Seroprevalence increases again in June but stabilizes throughout month: Canadian Blood Services

With the continued transmission of the Omicron variant in Canada, infection-acquired seroprevalence increased again within the blood donor community, from 46.3% at the end of May to 50.7% by the end of June. However, it was relatively stable throughout the month. An increase in the concentration of vaccine-induced (anti-S) antibodies in those over 60 was observed in May and continued into June, most likely due to high uptake of fourth doses within this older age group. Almost all the blood donors were still positive for vaccine-induced (anti-S) antibodies.

Key findings:

• There was minimal change in infection-acquired seroprevalence from week to week over the month of June: from 50.5% to 51.1% to 50.3% to 50.8%.
• Consistent with previous surveys, donors aged 17- to 24-years-old had the highest infection-acquired (anti-N) seroprevalence rate (66.29 (95% CI 64.81, 67.77)) compared to other age groups. However, the seroprevalence rate increased in all age groups compared to May.
• Infection-acquired (anti-N) seroprevalence increased in all provinces in June compared to May. While the rates stabilized in Ontario and the Western provinces, they rose significantly in the Atlantic provinces.
• In the Atlantic provinces, levels of COVID-19 infection became more comparable with the rest of Canada in June (although the sample sizes in these four provinces are small).
• The newest Canadian Blood Services data emphasize the persistent inequities in infection burden among racialized communities, which continued to have higher seroprevalence than self-declared white donors (58.03% vs 49.01%).
• The data highlight that there was not much difference between the percentage of people infected who had the most social contact (51.5%) and who had the least social contact (48%) (most and least socially deprived).
• The data highlight that disparities in the burden of infection between the most materially deprived and the least materially deprived population groups, which has been observed throughout the pandemic, has narrowed (51.13% vs 47.90%).
• Nearly all blood donors (100% CI, 99.79, 100.00) tested positive for vaccine-induced (anti-S) antibodies.
• In May and June, the median concentrations of vaccine-induced (spike) antibodies increased, especially among those over the age of 60, likely due to the administration of a fourth dose.

Conclusion

Vaccine-induced (anti-S) antibody concentrations were high among blood donors in June, however, as expected, there was evidence of waning antibody concentrations since the roll-out of the third shot in January (98.89%, 95% CI – 98.73-99.06). Consistent with the early roll-out of the fourth dose to older Canadians, vaccine-induced antibody concentrations in older adults began to increase again in May and June (100%).

The latest report builds on the May 2022 report and includes samples from 32,121  people over the age of 17 who donated blood between June 1^st and 30^th, 2022, in all of Canada, excluding Quebec and the Territories.

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.