Initial preliminary results from the Canadian Partnership for Tomorrow’s Health (CanPath) COVID-19 Antibody Study, based on close to 6,000 dried blood spot samples collected between February 8 and May 17, 2021, show a high degree of variability in the level of antibodies produced by a single dose of a COVID-19 vaccine. These findings highlight the importance of accelerating second doses as the Delta variant continues to spread, particularly with the vast majority of Canadians having received only a single vaccine dose. This is the first pan-Canadian study using samples from a wide range of participants to confirm evidence from vaccine manufacturers’ clinical trials, as well as findings in a recent preprint from the United Kingdom and other smaller studies.
“Our preliminary results, based on a sampling of adults from across Canada, show that the level of antibodies elicited in response to a first dose of different vaccine types varies,” says Dr. Philip Awadalla, National Scientific Director of CanPath. “Consistent with earlier reports, we also found that single doses of the mRNA (Pfizer-BioNTech and Moderna) vaccines produced short-term antibody levels over one and a half times greater than those produced by the viral vector vaccine (Oxford-AstraZeneca).”
CanPath is a national population health research platform that follows the health of 330,000 volunteer Canadians (or 1% of the population). The Government of Canada, through its COVID-19 Immunity Task Force (CITF), funded this pan-Canadian COVID-19 seroprevalence study, which invited more than 20,000 of its participants age 30 and older to take part
Early results from this study show a lag in the development of antibodies after a single vaccine dose. Through a supporting online questionnaire, CanPath recorded who had been vaccinated, the date of vaccination, and the vaccine type and name.
“Approximately 10% of participants who reported being vaccinated with a single dose of an mRNA vaccine and 30% of those vaccinated with a single dose of the viral vector vaccine did not show signs of antibodies above thresholds differentiating them from the population at large,” explains Dr. Awadalla. Additionally, and again consistent with recent international reports, antibody levels appeared lower in those over the age of 60 after a single dose of any vaccine type.
“The ‘first-doses-fast’ strategy adopted in Canada has worked extremely well so far in reaching more Canadians. We have seen a drastic reduction in case numbers and hospitalizations, despite the arrival of variants, allowing us to look forward to a better summer,” says Dr. Catherine Hankins, CITF co-chair. “That said, this study offers more evidence that Canadians considering only one dose need to know that they are not fully protected. Getting a second dose is vital now with the Delta variant on the rise.”
“Although participants may still be protected through other immune mechanisms, these findings underscore the fact that, after a single dose, antibody production is far from what is produced after two doses,” explains Dr. Awadalla. The study detected high levels of antibodies in participants with two doses of an mRNA vaccine, with levels almost twice as high as those after the first dose. Whereas other studies have shown waning of antibodies after single doses of mRNA vaccines, CanPath found that levels of antibodies after two doses of an mRNA vaccine appear to stabilize over a period of 95 days.
The initial results do not yet include individuals who had received two doses of the Oxford-AstraZeneca vaccine. “Clinical trials have shown that two doses of AstraZeneca provides good protection against COVID-19” adds Dr. Hankins.
“These preliminary study results reinforce the need for Canadians to get their second dose of COVID-19 vaccines. Completing a two-dose vaccine series provides stronger protection against symptomatic infection, severe illness and variants of concern including the Delta variant,” states Canada’s Chief Public Health Officer, Dr. Theresa Tam. “Vaccines, in combination with public health measures and individual precautions continue to be key in reducing the spread of COVID-19.”
“These initial results clearly highlight the importance of continuing to follow current public health guidelines. It is vital that Canadians continue to practice public health measures and ensure uptake of second doses because a single dose does not provide complete protection against the SARS-CoV-2 virus,” says Dr. Victoria Kirsh, Scientific Associate for the Ontario Health Study, one of CanPath’s regional cohorts.
Continued Sample Collection
Between February 8 and June 7, 2021, CanPath participants completed more than 23,000 online questionnaires and submitted close to 13,000 dried blood spot samples. So far, 5,678 of those samples have been tested and have been used to establish the above preliminary results. CanPath will continue to collect dried blood spot samples to reach the target of 20,000 samples. Further findings from the complete study are expected this summer.
About the Canadian Partnership for Tomorrow’s Health (CanPath)
The Canadian Partnership for Tomorrow’s Health (CanPath) is Canada’s largest population health study and a national platform for health research. Comprised of more than 330,000 volunteer participants, CanPath is a unique platform that allows scientists to explore how genetics, environment, lifestyle, and behaviour interact and contribute to the development of chronic disease and cancer. CanPath is hosted by the University of Toronto’s Dalla Lana School of Public Health with national funding from the Canadian Partnership Against Cancer. To learn more, visit www.canpath.ca.
About the COVID-19 Immunity Task Force
The Government of Canada established the COVID-19 Immunity Task Force in late April 2020. The Task Force is overseen by a Leadership Group of volunteers that includes leading Canadian scientists and experts from universities and healthcare facilities across Canada who are focused on understanding the nature of immunity arising from the novel coronavirus that causes COVID-19. To that end, the CITF is supporting numerous studies 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, develop improved antibody testing methods, and help monitor the effectiveness and safety of vaccines as they are rolled out across Canada. The Task Force and its Secretariat work closely with a range of partners, including governments, public health agencies, institutions, health organizations, research teams, other task forces, and engages communities and stakeholders. Most recently, the Task Force has been asked to support vaccine surveillance, effectiveness and safety as part of its overall objective to generate data and ideas that inform interventions aimed at slowing—and ultimately stopping—the spread of SARS-CoV-2 in Canada. For more information visit: www.covid19immunitytaskforce.ca
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