An age-stratified mathematical model to inform optimal measles vaccination strategies

Measles a highly transmissible viral disease continues to pose a major public health threat globally, despite the availability of an effective vaccine. The World Health Organisation recommends that the first dose of the measles vaccine be administered between 9-12 months of age, based on the risk of maternal antibody interference and the timing of an infants immune system maturity. However, infants younger than 9 months remain vulnerable to measles, particularly in areas with high transmission rates and low herd immunity. To address the interplay between maternal immunity decay, early infection risk, and vaccination effectiveness, this study considers a new age-stratified, multi-compartmental model of measles transmission that explicitly accounts for infections in children too young to be vaccinated. The novelty of the proposed model lies in incorporating the dynamics of maternal immunity loss and infection progression within the maternal immune compartment, addressing a crucial factor influencing disease susceptibility in very young age groups. The positivity of the solutions of the age-stratified model is proved, and an analytical expression is derived for the effective reproduction number. Moreover, for the model without age stratification, the existence of equilibrium points and the local and global stability are studied. Global sensitivity analysis is used to identify critical parameters that affect the incidence of measles. Using a bootstrap algorithm, the model estimates age-specific transmission rates by fitting it to yearly incidence data from six countries where measles is prevalent. Different vaccination strategies are thoroughly examined, classified by the age of administration for routine and supplementary doses, as well as by the frequency of supplementary doses. Furthermore, the impact of an additional zero-dose measles vaccination for children aged 6 to 9 months is extensively analyzed. The estimated number of averted cases and the projected year of elimination are noted in different strategies for the six countries. In addition to routine measles immunization activities, combining conventional supplementary immunization activities with an additional zero-dose measles vaccination for children aged 6 to 9 months emerges as the most effective strategy. The findings aim to inform vaccine policy updates and contribute to enhanced global measles eradication efforts.