Doxycycline pre-exposure prophylaxis prevents sexually transmitted infections without affecting vaginal bacterial flora in female sex workers.

Background: Bacterial sexually transmitted infections (STIs) like syphilis, chlamydia and gonorrhoea are often asymptomatic but can cause severe complications, including infertility and vertical transmission in cisgender women, particularly female sex workers (FSWs). Sex work is strongly associated with syphilis, with FSWs representing 38% of syphilis cases among Japanese women in 2021. Despite doxycycline's proven effectiveness in preventing bacterial STIs, its efficacy in high-risk cisgender women remains inconclusive, highlighting the need for targeted STI prevention strategies in this population.

Objectives: We investigated the effectiveness of doxycycline pre-exposure prophylaxis (doxyPrEP) in preventing STIs and its impact on vaginal flora among FSWs.

Participants and methods: This retrospective study included 40 FSWs aged ≥18 years who initiated doxyPrEP (100 mg/day) for STI prevention at a private clinic in Tokyo, Japan, between 1 October 2022 and 14 November 2023. Incidence rate ratios (IRR) for chlamydia, gonorrhoea, syphilis, bacterial vaginosis (BV), and vulvovaginal candidiasis (VVC) were estimated using fixed-effects Poisson regression models. Adherence, side effects, and satisfaction were evaluated through follow-up clinical evaluations.

Results: Overall STI incidence significantly declined from 232.3 to 79.2/100 person-years following doxyPrEP initiation (IRR = 0.33, P = 0.020). The reduction in chlamydia showed marginal statistical significance (IRR = 0.35, P = 0.056), and syphilis cases dropped to zero. Gonorrhoea, BV, and VVC incidence showed no significant changes. Follow-up clinical evaluations indicated high adherence to doxyPrEP, no serious adverse events, and high satisfaction with doxyPrEP.

Conclusions: DoxyPrEP significantly reduced the overall STI incidence among FSWs without increasing other vaginal infections.