Impact of mass vaccination campaigns on measles transmission during a nationwide outbreak in Guinea, 2017

Objective: To estimate the time-dependent measles effective reproduction number (Rt)as an indicator of the impact of three outbreak response vaccination (ORV) campaigns on measles transmission during a nationwide outbreak in Guinea. Methods: Rt represents the average number of secondary cases generated by a single primary case in a partially immune population during a given time period. Measles Rt was estimated using daily incidence data for 3,952 outbreak-associated measles cases in Guinea in 2017 for the time periods prior to, between, and following each of three ORV campaigns using a simple and extensible mathematical model. Results: Rt was estimated to be above the threshold value of 1 during the initial growth period of the outbreak until the first ORV campaign began on March 13 (Rt =1.60, 95% CI: 1.55–1.67). It subsequently dropped below 1 and remained < 1 through the end of the year (range: 0.71–0.91), although low levels of transmission persisted. Conclusions: Reduction in Rt coincided with implementation of the ORV campaigns, indicating success of the campaigns at maintaining measles transmission intensity below epidemic growth levels. However, persistent measles transmission remains an issue in Guinea due to insufficient levels of herd immunity. Estimation of Rt should be further leveraged to help decision makers and field staff understand outbreak progress and the timing and type of vaccination efforts needed to halt transmission