Over half of Canadian blood donors aged 17 to 24 were infected with SARS-CoV-2 by mid-April

Blood donors between 17 and 24 years old have persistently experienced the highest seropositivity rate from infection. Mid-April data from the Canadian Blood Services show that a majority (52%) in this age group had evidence of infection-acquired antibodies (antibodies targeting the nucleocapsid protein). Overall, 35.3% of donors had evidence of a past SARS-CoV-2 infection by mid-April, a substantial increase from 29.5% at the end of March. This trend is consistent with a resurgence of Omicron infections in the sixth wave. Importantly, the median concentration of spike antibodies was at its lowest since the beginning of this year, pointing to waning immunity.

Key findings:

• Consistent with previous surveys, self-declared racialized donors were disproportionally affected by SARS-CoV-2 in mid-April, as they exhibited a nearly 12 percentage point increase in seropositivity rate compared to white donors (seropositivity among racialized donors in mid-April: 44.5% vs. seropositivity among self-declared white donors: 33.0%).
• Nearly all blood donors (99.8%) continued to test positive for antibodies targeting the spike protein, a factor primarily driven by vaccination.
• Infection-acquired seropositivity (as evidenced by nucleocapsid antibodies) jumped over 18% throughout the study period, from 29.5% at the end of March to 35.3% by mid-April, consistent with the Omicron-fuelled sixth wave.
• The median concentration of spike antibodies, which increased substantially by February 2022 due chiefly to recent vaccination, decreased from those levels in the most recent report, particularly among donors aged 60 and older.

The latest interim report includes 14,341 people who donated blood from April 1 to 15, 2022, in all Canadian provinces excluding Quebec.

Early April seropositivity data exposes beginning of second Omicron surge

Compared to March’s modest week-to-week increase in infection-acquired seropositivity, the mid-April report showed a more pronounced increase. Over the study period, seropositivity jumped from 29.5% (March 24-31) to 32.6% (April 1-7) to 35.3% (April 8-15). As antibodies targeting the nucleocapsid protein of SARS-CoV-2¹ appear on average one-to-two weeks following symptom onset, this report likely captures infections sustained in the beginning of the sixth wave in Canada, which began in late March. Of note, the percentage of donors with infection-acquired antibodies in mid-April was six times greater than that of Canadian Blood Services reports before the emergence of Omicron in December 2021.

More than half of young donors have been infected

By mid-April, 52.7% of donors aged 17-24 had evidence of a past infection. Similar to previous reports, infection-acquired seropositivity was lower with each increasing age: 45.9% of donors aged 25-39, 35.2% of those aged 40-59, and 17.9% of those aged 60 and over were positive for nucleocapsid antibodies.

These estimates may be an underrepresentation of the true number of past infections due to the seroreversion (waning of antibodies) of nucleocapsid antibodies acquired earlier in the pandemic. However, as the number of Omicron infections represent the most recent pool of infections, most of these individuals will likely not have as yet experienced any seroreversion.

Spike antibodies have declined with time since most received their last dose

The median concentration of spike antibodies, which started to dip in early fall 2021, increased substantially by early 2022. This was most likely due to third doses of vaccine, as a rise in the concentration of spike antibodies is expected after vaccination. Increases in spike antibody concentrations may also be indicative of a recent infection. Median spike antibody concentrations began to decline in March and continued to do so in April. This drop was especially apparent in those aged 60 and older, who were among the first to receive a third dose and have only recently been eligible for a fourth dose.

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 Antibodies to the nucleocapsid protein are indicative of a past infection with the virus as COVID-19 vaccines approved and administered in Canada target the spike protein, thus permitting the distinction.