Impact of mass vaccination campaigns on measles transmission during an outbreak in Guinea, 2017.

Objective: To estimate the time-dependent measles effective reproduction number (R _t ) as an indicator of the impact of three outbreak response vaccination (ORV) campaigns on measles transmission during a nationwide outbreak in Guinea.
Methods: R _t represents the average number of secondary cases generated by a single primary case in a partially immune population during a given time period. Measles R _t was estimated using daily incidence data for 3952 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: R _t 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 (R _t  = 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 R _t 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 R _t 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.
Competing Interests: Declaration of Competing Interest The authors declare that there are no conflicts of interest.
(Copyright © 2020 The British Infection Association. Published by Elsevier Ltd. All rights reserved.)