Nearly 40% of Canadian adults have had an Omicron infection: Canadian Blood Services

Consistent with the ongoing transmission of the Omicron variant, infection-acquired seropositivity increased among blood donor throughout April, from 33% at the beginning of the month to 40% by the end. The average for the month, 36.7%, was higher than March’s average of 28.7%. The newest CBS data emphasize the persisting inequities in infection burden carried by young adults and racialized communities. In their latest report, Canadian Blood Services also estimate that 37% of unvaccinated blood donors had evidence of a recent infection with the virus. This, compared to about 22% of vaccinated donors.

Evidence of the combination of waning immunity since many Canadians were vaccinated and the nature of the Omicron wave, breakthrough infections among vaccinated donors remained low from June 2021 to December 2021, but increased from 5.19% in January 2022 to 21.99% in April 2022. These data support the need for people to get a third vaccine, and where recommended, even a fourth dose to enhance protection.

Key findings:

• Infection-acquired seropositivity, evidenced by antibodies targeting the nucleocapsid protein, rose throughout April, from 32.8% at the beginning to 40.4% at the end, averaging at 36.7% for the entire month. This is higher than March’s average of 28.7%
• Nearly all blood donors (99.7%) tested positive for antibodies targeting the spike protein, most likely induced by vaccination.
• Infection-acquired seropositivity increased in all provinces covered by Canadian Blood Services but doubled in the Atlantic provinces to 27.9% in April, compared to 13.9% in March. The biggest increases were seen in New Brunswick, Prince Edward Island and Newfoundland, where the percentage of people with infection-acquired antibodies increased five-fold. However, the exact percentage increase in these provinces may be skewed because of the small sample size.

• While infection-acquired seropositivity increased across all age groups, 17-24-year-olds, the age group that has consistently had the highest rate of infection-acquired seropositivity, saw the percentage of those showing evidence of a previous infection rise to 55.4%, up from 36.3% in February.
• Donors belonging to a racialized group were more likely to have evidence of a previous infection (45.0%) than donors who identified as white (34.8%).
• In a sub-study of repeat blood donors, new infections in unvaccinated donors increased from 1.53% in June 2021 to 9.12% in January 2022 to 37.19% in April 2022.
• Breakthrough infections among vaccinated donors remained low from June 2021 to December 2021, but increased from 5.19% in January 2022 to 21.99% in April 2022.

The latest report builds on the mid-April report, and now includes samples from 29,787 people over the age of 17 who donated blood between April 1 to 30, 2022, in all of Canada excluding Quebec and the Territories.

Weekly seropositivity rates tick upwards, but begin to stabilize

April’s average seropositivity rate due to infection of 36.7% – up from February’s rate of 28.7% – represents roughly triple January’s rate of 12.1%. Monthly seropositivity rates are likely an underestimation of the toll of past infections. Seroreversion may account for loss of detection of antibodies acquired due to an infection earlier in the pandemic. Consistent with the advance of the Omicron variant, seropositivity increased gradually on a weekly basis throughout April, from 32.8% to 35.5% to 37.6% to 40.0%.

High Omicron burden among the unvaccinated: A sub-study of repeat donors

Breakthrough infections in individuals who received at least one vaccine dose were infrequent in the fall, but in early 2022 this scenario changed. In December 2021, 0.7% of repeat vaccinated donors were believed to have had a breakthrough SARS-CoV-2 infection². In April, this rate surged to 21.2%. This increase is in line with the immune escape properties of the Omicron variant. 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).

In April, more than a third (37.2%) of repeat blood donors who did not have any antibodies to SARS-CoV-2 (i.e., neither vaccinated or previously infected) had evidence of a recent infection with the virus. This rate has risen steeply over the last few months, from 3.9% December 2021, to 9.0% in January, to 23.7% in February, to 29.5% in March.

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 As measured by the Material Deprivation Index, which makes use of postal code data.
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.