Measles was eradicated in Canada in 1998. Researchers conducted a sero-epidemiology research in Ontario, Canada, to assess population immunity to measles and identify populations at increased risk of outbreaks. They tested 1,199 residual sera from individuals aged 1–39 years using a previously developed modified enzyme immunoassay. Using a plaque reduction neutralization technique, they retested negative and equivocal samples. Findings were interpreted in the context of Ontario’s vaccination program and vaccine coverage statistics. Of the 1,199 sera tested, 1035 were positive for measles, 70 were ambiguous, and 94 were negative. The proportion of positive sera was highest in the 1–5 year age group, with 180/199 positive sera, and lowest in the 12–19 year age group, with 158/199 positive sera. Females were more likely than males to have antibody titers over the protective threshold when age was taken into account. The majority of the research cohort were eligible for two measles vaccination doses, and vaccine uptake in Ontario for school-aged cohorts is greater than 90%.

They found a higher-than-expected proportion of sera with antibody levels below the protective threshold, suggesting that immunity may be decreasing in some Ontario age groups despite high vaccination coverage.