Most Canadian blood donors exhibited some form of immunity against SARS-CoV-2 in January 2022, primarily due to vaccination. Nonetheless, infection-acquired seroprevalence increased sharply between December 2021 and the end of January 2022, nearly tripling from 6.4% to 16.3%. Almost a quarter of donors aged 17-24 were seropositive for infection-acquired antibodies (22.2%).
- Nearly all blood donors (98.9%) tested positive for antibodies targeting the spike protein, chiefly due to vaccination.
- Infection-acquired seropositivity (as evidenced by anti-nucleocapsid IgG antibodies) climbed throughout the month, from 7.2% at the beginning of January to 16.3% by the end of the month, consistent with the spread of Omicron.
- Infection-acquired seropositivity increased in almost all Canadian provinces (CBS data exclude Quebec).
- While infection-acquired seropositivity increased across all age categories, the 17-24 bracket has consistently had the highest rate, this month at nearly a quarter (22.2%). This is nearly double the rate in December (11.4%).
- In a sub-study of repeat blood donors, 9.0% of unvaccinated donors had evidence of a prior infection. By comparison, 5.3% of vaccinated donors had evidence of a presumed breakthrough infection.
The latest report builds on the January interim report released mid-month, and now includes samples from 32,505 people who donated blood between January 1 to 31, 2022, in all Canadian provinces excluding Quebec.
Young adults and racialized groups disproportionately affected in Omicron-fuelled fifth wave
Given that antibodies targeting the nucleocapsid protein of SARS-CoV-21 appear on average one-to-two weeks following symptom onset, this report captures antibody-evidence from infections leading up to mid-January. Beyond being substantially higher than December’s rate of 6.4%, January’s seropositivity rate of 12.1% is the highest yet recorded. These findings reveal Omicron’s impact at a time when PCR tests were largely unavailable, thus leading to an underrepresentation of the true toll of infection.
In line with previous pandemic waves, it appears that young donors, particularly those aged 17-24, were disproportionally affected. The age bracket with the second highest seropositivity rate was those aged 25-39, at 15.4%. Additionally, as in previous reports, evidence of a prior infection was almost twice as likely in self-identified racialized donors (18.3%) compared to white donors (10.7%). Differences in infection-acquired seropositivity between high- and low-income neighbourhoods were not significant (11.3% vs. 15.2%, respectively).
A boost in spike antibody concentrations thanks to boosters
The median concentration of spike antibodies (which reflect vaccine-induced antibodies) increased substantially across all age groups in January. This is consistent with the rollout of third (booster) vaccine doses, given that a rise in the concentration of spike antibodies is expected after vaccination. While boosters are the biggest player behind this rise in antibody concentration, recent infections may also be a contributing factor.
High rates of infection among the unvaccinated: A sub-study of repeat donors
Canadian Blood Services routinely monitors rates of new infections and breakthrough infections in repeat donors (that is, individuals who donate blood more than once a year). Breakthrough infections in individuals who have received at least one dose of vaccine were very infrequent in the fall, but in January 2022, this scenario changed. Among the 6,265 repeat donors vaccinated with at least one dose, 331 (5.2%) were believed to have had a breakthrough SARS-CoV-2 infection2. This represents a sharp increase compared to December, which saw a presumed breakthrough rate of about 0.7%. Moreover, of 4,182 donors without any profile of SARS-CoV-2 antibodies (i.e., neither vaccinated nor previously infected) prior to January, 9.0% were found to have had new infections. This rate is more than double of that recorded in December (3.9%).
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.
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.
2 As deduced by testing positive for only spike antibodies on a first donation and then testing positive for both spike and nucleocapsid antibodies on a later donation.