Highest levels recorded in Western Canada but mass national vaccination remains only route to achieving herd immunity
Canadian Blood Services and Canada’s COVID-19 Immunity Task Force (CITF) are releasing new results from their collaborative nine-province SARS-CoV-2 seroprevalence study. This analysis, based on 33,860 blood samples from blood donor centres across Canada (excluding Quebec and the Territories) collected in October and November 2020, suggests that 1.5 per cent of healthy Canadians had been exposed to SARS-CoV-2, the virus causing COVID-19.
“Although this represents a doubling in comparison to an initial report following the first wave in May and June, the overall seroprevalence – the number of people with antibodies, which suggests a previous COVID-19 infection – remains very low and Canada remains a long way from herd immunity,” says CITF Executive Director Tim Evans.
“Community transmission has been more intense in this sustained second wave,” states Dr. Catherine Hankins, CITF Co-Chair. “However, even using serology to add cases to our count that never came to light through formal diagnostic tests, it is clear that the vast majority of Canadians remain vulnerable to COVID-19. We have no choice but to keep up public health measures until vaccine-induced immunity grows substantially.”
Most growth in the Prairies and Western Canada
“These results show the greatest increases in the Prairies and Western provinces,” says Dr. Sheila O’Brien, associate director of epidemiology and surveillance at Canadian Blood Services. “Since the first wave in May and June, seroprevalence in the Prairies (Manitoba and Saskatchewan) increased five-fold, from 1.69% to 8.71%. Seroprevalence in British Columbia and Alberta has nearly tripled.”
In contrast, in Ontario and the Maritimes, there have been no detectable increases in seroprevalence amongst blood donors.
Higher seroprevalence among young adults and racialized Canadians
Donors aged between 17 and 24 years old had the highest seroprevalence rates (2.97%) in November and have seen the largest increase since the first wave compared to other age groups.
“These results are an important reminder that young adults, even though they are less likely to fall ill from infection, must pay attention to public health measures, not only for themselves but as they are vectors of transmission,” says Dr. David Naylor, CITF Co-Chair. “The clustering of infection amongst racialized Canadians also requires more tailored approaches to respond to the diverse challenges faced by these communities”.
“The data are also confirming that racialized Canadians are more likely to have antibodies against COVID-19 than white donors, and the disparity seems to be widening over time,” states Dr. O’Brien. Racialized Canadians continue to have higher seroprevalence rates (2.5%) compared to donors who self-identify as white (1.35%).
“This is vital information. Continued monitoring of seroprevalence will be foundational data to understanding infection-acquired immunity as vaccination rolls out,” she continues.
A dramatic change in some cities
“When we take a look at selected cities across the country, the second wave looks dramatically different,” states Dr. Hankins. Seroprevalence in Toronto and Ottawa – which had the highest seroprevalence levels in wave one – shows no significant change, whereas the cities of Edmonton and Winnipeg reveal a seven-fold increase in seroprevalence.” (See table 2 below for a breakdown of selected cities).
“Canadian Blood Services is proud to contribute to Canada’s fight against COVID-19 through this seroprevalence study,” says Dr. Graham Sher, CEO of Canadian Blood Services. “As an organization, we are uniquely positioned to perform this work thanks to our existing infrastructure, experience and expertise. We hope this data will help inform decision-makers as they forge ahead through the next phases of this pandemic.”
“Ongoing seroprevalence studies are important for Canada as we continue with the COVID-19 vaccine rollout,” says Chief Public Health Officer Dr. Theresa Tam. “As more Canadians are vaccinated, we will start seeing a significant increase in the proportion of the population with antibodies. These reports will help Canada with an estimate of the number of Canadians who may have some form of immunity to COVID-19, either acquired by a previous infection or through vaccines. These results will help inform further public health efforts.”
Canadian Blood Services is moving towards using a new assay to test blood samples that will allow them to differentiate between infection-acquired and vaccine-induced immunity.
The study is supported by the Government of Canada, through the CITF, which will be working with Canadian Blood Services to further validate these results by comparing results to other antibody tests over the coming weeks.
ABOUT THE PARTNERSHIP
When the Government of Canada established the CITF in late April 2020, Canadian Blood Services and Héma-Québec reached out to offer their assistance. Canadian Blood Services and Héma-Québec routinely test blood donations and both organizations also regularly contribute to ethics-approved research studies like this one.
