Measles seroprevalence by birth cohort across the lifespan: a population-based, cross-sectional serosurvey in British Columbia, Canada.

BACKGROUND: In 2025, the province of British Columbia (BC), Canada, experienced heightened measles activity, mostly involving unvaccinated communities. Publicly funded measles vaccination for children aged 12 months has been routine since 1969, with a second dose at age 18 months added in 1996, and rescheduled to 4 to 6 years (school entry) in 2012. We aimed to assess population-based measles seroprevalence in relation to historic measles endemicity and vaccination considerations. METHODS: In August 2024, we undertook a cross-sectional serosurvey, testing more than 1000 anonymized residual sera for measles antibody. We collected sera from outpatients attending a laboratory network in the Lower Mainland, BC, with equal numbers from 10 age groups, from 1 year to older than 80 years. RESULTS: Measles seropositivity was 89% (95% confidence interval [CI] 87% to 91%) overall and 93% (95% CI 91% to 94%) if equivocal results were also considered positive. Seropositivity was 90% or higher in all age groups except 10- to 19-year-olds (82% [95% CI 74% to 89%]), 20- to 29-year-olds (69% [95% CI 59% to 78%]), and 30- to 39-year-olds (73% [95% CI 63% to 81%]). Results remained below 80% for 20- to 39-year-olds, and significantly below all other age groups, even considering equivocal results as positive. In birth cohort analysis, seropositivity appeared lower among those due for their second vaccine dose during the COVID-19 pandemic or born during the postvaccination era to mothers with a higher likelihood of previous infection when measles was endemic. INTERPRETATION: Our measles serosurvey findings inform vaccine coverage and complement other case-based surveillance indicating robust population-level immunity outside of unvaccinated clusters or communities. In addition to showing age-related antibody decline, serosurveillance provides insights into potential cohort effects that may have implications for vaccination programs.