30% of Canadian Blood Services donors infected with SARS-CoV-2 by end of March 2022

Consistent with the ongoing transmission of the Omicron variant, infection-acquired seropositivity increased in the blood donor community gradually throughout March, from 27% to 30%. The newest CBS data emphasize the persisting inequities in infection burden among young adults, racialized communities, and those residing in lower-income neighbourhoods. In their latest report, Canadian Blood Services also estimate that 30% of unvaccinated blood donors had evidence of a recent infection with the virus. This, compared to about 18% of vaccinated donors.

Key findings:

• Infection-acquired seropositivity, evidenced by antibodies targeting the nucleocapsid protein, rose throughout March, from 27.0% at the beginning to 29.5% at the end, averaging at 28.7% for the entire month. This is higher than February’s average of 23.7%.
• Similar to previous surveys, nearly all blood donors (99.6%) tested positive for antibodies targeting the spike protein, most likely induced by vaccination.
• Compared to February, infection-acquired seropositivity increased in all provinces covered by Canadian Blood Services, except for Prince Edward Island and Newfoundland and Labrador.
• While infection-acquired seropositivity increased across all age groups, 17-24-year-olds, who are consistently the most impacted, saw a rate of 44.3%, up from 36.3% in February.
• Donors belonging to a racialized group were more likely to have infection-acquired antibodies (38.6%) than donors who identified as white (26.3%).
• Rates of infection-acquired seropositivity were greater among donors from lower income, compared to higher income neighbourhoods¹ (36.6% vs. 25.9%, respectively).
• In a sub-study of repeat blood donors, 29.5% of unvaccinated donors had evidence of a recent infection. By comparison, 17.5% of vaccinated donors had evidence of a recent breakthrough infection.
• Spike antibody concentrations (which show antibodies due to vaccination or infection) were still elevated in March but declined relative to their substantial increase in January and February 2022 following high booster uptake and growing infections.

The latest report builds on the mid-March report, and now includes samples from 26,026 people over the age of 17 who donated blood between March 1 to 31, 2022, in all of Canada excluding Quebec and the Territories.

Weekly seropositivity rates tick upwards, but begin to stabilize

March’s average seropositivity rate due to infection of 28.7% – up from February’s rate of 23.7% – represents more than double January’s rate of 12.1%. Monthly seropositivity rates are likely an underestimation of the toll of past infections. Seroreversion Seroreversion is the decline of antibody levels below the threshold of detection by an assay. This occurs naturally with the passage of time since initial exposure to a pathogen, in this case, the SARS-CoV-2 virus. 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 March, from 27.0% to 27.5% to 30.7% to 29.5%. Nonetheless, they were not as sharp as the weekly upticks seen in January and February.

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 March, this rate surged to 17.5%. 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 March, more than a quarter (29.5%) 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.

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.