Assessing measles immunity in individuals of childbearing age in Toronto, Ontario, Canada

OBJECTIVES: Measles is a highly infectious virus with potentially serious complications. Infants too young to be vaccinated depend on maternally-derived, transplacentally-transferred antibodies for early protection. We aimed to identify factors influencing maternal measles immunity, and determine predictors of maternal susceptibility and infant immunity during the first year of life. METHODS: Using a prospective cross-sectional study design, we collected data and samples from infant-mother pairs admitted to the Hospital for Sick Children between 2018 and 2020. Measles antibody titres were measured using the gold-standard plaque reduction neutralisation test. RESULTS: We recruited 258 mothers aged 16-45 years, of whom 80.6% (95% CI: 75.8,85.4) had protective antibody levels. The proportion with protective antibodies was lowest among mothers 40 years. Among the 233 individuals reporting vaccination status, 97.0% received at least one measles-containing vaccine. CONCLUSIONS: Nearly one-fifth of mothers in our study were seronegative to measles, increasing to nearly one-third of mothers under 30 years. The higher susceptibility observed in younger individuals within a highly-vaccinated cohort may suggest waning humoral immunity. Further research is needed to understand the significance of the high proportion of seronegative individuals in younger age groups.