Most Canadians now have hybrid immunity against SARS-CoV-2 through natural infection, primarily from the Omicron variant, and vaccination, but seropositivity varies by age and location, research indicates.
By March 2023, 76% of the Canadian population had detectable infection-induced antibodies, according to an analysis of more than 900,000 blood samples collected from March 2020 to March 2023.
Dr Bruce Mazer
“The results of this study are important for understanding what happened during the pandemic and informing efforts to plan for future pandemics,” study authors Bruce D. Mazer, MD, associate scientific director, and David L. Buckeridge, MD, PhD, scientific lead, both of the COVID-19 Immunity Task Force (CITF) and McGill University in Montreal, Quebec, told Medscape Medical News.
Dr David Buckeridge
“Serological data have allowed us to learn about the burden of illness in the young,” they said, “and the ability of healthy older people to protect themselves from infection — with the lowest infection rates in our study.”
The analysis also provided evidence that “people from racialized groups and living in poorer neighborhoods suffered higher rates of infection-acquired immunity throughout the pandemic, suggesting that tailored public health programs should be considered,” they said.
The study was published online August 14 in the Canadian Medical Association Journal.
Spread Among Youth
The investigators used a time-series approach to estimate trends in SARS-CoV-2 seroprevalence from infection and vaccination over the following three intervals: prevaccination (March to November 2020), vaccine rollout (December 2020 to November 2021), and the arrival of the Omicron variant (December 2021 to March 2023).
Data came from more than 900,000 samples collected in seven research studies
that were conducted in collaboration with the CITF.
By November 2021, 9% of Canadians had humoral immunity to SARS-CoV-2 from an infection. Seroprevalence increased rapidly after the emergence of the Omicron variant, however. It increased to 47% by mid-June 2022, with an average monthly increase of 6.4% between December 15, 2021, and July 2022.
Slower increases occurred during the summer. Then, infection-acquired seroprevalence rose rapidly in the latter half of 2022, plateauing in March 2023, when about 76% of the population had antibodies from infections.
During the Omicron waves, rates of seropositivity increased faster among younger age groups. By mid-June 2022, infection-acquired seroprevalence was highest in people younger than 25 years (57%), followed by those aged 25 to 39 years (51%), 40 to 59 years (40%), and 60 years and older (25%).
Trends were similar in provincial serosurveys, in which the highest seroprevalence rates were among children and adolescents. Infection-induced antibodies were most prominent in the Western provinces of Manitoba, Saskatchewan, Alberta, and British Columbia.
The Omicron wave caused similar increases in infection-induced humoral immunity in multiple countries, the study shows. “However,” the authors wrote, “given variations in population seropositivity by age and geography, the potential for waning antibody levels, and new variants that may escape immunity, public health policy and clinical decisions should be tailored to local patterns of population immunity.”
Stay Vigilant
Mazer and Buckeridge said that although the CITF’s mandate will end in March 2024, the team has developed the CITF Databank, which centralizes and harmonizes population-level seroprevalence estimates and individual-level data from CITF-funded studies. The databank currently holds data from 18 studies and is expected to include data from up to 70 studies in the future, which will allow Canada to be better prepared to respond to new emerging pathogens.
For now, they suggest that “clinicians need to be vigilant about those who proved to be vulnerable [during the pandemic], such as older people, and those with medical conditions that place them at risk for complications.
“Clinicians also need to understand the broader context of infection and immunity so that they can assess the risk faced by their patients.
“The high level of vaccination-induced immunity in Canada likely contributed to our lower mortality during the Omicron wave,” they said. “Omicron taught us that the virus keeps changing, like influenza, meaning we have to stay vigilant and ensure the vulnerable are vaccinated according to current guidelines.”
Similar in US?
Anna Bershteyn, PhD, an assistant professor of population health at NYU Grossman School of Medicine in New York City and formerly an advisor to the New York City Department of Health and Mental Hygiene on COVID-19 forecasting, commented on the findings for Medscape.
Dr Anna Bershteyn
“Like Canada, the US saw large increases in the percentages of people with SARS-CoV-2 infection-induced antibodies during the 2021–2022 winter Omicron wave,” she said. But in the US, the proportion of people with infection-induced antibodies was much higher even before Omicron arrived (33.5% compared with Canada’s 9%), possibly because of factors such as the larger and denser cities in the US. Bershteyn did not participate in the research.
“It’s important that this study not be misconstrued as vaccines somehow failing to protect the public from the Omicron wave,” she said. “From the get-go, vaccines were intended to reduce severe disease outcomes such as hospitalization and death…. It’s clear that vaccines saved lives and contributed to the hybrid immunity most Canadians and Americans have today.”
The projects were supported by funding from the Public Health Agency of Canada (PHAC) through the COVID-19 Immunity Task Force. Buckeridge has received funding from PHAC in support of the present research. Mazer and Bershteyn have disclosed no relevant financial relationships.
CMAJ. Published online August 14, 2023. Full text
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