Target immunity levels for achieving and maintaining measles elimination

Vaccination has reduced the global incidence of measles to the lowest rates in history. Local interruption of measles transmission, however, requires sustained high levels of population immunity that can be challenging to achieve and maintain. The herd immunity threshold for measles is typically stipulated at 90-95%. This figure, however, does not easily translate into required immunity levels across all age groups that would be sufficient to interrupt transmission. Previous estimates of such levels were based on speculative contact patterns based on historical data from high-income countries. The aim of this study was to determine age-specific immunity levels that would ensure elimination of measles using observed contact patterns from a broad range of settings. We combined recent observations on age-specific mixing patterns with scenarios for the distribution of immunity to estimate transmission potential. We validated these models by deriving predictions based on serological studies and comparing them to observed case data. We found that 95% immunity needs to be achieved at the time of school entry to guarantee elimination. The level of immunity found in the 5-to-9 year old age group in serological studies was the strongest predictor of future case load. Higher levels of immunity in 5-to-9 year olds are required than the previously derived target of 90% to interrupt transmission. While such high levels can be difficult to achieve, school entry provides a clear opportunity to ensure sufficient levels of immunity.