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 fill 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 findings 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. Funding: This work was supported by Faculty of Tropical Medicine (MCTM, ICTM grant), Mahidol University (to T.N.) and APC fee was supported by Mahidol University (to T.N.). S.M. and W.P. were funded in whole, or in part, by the Wellcome Trust [Grant number 220211]. For the purpose of open access, the authors have applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.