Optimal testing frequency for sexually transmitted infections among men who have sex with men and transgender women who use HIV pre-exposure prophylaxis in Australia, Brazil and Thailand: a cost-effectiveness analysis

Background: (NG) and syphilis. However, the optimal and most cost-effective frequency for separate and combined STI testing in PrEP users has yet to be established. Methods: We constructed dynamic transmission models and simulated the epidemic trajectories of three STIs (CT, NG and syphilis) among MSM/TGW using PrEP in Australia, Brazil and Thailand. Two testing scenarios were considered: single pathogen testing for CT, NG or syphilis, and combined testing for all three STIs simultaneously (syphilis testing alone plus dual testing for CT/NG). From a healthcare system perspective, cost-effectiveness analysis and incremental cost-effectiveness ratio (ICER) was used to explore the optimal testing strategy for STIs across three countries over a 5-year time horizon. Findings: For the single CT/NG testing scenario, either maintaining status quo, or every 6 or 12 months was more cost-effective for Australian MSM, Brazilian MSM and Thai MSM/TGW, while 3-monthly CT/NG testing was optimal for Brazilian TGW. For the single syphilis testing scenario, 3-monthly testing was more cost-effective for Australian, Brazilian MSM/TGW and Thai TGW, while 6-monthly testing was optimal for Thai MSM. For the combined testing strategy, 3-monthly dual testing for CT/NG, along with 3-monthly syphilis testing for Australian MSM and Brazilian TGW, was a cost-saving and optimal strategy, but exceeded the willingness-to-pay threshold for Brazilian MSM (ICER = $23,187/QALY gained), Thai MSM (ICER = $477,442/QALY gained) and Thai TGW (ICER = $37,697/QALY gained). Interpretation: Our study suggests that more frequent (3-monthly) syphilis testing for PrEP users provides enhanced economic value among both MSM and TGW. However, less frequent CT and NG testing may only be required, depending on the background disease burden and the costs associated with testing and management in each country. Funding: Supported by the World Health Organization (Grant Number: INV-035239).