COVID-19 pandemic and waning immunity disrupted measles population immunity and strategies to close immunity gaps.

Widespread measles cases following the COVID-19 pandemic have threatened human health and progress towards measles elimination. While we know that the pandemic has disrupted routine measles vaccination worldwide, most settings still lack quantification of post-pandemic population immunity gaps and the feasibility of closing these gaps through supplementary immunisation activities. Using serological data (15,405 antibody measurements from 3,674 individuals) in southern China between 2013-2024, we analyse pre- and post-pandemic susceptibility profile changes in the pediatric population, and explore underlying differences and mechanisms of measles-specific immunity. We show that childbearing-age women born at a time of low measles infection risk (born between 1990-2005) had 0.39 (95% CI 0.38-0.42) log mIU/ml lower mean log-concentration in 2021-2024 compared to those born during high-risk periods (born between 1984-1989). This in turn resulted in a 0.29 (95% CI 0.27-0.52) log mIU/ml lower mean log-concentration at birth and faster antibody waning in their infants. We also observe that 9.5-10.7% pediatric susceptibility was attributed to waning vaccine-induced immunity during 2013-2024. We project a 23.5-50.0% immunity increase (from 66.6% to 92.7%) in children under 15 years following non-selective supplementary immunisation activities, supporting the key role of supplementary immunisation activities in restoring robust population immunity.