Disentangling Between- and Within-Person Effects of HIV PrEP and Treatment Attitudes on Condom Use and Bacterial STIs...

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.