Doxycycline Postexposure Prophylaxis and Sexually Transmitted Infection Trends.

Importance: Increasing rates of sexually transmitted infections (STIs) have been associated with rises in serious morbidity. While doxycycline postexposure prophylaxis (doxyPEP), a strategy in which individuals take doxycycline, 200 mg, after condomless sex to prevent bacterial STIs, has been shown to be efficacious in randomized clinical trials, doxyPEP's potential effect on population-level STI incidence is unknown. Objective: To assess the association of citywide doxyPEP guideline release with reported chlamydia, gonorrhea, and early syphilis cases in men who have sex with men (MSM) and in transgender women in San Francisco, California. Design, Setting, and Participants: This population-level interrupted time series analysis of reported San Francisco STI cases measured monthly cases of chlamydia, gonorrhea, and early syphilis prior to (July 2021-October 2022) and after (November 2022-November 2023) release of citywide doxyPEP guidelines in October 2022. All reported chlamydia, gonorrhea, and early syphilis cases among MSM and transgender women in San Francisco during the period of analysis were included. Data were analyzed November 2023 to July 2024. Exposure: Release of doxyPEP citywide guidelines. Main Outcomes and Measures: The primary outcome was the percentage change between projected and observed chlamydia, gonorrhea, and early syphilis cases in the 13-month postexposure period. Results: Citywide, there were 6694 cases of chlamydia, 9603 cases of gonorrhea, and 2121 cases of early syphilis among MSM and transgender women during the analytic period. STI cases among MSM and transgender women decreased significantly compared with model projections for chlamydia (-6.58% per month; 95% CI, -7.99% to -5.16%) and early syphilis (-2.68% per month; 95% CI, -3.75% to -1.60%) after doxyPEP implementation. By the end of the 13-month postperiod in November 2023, chlamydia and early syphilis cases decreased -49.64% (95% CI, -59.05% to -38.06%) and -51.39% (95% CI, -58.21% to -43.46%), respectively, compared with projected cases. There was a significant increase in monthly gonorrhea cases compared with projections (1.77% per month; 95% CI, 0.87% to 2.67%). Conclusions and Relevance: This study suggests that San Francisco's doxyPEP guideline release was associated with decreases in reported cases of chlamydia and early syphilis, but not gonorrhea, among MSM and transgender women in San Francisco. Further analyses are needed to assess whether declines are sustained and monitor for adverse consequences, including antimicrobial resistance. Supporting doxyPEP implementation for MSM and transgender women at risk for STIs could have a significant impact on the nationwide STI epidemic.