The latest results from the Canadian Blood Services’ monthly serosurvey indicate that 95% of blood donors sampled in July 2021 had antibodies against SARS-CoV-2, acquired predominantly through vaccination. Not only did vaccine-induced seroprevalence increase among all sociodemographic groups and across all geographic regions, but infection-acquired seroprevalence dropped compared to June 2021. Donors identifying as part of a racialized group and those aged 17 to 24 remained most likely to have antibodies due to infection.
The latest Canadian Blood Services data included 8,457 people who donated blood between July 14 and 23, 2021, in all Canadian provinces, excluding Quebec.
- In July 2021, antibodies to SARS-CoV-2 among blood donors:
- Climbed to 94.7%, up from 90.8% in June, largely due to vaccines.
- Increased at the same rate across blood donors from all socioeconomic groups, including racialized communities and lower-income neighbourhoods.
- In July 2021, antibodies due to SARS-CoV-2 infection among blood donors:
- Dropped to 4.1% from 4.5% in June.
- Were two times higher in racialized compared to self-declared white donors, similar to previous reports.
- Were no different among donors from richer or poorer neighbourhoods, contrary to previous reports.
- Were highest amongst blood donors aged 17-24 at 6.7%, although considerably lower than in June (9.3%).
Blood donor data mirrors widescale vaccination in Canada
Overall, the proportion of Canadians with anti-SARS-CoV-2 antibodies in July was 94.7%, primarily due to vaccination. This represents a ~4% increase from June 2021, which saw 90.8% of blood donors with immunity against SARS-CoV-2. Additionally, while the level of antibodies against the spike protein of SARS-CoV-2 remained consistently low from January to May 2021 – rising moderately in June 2021 – they jumped to significantly higher levels in July 2021, further suggesting widespread vaccine-driven immunity.
Consistent with the June report, seroprevalence did not vary significantly among socioeconomic groups, as was observed in April and May 2021. In fact, in July, 95.0% of blood donors who self-identified as white had anti-SARS-CoV-2 antibodies primarily due to vaccination, no different than those who self-identified as part of a racialized group (93.8%). Similarly, donors living in lower-income neighbourhoods1 were almost as likely to have antibodies (92.9%) as those living in higher-income neighbourhoods (96.7%): an improvement compared to June 2021 when the gap was larger (88.3% in lower-income and 93.7% in higher-income neighbourhoods).
Infection rates decline, but remain highest among 17–24-year-olds
Canada’s levels of seroprevalence due to a past SARS-CoV-2 infection remained low in July 2021, at 4.1%. This is the first report in which the rate has dropped relative to the previous month:
- Infection-acquired seroprevalence had been gradually rising since April (3.2% in April, 4.0% in May, and 4.5% in June).
- While seroprevalence due to a past infection remained highest in the youngest age bracket (17-24-years-old: 6.7%), this number decreased by 2.6% since June.
- In line with previous surveys, seroprevalence in youths aged 17 to 24 was highest in Alberta (11.9%), though this rate is only 2/3rds as high as the level recorded in that province in June (17.5%).
- In contrast, in British Columbia, antibodies arising from infection more than doubled in July among 17-24-year-olds (9.9%) compared to June (4.6%).
Underscoring gaps in race and affluence in infection rates
The rate of seroprevalence due to past infection with SARS-CoV-2 in self-declared racialized donors (7.3%) was more than double that of donors who identified as white (3.3.%). However, in contrast to previous reports, donors living in lower-income neighbourhoods were only 1.2 times more likely to have evidence of a past infection with SARS-CoV-2 (4.6%) compared to donors living in higher-income neighbourhoods (3.9%). These latest findings indicate that the gap in infection burden between donors living in less affluent versus more affluent neighbourhoods is narrowing, while the gap between racialized and non-racialized donors persists.
Repeat testing of frequent donors: a sub-study
Canadian Blood Services reports that of the 14,201 people who donated blood more than once since January 2021, a little over half (51.2%) tested negative for a past infection or vaccination during their initial blood donation2, but their latest test results suggest that they had been subsequently vaccinated3. Additionally, five donors who were vaccinated based on a positive test for only the spike protein antibody, later tested positive for both spike and nucleocapsid protein antibodies, suggesting potential breakthrough SARS-CoV-2 infection.
1 Measured using the Material Deprivation Index based on postal codes.
2 Tested negative for SARS-CoV-2 spike and nucleocapsid, two of the proteins used to measure the presence of COVID-19-specific antibodies.
3 Tested positive for SARS-CoV-2 spike and negative for nucleocapsid, learn more about this here.