Assessment of population immunity to measles in Ontario, Canada: a Canadian Immunization Research Network (CIRN) study

Hum Vaccin Immunother. 2019;15(12):2856-2864. doi: 10.1080/21645515.2019.1619402. Epub 2019 Jul 16.

Abstract

Canada eliminated measles in 1998. We conducted a sero-epidemiology study to estimate population immunity to measles in the province of Ontario, Canada and to identify groups at higher risk of outbreaks. We used a previously developed modified enzyme immunoassay to test 1,199 residual sera from patients aged 1-39 years. We re-tested negative and equivocal sera using a plaque reduction neutralization assay. We interpreted our results in the context of Ontario's immunization program and vaccine coverage data. Of 1,199 sera, 1035 (86.3%, 95% confidence interval (CI) 84.4, 88.2) were above the measles threshold for protection, 70 (5.8%, 95% CI 4.5, 7.2) were equivocal and 94 (7.8%, 95% CI 6.3, 9.4) were negative. The proportion of positive sera was highest for those 1-5 years, with 180/199 (90.5%, 95% CI 86.4, 94.5) positive sera, and lowest for those age 12-19 years, at 158/199 (79.4%, 95% CI 73.8, 85.0). Adjusted for age, females were more likely than males to have antibody titers above the threshold of protection (odds ratio = 1.60, 95% CI 1.14, 2.24). Most of the study cohort were eligible for two measles vaccine doses, and vaccine uptake in Ontario is >90% for school-aged cohorts. We observed a higher than expected proportion of sera with antibody levels below the threshold of protection, suggesting that immunity in some Ontario age-groups may be waning, despite high vaccine coverage. Alternatively, the traditional measles correlates of protection may not be an appropriate measure of population protection in measles-eliminated settings.

Keywords: Canada; Measles; Ontario; herd immunity; measles immunity; sero-epidemiology; threshold of protection.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Antibodies, Viral / blood*
  • Child
  • Child, Preschool
  • Cohort Studies
  • Epidemiological Monitoring*
  • Female
  • Humans
  • Immunization / statistics & numerical data*
  • Immunoenzyme Techniques
  • Infant
  • Male
  • Measles / epidemiology*
  • Measles / immunology
  • Measles Vaccine / administration & dosage
  • Risk Factors
  • Seroepidemiologic Studies
  • Vaccination Coverage / statistics & numerical data
  • Young Adult

Substances

  • Antibodies, Viral
  • Measles Vaccine

Grant support

This work was supported by the Canadian Immunization Research Network and Public Health Ontario.