The impact of local vaccine coverage and recent incidence on measles transmission in France between 2009 and 2018

Abstract

AbstractBackgroundDespite high levels of vaccine coverage, sub-national heterogeneity in immunity to measles can create pockets of susceptibility, which are hard to detect and may result in long-lasting outbreaks. The elimination status defined by the World Health Organization aims to identify countries where the virus is no longer circulating and can be verified after 36 months of interrupted transmission. However, since 2018, numerous countries have lost their elimination status soon after reaching it, showing that the indicators used to define elimination may not be predictive of lower risks of outbreaks.Methods and FindingsWe quantified the impact of local vaccine coverage and recent levels of incidence on the dynamics of measles in each French department between 2009 and 2018, using mathematical models based on the ‘Epidemic-Endemic’ regression framework. High values of local vaccine coverage were associated with fewer imported cases and lower risks of local transmissions. Regions that had recently reported high levels of incidence were also at a lower risk of local transmission, potentially due to additional immunity accumulated during these recent outbreaks. Therefore, all else being equal, the risk of local transmission was not lower in areas fulfilling the elimination criteria (i.e., low recent incidence). After fitting the models using daily case counts, we used the parameters’ estimates to simulate the effect of variations in the vaccine coverage and recent incidence on future transmission. A decrease of 3% in the three-year average vaccine uptake led to a five-fold increase in the number of cases simulated in a year on average.ConclusionsSpatiotemporal variation in vaccine coverage because of disruption of routine immunisation programmes, or lower trust in vaccines, can lead to large increases in both local and cross regional transmission. The association found between local vaccine coverage and incidence suggests that, although regional vaccine uptake can be hard to collect and unreliable because of population movements, it can provide insights into the risks of imminent outbreak. Periods of low local measles incidence were not indicative of a decrease in the risks of local transmission. Therefore, the incidence indicator used to define the elimination status was not consistently associated with lower risks of measles outbreak in France. More detailed models of local immunity levels or subnational seroprevalence studies may yield better estimates of local risk of measles outbreaks.