A study of female sex workers in Japan found that doxycycline pre-exposure prophylaxis (doxyPrEP) is tied to significantly reduced overall incidence of sexually transmitted infections (STIs) without increasing other vaginal infections, researchers reported today in JAC-Antimicrobial Resistance.

The study, conducted by researchers with the Japan Institute for Health Security, involved 40 cisgender female sex workers who initiated a daily prophylactic 100-milligram dose of doxycycline for STI prevention from October 1, 2022, to November 14, 2023. Data on incidence of chlamydia, gonorrhea, syphilis, bacterial vaginosis (BV), and vulvovaginal candidiasis (VVC) were collected before and after doxyPrEp initiation. The study authors note that the participants were also given the choice of doxyPEP, a strategy that involves taking a higher dose of the antibiotic after unprotected sex, but they chose doxyPreP because of its convenience, a potentially lower incidence of side effects, and improved adherence.

The overall STI incidence rate after doxyPrEP initiation fell from 232.3 to 79.2 per 100 person-years (incidence rate ratio [IRR], 0.33 years; 95% confidence interval [CI], 0.13 to 0.84). The reduction was driven primarily by a drop in syphilis infections to zero, while the reduction in chlamydia incidence showed marginal statistical significance and gonorrhea showed no statistically significant reduction. Similarly, no significant changes were observed in BV or VVC incidence.

Follow-up clinical evaluations on 22 participants found that adherence to doxyPrEP was high, with most participants reporting no missed doses, and that satisfaction with the strategy was likewise high. No serious adverse events were reported. 

Further studies are needed

The findings are noteworthy, because a previous randomized controlled trial in Kenya found that doxyPEP was not effective in preventing bacterial STIs in cisgender women. Experts have suggested that those results might be attributed to low adherence. In this study, adherence was high.

The study authors say, however, that the results of their small, single-center study should be interpreted with caution.

"Further large-scale, controlled studies are needed to validate these findings, assess long-term effectiveness, and evaluate the potential impact on antibiotic resistance and broader public health implications," they wrote.

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