Third and fourth vaccine doses effectively raise antibody levels in older blood donors by mid-May: Canadian Blood Services

Consistent with the ongoing transmission of the current Omicron variants, infection-acquired seropositivity (as measured by anti-N antibodies) increased in the blood donor community gradually through the beginning of May, from 40% at the end of April to 46% by mid-May, according to the latest data from Canadian Blood Services. Since January, median anti-S antibodies, which are indicative of either vaccination or infection with SARS-CoV-2, rose in older adults – especially those over the age of 60 – coinciding with the administration of 3^rd and, eventually, 4^th doses. This is consistent with policies adopted across Canada to vaccinate older age groups earlier. Younger donors (aged 17 to 24) continue to have the highest seroprevalence due to infection (65%) of all age groups.

Key findings:

• Infection-acquired seropositivity has increased from 40% at the end of April to 46% by mid-May, consistent with the second wave of the current Omicron variants in late March and April.
• Nearly all blood donors (100%) tested positive for anti-S antibodies, most likely due to vaccination.
• In the beginning of May, median anti-spike antibody concentrations increased, especially in the 60-69 and 70+ age groups, likely due to the administration of 4^th doses of vaccine.
• Consistent with previous surveys, donors aged 17- to 24-years-old had the highest anti-N (infection-acquired) seroprevalence rate compared to other age groups (65%).
• Infection-acquired antibodies increased in all provinces, however, fewer samples were tested from some Atlantic provinces.
• Racialized groups continued to have higher infection-acquired seroprevalence than self-declared white donors (54.95% vs 43.89%).
• By mid-May 2022, the infection-acquired antibody rate was more than eight times the seroprevalence rate observed prior to the emergence of the Omicron variant.

Vaccine-induced antibody concentrations (anti-S) are generally high among blood donors. A decrease in anti-S antibody concentrations was shown at the beginning of September 2021, but began to rise again in December among older donors. As expected, there is evidence of waning anti-S antibody concentrations as time passed since the roll-out of the 3^rd dose in January. However, consistent with the administration of 4^th doses to older Canadians in spring 2022, concentrations in older adults began to increase again.

The latest report includes samples from 15,958 people over the age of 17 who donated blood May 1-15, 2022 in all Canadian provinces, 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.