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

Estimating the durability of immunity from vaccination is complicated by unreported re-vaccination, 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=2,530) for 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 one-dose vaccination schedule. We found that measles seroprevalence has generally decreased over this time period. Furthermore, we conducted two 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 that vulnerabilities might emerge during periods of limited measles circulation for countries with a one-dose schedule due to both the build-up of susceptible individuals and waning titers.