METHODS: We used the synthetic control method (SCM) under assumption of partial interference to evaluate the direct and spillover PEs of Wolbachia-mediated introgression in a long-running operational trial of the intervention in Malaysia. Synthetic controls (SCs), which comprise of a weighted sum of non-spillover controls, were constructed for each directly-treated and spillover site in the pre-intervention period to account for historical imbalances in dengue risk and risk trajectories. SCs were compared to directly/spillover-treated sites to estimate the impact of Wolbachia-introgression on dengue incidence across each site, calendar year and intervention time. Robustness checks, including visual inspections, root-mean-square error (RMSE) calculations, in-space and in-time placebo checks, and permutation tests, were used to inspect the model's ability in attributing dengue incidence reductions to the Wolbachia interventions.

FINDINGS: The direct and spillover PEs of Wolbachia on dengue incidence were expressed as a percentage reduction of dengue incidence, or the absolute case reductions, by comparing SCs to actual intervention/spillover sites. Findings indicate a direct reduction in dengue incidence by 64.35% (95% CI: 63.50-66.71, p mosquitoes as a sustainable, cost-effective strategy against dengue.

METHODS: The prevalence of Wolbachia in Culicinae mosquitoes was assessed via PCR with wsp primers. For some of the mosquitoes, in which the wsp primers failed to amplify a product, Wolbachia screening was performed using nested PCR targeting the 16S rRNA gene. Wolbachia sequences were aligned using Geneious 9.1.6 software, analyzed with BLAST, and the most similar sequences were downloaded. Phylogenetic analyses were carried out with MEGA 7.0 software. Graphs were drawn with GraphPad Prism 8.0 software.

RESULTS: A total of 217 adult mosquitoes representing 26 mosquito species were screened. Of these, infections with Wolbachia were detected in 4 and 15 mosquito species using wsp and 16S rRNA primers, respectively. To our knowledge, this is the first time Wolbachia was detected using 16S rRNA gene amplification, in some Anopheles species (some infected with Plasmodium), Culex sinensis, Culex vishnui, Culex pseudovishnui, Mansonia bonneae and Mansonia annulifera. Phylogenetic analysis based on wsp revealed Wolbachia from most of the mosquitoes belonged to Wolbachia Supergroup B. Based on 16S rRNA phylogenetic analysis, the Wolbachia strain from Anopheles mosquitoes were more closely related to Wolbachia infecting Anopheles from Africa than from Myanmar.