The latest Canadian Blood Services (CBS) report, from October 2021, indicates that 98% of blood donors sampled had antibodies to SARS-CoV-2. This number is largely driven by immunization with at least one vaccine dose, as infection-acquired seroprevalence stayed low, at 4.3%. Results also reveal a downward trend in the mean levels of antibodies across all age groups between September and October, supporting the need for a third (booster) dose.
Breakthrough infections in individuals vaccinated with at least one dose remained infrequent during this period. It is important to note that the data predate the current surge in the Omicron variant and was collected prior to when most donors were eligible for a booster.
The latest CBS report included 9,627 people who donated blood between October 14th and 23rd, 2021 in all of Canada, excluding Quebec and the Territories.
Key results from October 2021:
- Antibodies to SARS-CoV-2 from vaccines or infection among blood donors:
- were 98.0%, up one percent compared to September and were largely acquired through vaccination;
- were present in almost all blood donors residing in higher-income neighbourhoods1 (99.3%), slightly more than among those residing in lower-income neighbourhoods (97.1%), consistent with previous reports;
- were equally prevalent in donors identifying as white (98.2%) as those from racialized groups (98.7%).
- Antibodies due only to prior SARS-CoV-2 infection among blood donors:
- were 4.3%, similar to the previous four months: September (4.4%), August (4.4%), July (4.1%), and June (4.5%);
- were highest among blood donors aged 17-24 (7.5%), in line with previous reports;
- were almost twice as high among racialized donors (6.2%) compared to self-identified white donors (3.9%) across all geographic areas sampled, a narrower gap compared to previous observations.
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.
Downward trend in antibody levels detected
The concentration of antibodies against the spike protein of SARS-CoV-2, meaning in people who have been vaccinated, which rose dramatically with the widespread roll-out of vaccines in July (higher than 2,500 arbitrary units/millilitre), has started to decline in October. This does not come as a surprise since spike antibody concentrations are expected to diminish over time following vaccination. Antibodies had waned the most in those 70 and older, who were among the first people eligible to receive a vaccine. These findings further support the timely roll-out of third (booster) vaccine doses for adults in order to help elevate antibody levels.
Breakthrough infections uncommon in October, ahead of Omicron arrival
In the latest report, 0.3% (15/5,605) of individuals who donated blood more than once since January 2021 were surmised to have had a breakthrough SARS-CoV-2 infection. These donors were presumed to be vaccinated with at least one dose as defined by positive testing for anti-spike antibodies only, and on a later donation, tested positive for both spike and nucleocapsid antibodies. This corresponds to the rate of breakthrough infections measured in the September report (12/4,330; 0.3%).
1 Measured using the Material Deprivation Index based on postal codes.