In their latest report, Canadian Blood Services reveals that 97% of blood donors sampled in September had evidence of antibodies against SARS-CoV-2 acquired through immunization with at least one vaccine dose and/or a past infection. This number is largely driven by vaccination as infection-acquired seroprevalence remained low in September – at 4.4% – despite the ongoing fourth wave. September results show evidence of antibody wane in older adults supporting the need for boosters, but breakthrough infections in individuals who have received at least one dose of vaccine are infrequent.

The latest Canadian Blood Services report included 9,363 people who donated blood between September 14th and 24th, 2021, in all Canadian provinces, excluding Quebec.

Key results in September 2021:

  • Antibodies to SARS-CoV-2 (from vaccines or infection) among blood donors:
    • were 97.0%, up slightly compared to August (96.1%) and were largely due to vaccination;
    • were more prevalent in blood donors residing in higher-income neighbourhoods1 (97.6%) than lower-income neighbourhoods (94.7%), as was the case in previous reports.
    • were equally prevalent in donors identifying as white (97.0%), compared to those from racialized groups (98.0%).
  • Antibodies due to previous SARS-CoV-2 infection among blood donors:
    • remained the same as in August (4.4%), and not much changed from July (4.1%) and June (4.5%);
    • were highest among blood donors aged 17-24 (8.7%), in line with previous reports;
    • were on average 2-times higher among racialized donors (7.6%) compared to self-declared white donors (3.7%) across all geographic areas sampled, similar to previous observations.
    • increased in donors aged 60 and older in September (2.8%) compared to August (1.6%); the other age groups saw no change.

It should be noted that individuals who choose to donate blood are generally in good health and are more likely to live in populous urban areas than in less populous rural areas.

Antibody wane started to be observed

Blood donor data shows that the concentration of antibodies against the spike protein of SARS-CoV-2 continues to rise drastically (higher than 2,500 arbitrary units/millilitre since July), suggesting wide-ranging vaccine-driven immunity. Spike antibody concentrations are anticipated to diminish as time elapses following vaccination. The September report, indeed, shows a modest decline in peak spike antibody concentrations in older individuals (70 years old), who were prioritized in Canada’s initial COVID-19 immunization efforts. These observations further support the timely introduction of third vaccine doses in this age group.

Repeat testing of frequent donors showed that breakthrough infections remain infrequent

Among the 21,727 donors who donated blood more than once since January 2021, the most common antibody test profile (seen in 54.0% of donors) was unvaccinated at the time of their first donation2 and vaccinated (with at least one dose) by their most recent donation3. Twelve blood donors (0.3%) were believed to have had a breakthrough SARS-CoV-2 infection (based on 4,330 donors presumed to be vaccinated as defined by positive testing for anti-spike antibodies only, and later testing that were positive for both spike and nucleocapsid antibodies).

Explore our interactive webpage featuring the latest aggregated data gathered by Canadian Blood Services and Héma-Québec on SARS-CoV-2 seroprevalence in Canada.

1 Measured using the Material Deprivation Index based on postal codes.
2 Tested negative for SARS-CoV-2 spike and nucleocapsid, two of the proteins used to measure the presence of COVID-19-specific antibodies.
3 Tested positive for SARS-CoV-2 spike and negative for nucleocapsid, learn more about this here.