Abstract
HIV treatment attitudes and HIV PrEP are associated with sexual behavior that elevates the risk for bacterial STIs (B-STIs) among gay, bisexual and other men who have sex with men (GBM). Using a longitudinal multi-site study (Engage Cohort Study, 2017-2024), we modelled direct and indirect within- and between-person effects of HIV treatment attitudes and PrEP use on condomless anal sex (CAS) and incident B-STIs among 1863 HIV-negative GBM. We modelled up to four years of follow-up visits using multi-level mediation analysis, with CAS as a mediating variable between HIV treatment attitudes and PrEP use on incident B-STIs. We found direct within-person effects of HIV treatment attitudes and PrEP use on incident B-STIs, such that participants were more likely to report incident B-STIs at time points if they reported using PrEP and being more optimistic about HIV treatments; there were no indirect within-person effects through CAS. In contrast, we found both direct and indirect between-person effects. Specifically, participants who reported more PrEP use or who were more optimistic about HIV treatments over the course of the study were more likely to report incident B-STIs. Further, these effects were partially mediated by CAS. Given that we found within-person PrEP use is associated with increased CAS and STIs, and the general increase in adoption of PrEP over time, there is a need to expand biomedical intervention programs for B-STIs prophylaxis by integrating them with social and informational interventions about B-STIs risks, especially for GBM who use HIV PrEP.