This is a summary, written by members of the CITF Secretariat, of the five presentations given at the CITF Scientific Meeting in Vancouver, March 8-10, 2023, during the breakout session entitled, Pan-Canadian serosurveillance.

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

CITF-funded studies monitoring both infection-acquired and vaccine-induced seroprevalence over time have been – and will continue to be – key to understanding the spread of the virus. They are also crucial to identify populations potentially more susceptible to infection, severe disease, and death. Here, we summarize results from the five presentations given during the “Pan-Canadian serosurveillance” breakout session at the CITF Scientific Meeting in Vancouver, March 8-10, 2023. The study teams presented seroprevalence data from different age groups and regions, distinct pandemic phases, and in relation to the number of vaccine doses received, offering a broad view of seroprevalence across the country.

1. Presenter and CITF PI: Dr. Tanya Murphy, The evolution of population immunity to SARS-CoV-2 – a time-series study of seroprevalence in Canada, 2020-2022

The CITF collaborated with seven large population projects across Canada to aggregate seroprevalence data. Seropositivity estimates for anti-N (infection-acquired) and anti-S (vaccine-induced and infection-acquired) antibodies were estimated using a Bayesian multi-level model assuming beta bi-nominal distributions.

    • By November 2021, 9% of Canadians had infection-acquired immunity to SARS-CoV-2. Seroprevalence increased rapidly after the arrival of Omicron: by March 15, 2023, 76% of Canadians had detectable antibodies from infection.
    • The rate of increase in the Omicron era varied from a peak of 11% per month in late-January 2022, to lows of approximately 3% to 4% per month in March and June 2022.
    • The rapid rise in infection-acquired antibodies occurred across Canada and was most pronounced in younger age groups (17-24 years) and in the Western provinces of Manitoba, Saskatchewan, Alberta, and British Columbia.

2. Presenter and CITF PI: Dr. Sheila O’Brien, To monitor infection and vaccination mediated SARS-CoV-2 antibodies in a healthy population over the course of the pandemic

Canadian Blood Services reported seroprevalence from 631,011 unique adults who donated blood between May 2020 and December 2022, across Canada (excluding Quebec and the Territories).

    • Nearly all donors were vaccinated in 2021. Anti-S seropositivity reflected high vaccine uptake.
    • SARS-CoV-2 seroprevalence due to infection was low until 2022. Despite the high vaccination rates, infection-acquired seroprevalence increased rapidly when the Omicron variant arrived. Younger age groups (17-24) had the highest infection-acquired seroprevalence, followed by those 25-39, 40-59, and then 60+ between May 2020 and December 2022.
    • Infection-acquired seroprevalence was higher in the Western provinces.
    • Racialization and material deprivation were important predictors of higher infection rates.
    • Ongoing monitoring of seroprevalence is important for public health policies.

Individuals who choose to donate blood are generally in good health and are more likely to live in populous urban areas. Percentages were adjusted for test characteristics and population distribution.

3. Presenter Dr. Patrick Brown and CITF PI Dr. Prabhat Jha: Seroprevalence of SARS-CoV-2 in a representative Canadian adult cohort during the Omicron waves

Approximately 30,690 individuals were tested for infection-acquired and vaccine-induced seroprevalence during six phases of the CITF-funded Ab-C Study, from March 2020 to December 2022 from across Canada.

    • Only about 13% of adults had infection-acquired antibodies before Omicron.
    • About 36% of participants had antibodies from Omicron BA.1/1.1 variant infection.
    • The Omicron BA.2/5 variant caused a sharp increase in infections, with a cumulative seroprevalence of almost 78%.
    • About 25 million Canadian adults were found to have been infected, despite more than 30 million having received two or more doses of a vaccine. Hybrid immunity levels are therefore high.
    • COVID death rates per week were lower during Omicron BA.2/5 than BA.1/1.1.
    • Vaccination >6 months ago, especially in older adults, remained the key vulnerability for infections.
    • Ongoing epidemiological studies (serosurveys, mortality studies) are essential to guide vaccine programs and document long-COVID.

4. Presenter and CITF PI: Dr. Maureen Anderson, Saskatchewan SARS-CoV-2 population-based seroprevalence study: Highlights and lessons learned

Residual serum samples from participants (n = 21,907) in Saskatchewan were tested for antibodies to SARS-CoV-2 during three time periods (May-Dec 2020; Oct-Dec 2021; Jan-Jun 2022). All samples were tested for IgG antibodies to SARS-CoV-2 spike and nucleocapsid proteins. Additionally, microneutralization testing was conducted on 580 samples against ancestral SARS-CoV-2, Delta and Omicron strains.

    • Higher Anti-S and Anti-N antibodies titers lead to better neutralization of ancestral SARS-CoV2-2, Delta and Omicron strains .
    • Differences were observed in the Anti-S antibody titers and neutralization between different waves of infections. This could be due to a combination of vaccination and infections.
    • Samples positive for both Anti-S antibodies and Anti-N antibodies had the strongest antibody immune responses and neutralization titres compared to samples positive to spike alone, suggesting superiority of hybrid immunity in neutralizing the virus.

5. Presenter and CITF PI: Dr. Derek Stein, Riding high: SARS-CoV-2 seroprevalence, vaccination, and breakthrough infections in Manitoba, Canada

The proportion of Manitobans exposed to COVID-19 was estimated based on sex, age, and geography throughout the pandemic. Vaccination rollout and waning immunity were also monitored to estimate the proportions of breakthrough infections during the Delta and Omicron waves.

    • Infection-acquired seroprevalence was low until 2021, followed by a sudden spike in early 2022, which coincided with the Omicron wave. Children ages 0-19 had the highest seroprevalence of all age groups in 2022 compared to all age groups. Individuals aged 60+ years had the lowest seroprevalence during this time.
    • Breakthrough infections and hospitalizations were associated with waning antibodies during the Delta wave. Individuals experiencing breakthrough infections had a median antibody level ≤645BAU/ml and those hospitalized had a median antibody level ≤150.