The latest data from Canadian Blood Services show the rapid increase in infection-induced immunity began slowing by mid-March. The data show infection-acquired seropositivity increased moderately between the end of February and mid-March, up to 27.5% from 25.3%. The growth of infections continues to be concentrated in younger age groups with nearly half of all donors aged 17 to 24 (44.8%) showing evidence of a previous SARS-CoV-2 infection. The median concentration of spike antibodies acquired via immunization and/or infection, which increased substantially in January and February due to recent vaccinations and/or infections, decreased across all age groups by mid-March.
Key findings:
- Infection-acquired seropositivity (as evidenced by anti-nucleocapsid antibodies) climbed steadily throughout study period, from 25.3% at the end of February to 27.3% the first week of March to 27.5% by mid-March.
- Self-declared racialized donors continued to exhibit higher rates of infection-acquired seropositivity compared to self-declared white donors, 37.8% vs. 25.2%, respectively.
- Donors in the youngest age bracket, between 17 and 24, have consistently seen the highest seropositivity rate from infection across all age brackets, approaching the halfway mark (44.8%) during this period.
- Nearly all blood donors (99.5%) tested positive for antibodies targeting the spike protein, a factor primarily driven by immunization.
- The median concentration of spike antibodies, which increased substantially in January and February 2022 due chiefly to recent vaccination, was beginning to drop by mid-March.
The latest interim report includes 13,571 people who donated blood from March 1 to 15, 2022, in all Canadian provinces excluding Quebec.
Early March seropositivity data steadied compared to prior months
In contrast to the January and February reports, the mid-March report shows a more modest week-to-week increase in infection-acquired seropositivity, from 27.3% (March 1-7) to 27.5% (March 8-15). As antibodies targeting the nucleocapsid protein of SARS-CoV-21 appear on average one-to-two weeks following symptom onset, this report likely captures infections acquired during in the bulk of the fifth wave, which was driven by the Omicron BA.1. and BA.2 variants.
Nearly half of young donors were previously infected
By mid-March, 44.8% of donors aged 17-24 had evidence of a past infection with the virus. Infection-acquired seropositivity decreased with each advancing age group: 34.2% of donors aged 25-39, 27.8% of those aged 40-59, and 14.2% of those aged 60 and over.
Spike antibody concentrations on the decline
The median concentration of spike antibodies, which started to dip to less than 5,000 units per millimetre (U/mL) in September 2021, increased substantially by January and February 2022 to approximately 25,000 U/mL in younger donors, but about 15,000 U/mL in older donors. This was chiefly due to recent vaccination, most likely third doses, as a rise in the concentration of spike antibodies is expected after vaccination. Increases in spike antibody concentrations may also be indicative of a recent infection. While median spike antibody concentrations are still high by mid-March, they were beginning to drop across all age groups. This drop was especially apparent in those aged 70 and older. This points to waning antibodies from the time most Canadians received their last vaccine.
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. Percentages were adjusted for test characteristics and population distribution.
Explore our interactive webpage updated every month, featuring the latest aggregated data gathered by the Canadian Blood Services and Héma-Québec on SARS-CoV-2 seroprevalence in Canada.
1 Antibodies to the nucleocapsid protein are indicative of a past infection with the virus as COVID-19 vaccines approved and administered in Canada target the spike protein, thus permitting the distinction.