Unveiling immunity gaps and determining a suitable age for a third dose of the measles-containing vaccine: a strategic approach to accelerating measles elimination

Background In highly measles immunized countries, immunity gaps in adolescents and young adults are a key issue posing an obstacle to measles elimination. This study aims to identify the gaps by estimating the age-stratified probability of seropositivity, and to ascertain a suitable age for the administration of a third dose of a measles- containing vaccine (MCV3) to effectively fi ll these gaps. Methods We retrospectively obtained measles serological results from hospital setting among among individuals aged 13-39 years and developed a serocatalytic dynamic probability model, stratifying seropositivity due to vaccination or natural infection. We calibrated the model to age-stratified seropositivity data within a Bayesian setting using the Metropolis-Hastings algorithm. A scenario analysis to determine a suitable age for MCV3 administration was also performed. Findings The overall prevalence of measles seropositivity was 65.6% (95% confidence interval [CI]: 61.5-69.6). Posterior predictive curves for the age-stratified seroprevalence exhibited a decreasing trend from ages 13-20 years but an upward trend from 26 to 30 years. The age at which a given individual's serostatus reached a 50% probability of seronegativity was found to be approximately 18-20 years depending on the annual measles force of infection. Interpretation Our fi ndings highlight a significant measles immunity gap in young adults aged 20-26 years, posing an increased risk of transmission. A MCV3 at the age of 18-20 years potentially closes the gap and aids measles elimination programmes.