the results:

  • The results of an antibody test can sometimes be positive even for someone who was not previously infected with SARS-CoV-2. This is known as a false-positive result and can make it look like more people have been infected than really were. This is more common when analysing results from an area with low infection rates.
  • It is possible for someone to be infected by SARS-CoV-2 but not make detectable antibodies. There is a period soon after infection, when antibodies are not detectable. It typically takes 1 to 3 weeks after infection until a person would have enough antibodies for detection. Furthermore, antibody levels were shown to decrease over time. When this happens, it can make it look like fewer people have been infected than really were.

Read the manuscript accepted for publication: