A NEW Swiss study has found that doxycycline post-exposure prophylaxis (DoxyPEP) substantially reduced rates of bacterial sexually transmitted infections (STI) in men living with HIV, although it did not appear to protect against gonorrhoea. The findings, presented at the 2025 European AIDS Clinical Society (EACS) Congress, provide important real-world evidence on the impact of DoxyPEP and the four-component meningococcal B vaccine (4CMenB) among high-risk populations.
The observational study, conducted within the Swiss HIV Cohort Study, included men who have sex with men (MSM) living with HIV. Eligible participants reported condomless sex with occasional partners or at least one bacterial STI diagnosis within the past 3 years. During routine clinical visits, participants who agreed to use DoxyPEP and/or 4CMenB as off-label prevention methods were prospectively followed, with STI cases compared to those recorded in the preceding 3 years.
Doxycycline Reduces Chlamydia and Syphilis Incidence
From November 2023 to December 2024, 157 participants were enrolled and followed for a median of 253 days. Overall STI incidence dropped from 56.26 to 30.23 cases per 100 person-years following the introduction of DoxyPEP, corresponding to an incidence rate ratio (IRR) of 0.53 (p<0.005). The reductions were most pronounced for Chlamydia trachomatis (IRR: 0.20; p<0.005) and Treponema pallidum (syphilis; IRR: 0.30; p<0.01). These findings are consistent with earlier clinical trial data showing that doxycycline prophylaxis can effectively prevent certain bacterial STIs in high-risk groups.
Doxycycline and Meningococcal B Vaccine Show No Effect on Gonorrhoea
By contrast, neither DoxyPEP alone nor in combination with 4CMenB reduced the incidence of Neisseria gonorrhoeae. Among DoxyPEP users, the IRR for gonorrhoea was 0.97 (p≥0.05), while among those receiving both DoxyPEP and 4CMenB, the IRR was 1.09 (p≥0.05). These results echo previous mixed findings on the potential off-target protective effects of 4CMenB against gonorrhoea and underline the continued challenge of addressing antimicrobial resistance in this pathogen.
Researchers concluded that doxycycline prophylaxis remains an effective strategy to prevent chlamydia and syphilis among MSM living with HIV, but additional preventive measures are needed to tackle ongoing gonorrhoea transmission.
Reference
Wimmersberger D et al. Doxycycline postexposure prophylaxis and the 4-component meningococcal B vaccine among persons living with HIV at high risk for bacterial sexually transmitted infections in Switzerland. PS15.2. EACS 2025, 15-18 October, 2025.
