Infection-acquired seropositivity increased throughout February 2022, averaging out to 23.7% for the month – double January’s rate of 12.1% – according to data from Canadian Blood Services. Nearly a quarter of repeat donors who were unvaccinated had evidence of a recent SARS-CoV-2 infection by the end of February compared to 15.6% of repeat donors vaccinated with at least one dose. More than a third (36.3%) of donors aged 17-to-24 were seropositive for infection-acquired antibodies, as were a third (33.5%) of donors belonging to a racialized group (vs. 21.2% in white donors).
- Nearly all blood donors (99.6%) tested positive for antibodies targeting the spike protein, suggesting they had vaccine-induced or infection-acquired antibodies, but most likely the high rate is due to a high vaccination rate.
- Infection-acquired seropositivity, evidenced by antibodies targeting the nucleocapsid protein, rose throughout the month, from 21.4% at the beginning to 25.3% at the end, averaging at 23.7% for the entire month.
- Infection-acquired seropositivity increased in all Canadian provinces compared to January (CBS data excludes Quebec).
- While infection-acquired seropositivity increased across all age groups, the 17-24 bracket, which has consistently experienced the highest rate, saw a rate of 36.3% in February, its highest monthly rate yet.
- In a sub-study of repeat blood donors, 23.7% of unvaccinated donors had evidence of a recent infection. By comparison, 15.6% of vaccinated donors had evidence of a recent breakthrough infection.
- A third (33.5%) of donors belonging to a racialized group were seropositive for infection-acquired antibodies, compared with 21.2% of donors who identified as white.
- Spike antibody concentrations increased further in February, which is reflective of high vaccine coverage as well as the growing number of recent infections.
The latest report builds on the mid-February report, and now includes samples from 28,616 people who donated blood between February 1 to 28, 2022, in all Canadian provinces excluding Quebec.
February’s seropositivity rate twice that of January 2022
The latest data reveal the extent of Omicron’s sweep through the Canadian population earlier this year. February’s average seropositivity rate due to infection of 23.7% was twice that seen in January (12.1%). It also constituted a big jump from December 2021’s seropositivity rate of 6.4%. While February’s seropositivity rate is the Canadian Blood Services’ highest recorded so far, it may be an underestimation of the toll of past infections because of seroreversion, which is the decline of antibody levels below the threshold of detection by an assay. This occurs naturally with the passage of time since exposure to the SARS-CoV-2 virus. In this case, seroreversion may account for loss of detection of antibodies acquired due to an infection earlier in the pandemic. Infection-acquired antibodies appear on average one-to-two weeks following symptom onset. As such, this report likely captures antibody-evidence from infections leading up to mid-February.
In line with previous reports, donors aged 17-24 exhibited the highest seropositivity rate. This month, more than a third (36.3%) were found to have recovered from an infection. Even so, all age brackets experienced a noticeable increase in anti-nucleocapsid seropositivity in February compared to January. Additionally, evidence of a prior infection was higher among self-identified racialized donors (33.5%) compared to self-identified white donors (21.2%). Infection-acquired immunity was slightly higher among donors from lower income compared to higher income neighbourhoods1 (22.4% vs. 28.9%, respectively).
Spike antibody concentrations reflect high vaccine coverage and growing infections
The median concentration of spike antibodies (garnered through vaccination or infection) remained high across all age groups in February. These concentrations, which declined in early fall due to time elapsed since vaccine roll-out, increased, first among those 70+ in December, and then among all groups starting in January. This suggests that many individuals have benefitted from recent vaccination, most likely third (booster) doses. While vaccination is the largest contributor to this rise in spike antibody concentrations, recent infections may also play a part.
Omicron targets all, but favours the unvaccinated: A sub-study of repeat donors
Breakthrough infections in individuals who received at least one dose of vaccine were very infrequent in the fall, but in early 2022, this scenario changed. In January 2022, 5.2% of repeat donors vaccinated with at least one dose were believed to have had a breakthrough SARS-CoV-2 infection2. In February, this rate tripled to 15.6% (1,099/7,065). This is a sharp increase from December 2021’s presumed breakthrough rate of about 0.7%. 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).
Infections among unvaccinated donors were also on the rise. In February, 23.7% of repeat blood donors who previously 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 is more than double that recorded in January (9.0%) and six times what was seen in December 2021 (3.9%).
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.
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.