by Caroline Mburu, John Ojal, Rose Selim, Rose Ombati, Donald Akech, Boniface Karia, James Tuju, Antipa Sigilai, Gaby Smits, Pieter van Gageldonk, Fiona van der Klis, Eunice Kagucia, Anthony Scott, Ifedayo Adetifa, Stefan FlascheBackground Measles outbreaks continue to cause a large burden of disease in Africa including Kenya. We used information from regular serological surveys in Kilifi Health and Demographic Surveillance System (KHDSS) in combination with mathematical modelling to estimate the relative contribution of the vaccination programme to current measles immunity. Methods We developed a static birth cohort model to track the proportion of children who are either measles naïve or seroconverted due to natural infection or vaccination through first dose of measles-containing vaccine (MCV1), the second dose (MCV2), or supplementary immunisation activities (SIAs). We fitted the model to biennial paediatric serological survey and case notification data and used vaccination coverage estimates from the KHDSS to estimate the relative contributions of vaccination and infection to measles immunity in Kilifi. Results We estimated that between 2009 and 2021, 60% (95%CI 55–64%) of measles seroconversion in Kilifi was attributable to MCV1, with MCV2 contributing 1.0% (95%CI 0.9-1.1%) since its introduction. Natural infection and SIAs accounted for 24% (95%CI 17–31%) and 16% (95%CI 14–19%), respectively. A hypothetical 10% increase in MCV1 coverage increased the seroconversion attributed to MCV1–67% (95%CI 63–71%), with concurrent reductions in seroconversion from natural infection and SIAs to 13% (95%CI 9–18%) and 10% (95%CI 9–12%), respectively. Importantly, this same 10% increase in MCV1, if administered promptly at 9 months, could potentially reduce seroconversion from natural infection further from 24% to 11% (95%CI 07–15%) and reliance on SIAs from 16% to 8% (95% CI 7–10%). Conclusion Optimizing routine coverage timing and uptake is crucial for reducing SIAs dependence and measles susceptibility. A 10% MCV1 coverage increase could have halved susceptibility and lessened SIA demand, highlighting the potential of minor improvements in coverage to alleviate measles and reduce costly SIAs.