Measles population immunity in Hunan, China: a serological assessment

Background: Assessing the measles immunity profile is critical for developing effective nationwide or regionwide supplementary immunization activities (SIAs). This study aims to assess measles population immunity levels in China and investigate factors contributing to age-specific heterogeneities. Methods: We conducted a cross-sectional population-based serological study in southern China between June and October 2021. We determined the population mean antibody concentration and seroprevalence by age and over time, along with their associated determinants. Moreover, we estimated the contact-adjusted immunity levels by considering both age-specific immunity levels and population contact rates. Results: Among the 1015 study participants (aged 0-95 years), the overall seroprevalence was estimated at 80.9% (95% confidence interval [CI], 78.3-83.3). When adjusting for the age-specific contact rates, the contact-adjusted immunity was estimated at 66.8% (95% CI, 56.6-75.1). Individuals younger than age 30 years showed significantly lower antibody concentration and seroprevalence (mean log concentration: 5.9, seroprevalence: 73.6% [95% CI, 69.9-77.3]) compared to those older than age 50 years (mean log concentration: 6.8, seroprevalence: 96.8% [95% CI, 94.7-98.9]). In particular, individuals born after the last SIA (2010) showed a significantly faster waning of immunity. Conclusions: Our findings highlight that immunity levels in the general population remain below the 95% threshold, underscoring the need for continued monitoring of immunity dynamics, especially for individuals born in a near-elimination setting and without subsequent SIAs.