The latest report from Canadian Blood Services, covering the early days of the Omicron wave, shows that, notwithstanding the nearly universal presence of anti-SARS-CoV-2 antibodies among blood donors, the number of donors who had antibodies due to prior infection rose.
- The proportion of donors with evidence of a prior infection was 6.4% in December, up from 5.1% in November. Importantly, this percentage increased over the 17-day study period from 5.6% to 6.6% to 7.5%, in accordance with the growing rise of infections due to Omicron.
- In repeat donors, infections were more likely to occur in those unvaccinated (3.9%) than vaccinated with at least one dose (0.7%).
- Infection-acquired seropositivity was more common in those aged 17-24 (11.4% vs. other age groups 8% or lower).
- The rise of the infection-acquired seropositivity rate was most noticeable in Alberta, where 12.9% of blood donors tested positive for a previous infection, compared to 8% or lower in other provinces.
- Infection-acquired seropositivity was twice as high among racialized blood donors than among in self-declared white donors (10.4% vs. 5.2%).
- The proportion of donors with SARS-CoV-2 antibodies acquired by prior infection or vaccination was 100% (adjusted seroprevalence). As in recent reports, this was mainly due to vaccination.
Study captures the early days of Omicron
The latest report includes 16,816 people who donated blood in the second half of month (December 14 to 30, 2021) in all Canadian provinces excluding Quebec.
Antibodies targeting the nucleocapsid protein of the SARS-CoV-2 virus1 appear on average one-to-two weeks following the onset of symptoms. Hence, this report encapsulates antibody-evidence from infections up to early, and potentially mid-, December. It should also be noted that donors with a known active SARS-CoV-2 infection are not eligible to donate blood until their infection and symptoms have cleared (at least 14 days). December’s infection-acquired seropositivity of 6.4% is the highest Canadian Blood Services has recorded so far.
Boosters rise spike antibody levels
The concentration of spike antibodies (arbitrary units/millilitre) started to decline in September, due to time elapsed since last vaccination. This decline had been particularly noticeable in older age groups as they were among the first be offered vaccines. In December however, the concentration of spike antibodies increased; for those above 70, the concentration rose above that seen in September. This increase likely reflects the timely administration of third (booster) doses of vaccine, which, as the name suggest, help to boost antibody levels and immune memory.
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 a different part of the virus, namely the spike protein, thus permitting the distinction.