Novel COVID-19 vaccines have helped combat the COVID-19 pandemic by curbing transmission and decreasing the risk of severe disease. In randomised clinical trials (RCTs), COVID-19 vaccines were shown to produce high levels of antibodies against SARS-CoV-2. However, RCTs often do not include older or younger individuals or vulnerable people, such as the immune compromised. In addition, pragmatic vaccine roll-out in Canada resulted in extended dosing schedules beyond the time periods used in clinical trials and the mixing of different vaccine types.

In September 2020, we formed the Stop the Spread Ottawa Cohort and recruited just over 1,000 people living in the Ottawa, Canada region. Our goal was to characterize vaccine-elicited antibodies and vaccine safety in a real-world setting. We continue to study the impact of extended vaccine schedules, vaccine type mixing, older age, and immune-compromising conditions on antibody production following multiple COVID-19 vaccine doses.

Participants are primarily women, white, and employed in public facing occupations. The mean age was 45 years at the time of enrolment. Approximately a third of participants had an immune-compromising condition or reported two or more comorbidities. We observed that over 95% of participants had detectable protective antibodies against SARS-CoV-2 three months after the second vaccine dose. Older age and severely immune-compromising conditions (e.g., cancer, active HIV, and transplant recipients) predicted low antibody levels three months after the second dose. Prior SARS-CoV-2 infection was strongly associated with higher antibody levels after vaccination. Increasing the time between dose 1 and 2 predicted higher antibodies at six months. Vaccine symptoms were similar for all doses and included fatigue, injection site pain, headache, fever/chills, and body aches.

Males and females, many different immune-compromising conditions, and a wide range of ages are well represented in the cohort. Non-white individuals were not as well represented in our cohort. Overall, we are finding that serial COVID-19 vaccine doses are safe, well-tolerated and effective at producing and maintaining antibody levels.

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Results of the Stop the Spread Ottawa (SSO) cohort study: a Canadian urban-based prospective evaluation of antibody responses and neutralisation efficiency to SARS-CoV-2 infection and vaccination. Alexa Keeshan, Yannick Galipeau, Aliisa Heiskanen, Erin Collins, Pauline S. McCluskie, Corey Arnold, Raphael Saginur, Ronald Booth, Julian Little, Michaeline McGuinty, C. Arianne Buchan, Angela Crawley, Marc André Langlois, and Curtis Cooper. BMJ Open. 2023.10.31;