Revealing trends in measles immunity during periods of varying measles circulation in Madagascar.

Estimating the durability of immunity from vaccination is complicated by unreported revaccination and unobserved natural infection or reexposure, which could result in overestimation of protection longevity. We tested serial, cross-sectional serum samples from 2005 to 2015 (n = 2530) for immunoglobulin G (IgG) to examine measles seroprevalence, spatiotemporal patterns of titers across regions, and antibody dynamics among children aged 1-9 years who grew up during varying measles circulation in Madagascar under a 1-dose vaccination schedule. We found that measles seroprevalence generally decreased over this period. Furthermore, we conducted 2 nested serological surveys, analyzing 393 samples taken in 2005 (n = 158), a time point preceded by high levels of measles circulation, and 2015 (n = 235), a time point preceded by low levels of measles circulation. Among children alive during periods of limited measles circulation, we found lower measles seroprevalence in all age groups and lower antibody titers in children aged 7-9 years old. Notably, titers among children aged 7-9 dipped near the threshold of protection, highlighting the importance of additional measles vaccine doses. Our findings suggest vulnerabilities might emerge during periods of limited measles circulation for countries with a 1-dose schedule, due to both the buildup of susceptible individuals and waning titers.