While the donation selection criteria ensure blood donors are healthy, caution should be exercised in extrapolating findings to all healthy adult Canadians because blood donors self-select to be blood donors, in some areas access to a donation clinic may be limited, and there are fewer elderly donors who donate blood compared to the general population.
ABOUT THE COVID-19 IMMUNITY TASK FORCE
The Government of Canada established the COVID-19 Immunity Task Force (CITF) in late April 2020 to determine the extent of SARS-CoV-2 infection in Canada (in the general population as well as in specific communities and priority populations), understand the nature of immunity following infection, and now, help monitor the effectiveness and safety of vaccines as they are rolled out across Canada. The Task Force regroups leading Canadian scientists and experts from universities and hospitals across Canada. It works closely with federal, provincial, and territorial public health officials, as well as health organizations, researchers, implementing partners, other task forces, communities, and stakeholders from inception through to dissemination of findings in order to best serve all Canadians. For more information visit: www.covid19immunitytaskforce.ca
ABOUT CANADIAN BLOOD SERVICES
Canadian Blood Services is a not-for-profit charitable organization. Regulated by Health Canada as a biologics manufacturer and primarily funded by the provincial and territorial ministries of health, Canadian Blood Services operates with a national scope, infrastructure and governance that make it unique within Canadian healthcare. In the domain of blood, plasma, and stem cells, Canadian Blood Services provides services for patients on behalf of all provincial and territorial governments, except Quebec. The national transplant registry for interprovincial organ sharing and related programs reaches into all provinces and territories, as a biological lifeline for Canadians. For more information visit: blood.ca
MEDIA CONTACTS
COVID-19 Immunity Task Force
media@covid19immunitytaskforce.ca
Rebecca Burns, Cell: +1.438.871.8763
Caroline Phaneuf, Cell: +1.514.444.4532
Canadian Blood Services
media@blood.ca
1-877-709-7773
Table 1: Rate of antibody detection in Blood Donors in Canada
| Values % (95% confidence Interval) | Values % (95% confidence Interval) | Values % (95% confidence Interval) | |
| Dates | May-July | October | November |
| Canada | 0.70% (0.63-0.77) | 0.88% (0.73-1.04) | 1.51% (1.31-1.71) |
| British Columbia | 0.56% (0.41-0.70) | 0.86% (0.50-1.23) | 1.51% (1.04-1.97) |
| Alberta | 0.48% (0.33-0.62) | 0.76% (0.38-1.14) | 1.79% (1.24-2.34) |
| Saskatchewan | 0.53% (0.23-0.83) | 0.17% (0.00-0.59) | 4.17% (2.57-5.77) |
| Manitoba | 0.59% (0.30-0.88) | 2.96% (1.70-4.23) | 8.56% (6.51-10.62) |
| Ontario | 0.88% (0.78-0.99) | 0.87% (0.65-1.08) | 0.77% (0.56-0.97) |
| Quebec* | 1.06% (0.82-1.30) | ||
| New Brunswick | 0.23% (0.00-0.49) | 0.17% (0.00-0.66) | 0.49% (0.00-1.20) |
| Nova Scotia | 0.69% (0.33-1.05) | 0.98% (0.11-1.85) | 0.19% (0.00-0.65) |
| Prince Edward Island | 0.04% (0.00-0.42) | 0% | 0% |
| Newfoundland | 0.44% (0.04-0.84) | 0.06% (0.00-0.52) | 0.95% (0.00-2.09) |
*Québec showed a seroprevalence of 2.23% in a study by Héma-Québec, with blood samples taken between May 25 and July 9.
Table 2: Rate of antibody detection in Blood Donors in selected Canadian cities
| Values % (95% confidence Interval) | Values % (95% confidence Interval) | Values % (95% confidence Interval) | |
| Dates | May-July | October | November |
| Vancouver | 0.60 | 1.19 | 1.42 |
| Calgary | 0.43 | 0.99 | 2.22 |
| Edmonton | 0.29 | 0.69 | 2.23 |
| Ottawa | 1.29 | 0.93 | 0.88 |
| Toronto | 1.07 | 1.16 | 0.98 |
| Winnipeg | n/a | 0.75 | 5.09 |
Please visit our website here to view more detailed data.