Remarkably, nearly all Canadian Blood Services blood donors sampled in November tested positive for antibodies against the SARS-CoV-2 spike protein. While this was mainly due to vaccination with at least one vaccine dose, seroprevalence from a prior infection with COVID-19 stood at 5.1%, higher than what was seen in previous months.
In a subset of repeat donors, new infections occurred mostly in individuals not yet vaccinated. Breakthrough infections, on the other hand, remained infrequent during this period. These data were collected prior to the Omicron variant sweeping across Canada.
The latest report included 9,018 people who donated blood between November 13 and 24, 2021 in all Canadian provinces excluding Quebec.
Key results from November 2021:
- Antibodies to SARS-CoV-2 from vaccines or infection among blood donors:
- were present in nearly all blood donors (100.0%). This was higher than in October (98.0%) and was largely driven by vaccination;
- were equally prevalent in blood donors residing in higher-income neighbourhoods1 (100.0 %) and lower-income neighbourhoods (99.3%) as well as among donors self-identifying as white (100.0%) and those from racialized groups (99.99%).
- Antibodies due only to prior SARS-CoV-2 infection among blood donors:
- were present in 5.1% of donors sampled, a considerable jump from previous months: October (4.3%), September (4.4%), August (4.4%), and July (4.1%);
- were highest among blood donors aged 17-24 (9.4%), in line with previous reports;
- were almost twice as high among racialized donors (8.3%) compared to self-identified white donors (4.6%) across all geographic areas sampled.
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.
Uptick in infection-acquired seroprevalence
Antibodies targeting the nucleocapsid protein of the SARS-CoV-2 virus appear on average one-two weeks following the onset of symptoms. While the Omicron variant made its debut in Canada in late November – setting into motion an exponential increase in daily case counts – antibody evidence will lag. Thus, the data presented here don’t begin to reflect its impact across Canada. Even so, November’s infection-only seroprevalence of 5.1% stands in contrast to the five months prior, which hovered around 4.3%.
Infection most common in those not yet vaccinated
New infections in unvaccinated donors have steadily increased from 1.5% in June to 3.2% in November. These findings are based upon a subset of individuals who donated blood more than once in the last year. Vaccine breakthrough infections, however, have remained rare. In November, 0.6% of repeat donors were presumed to have had a breakthrough infection2.
Spike antibody concentrations drop even further
As was the case in the previous report, the concentration of antibodies against the spike protein of SARS-CoV-2 have declined further. This concentration – which rose dramatically with the widespread roll-out of vaccines in July (higher than 2,500 arbitrary units/millilitre) – was expected to drop over time following vaccination. While these antibody concentrations are still high, they have waned the most in those 70 and older. This age group was among the first to be recommended to receive a third (booster) dose, which was administered late in 2021. Boosters will help to elevate the concentration of spike antibodies.
1 Measured using the Material Deprivation Index based on postal codes.
2 These donors were presumed to be vaccinated with at least one dose as defined by a positive test for anti-spike antibodies only (with no evidence of a prior infection), and on a later donation, testing positive for both spike and nucleocapsid antibodies (i.e., evidence of a new infection).