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

Abstract

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.

Figure 1:

Measles seroprevalence by age group, Lower Mainland, British Columbia, Canada, August 2024. Results are based on originally sampled age groups (n = 1097). Age at the time of specimen collection and derived birth years are shown in the X-axis. The 95% confidence intervals (CI) are for seropositive estimates only; for 95% CI around findings if equivocal specimens are considered positive, and for precise values, see Table 1.

Figure 2:

Measles seroprevalence by birth cohort in relation to measles vaccination program and endemicity considerations, Lower Mainland, British Columbia, Canada, August 2024. Displayed are findings based on derived birth cohorts (n = 1097). Birth cohorts were defined in relation to measles vaccination program and endemicity considerations,,,, with other potentially relevant perturbations (e.g., COVID-19 pandemic), as annotated. Derived year of birth and actual age at the August 2024 serum collection are displayed on the X-axis. The 95% confidence intervals (CI) are for seropositive estimates only; for 95% CI around findings if equivocal specimens are considered positive, and for precise values, see Table 2.*COVID-19 pandemic mitigation measures spanning March 2020 to March 2022. Includes school closures from March to May 2020, and other disruptions to social interactions, including limited in-person health care visits and with program impacts potentially longer lasting for increased vaccine hesitancy perspectives.,,, †Measles circulation persisted during the initial years after the vaccination program launch, but the longer before publicly funded vaccination programs began in BC, the greater the likelihood of infection-induced immunity and/or boost during the measles endemic period. ‡Among those aged 7–9 years (derived birth years 2015–2017; n = 123), seropositivity was significantly lower than in 4- to 6-year-olds, at 88% (95% CI 81%–93%) (p = 0.03), with 9% equivocal. §Among 10- to 12-year-olds (derived birth years 2012–2014; n = 21) for whom second vaccine dose was scheduled at age 4–6 years, 86% (95% CI 65%–95%) seropositive (none equivocal). Among 13- to 19-year-olds (derived birth years 2005–2011; n = 96) with second dose scheduled at age 18 months, 81% (95% CI 72%–88%) seropositive (9% equivocal). ¶Starting in 1981, measles, mumps, and rubella (MMR) combined product was provided to children aged 12 months or older. **Starting in 1969, first dose was given as measles-only vaccine at 12 months; catch-up program in 1986 provided MMR vaccines to all children from kindergarten (K) to grade 12 who had either received a measles-only vaccine at age 12 months, or who had not yet received any vaccine. ††Individuals born before 1970 are generally assumed to have acquired wildtype measles infection and are considered immune. In BC, however, those who self-identify without a history of measles or mumps vaccine or disease and taking into account other vaccine components (i.e., mumps, rubella), these individuals may be considered susceptible and offered 1 dose of MMR vaccine (2 doses if health care workers)., In BC, in 2013, the assumed age of having acquired wildtype measles infection was revised from 1957 to 1970, except in health care workers, where it remained at 1957. ‡‡Live vaccine licensed in Canada in 1963; publicly funded vaccination program in BC launched in 1969. §§Second dose changed to be given at age 4–6 years beginning in 2012 using MMR, product change to MMR and varicella (MMRV) for the second dose at the beginning of 2014. ¶¶Second dose of measles-containing vaccine at 18 months introduced to routine schedule in 1996, provided as MMR. Concurrent catch-up campaign encompassed all children aged 19 months and older, as well as high school and postsecondary students and staff. Catch-up doses provided as measles and rubella (MR). Coverage across target age groups 85.3% by end July 1996. ***Assumes average maternal age of 30 years.