Estimating the impact of the wMel release program on dengue and chikungunya incidence in Rio de Janeiro, Brazil

Background Introgression of the insect bacterium Wolbachia into Aedes aegypti mosquito populations been shown in randomised and non-randomised trials to reduce the incidence of dengue in treated communities, however evidence for the real-world effectiveness of large-scale Wolbachia mosquito deployments for arboviral disease control in endemic settings is still limited and no effectiveness studies have been conducted for chikungunya virus. A large Wolbachia ( w Mel strain) program was implemented in 2017 in Rio de Janeiro, Brazil. Here we assess the impact of the release program on dengue and chikungunya incidence. Methods and findings The program released 67 million w Mel infected mosquitoes across 28,489 release locations over a 86.8km2 area in Rio de Janeiro between August 2017 and the end of 2019. Following releases, mosquitoes were trapped and the presence of w Mel determined. To assess the impact of the release program on dengue and chikungunya incidence, we used spatiotemporally explicit models applied to geocoded dengue (N=194,330) and chikungunya cases (N=58,364) from 2010 (2016 for chikungunya) to 2019 from across the city. On average, 32% of mosquitoes collected from the release zones between 1 and 29 months after releases were positive for w Mel. Reduced w Mel introgression occurred in locations and seasonal periods when dengue and chikungunya cases were historically high. Despite incomplete introgression, we found that the releases were associated with a 38% (95%CI: 32-44%) reduction in dengue incidence and a 10% (95%CI: 4-16%) reduction in chikungunya incidence. Conclusions Stable establishment of w Mel in this diverse, urban setting appears more complicated than has been observed elsewhere. However, even intermediate levels of w Mel appear to reduce the incidence of two different arboviruses.