Background
HIV pre-exposure prophylaxis (PrEP) and doxycycline post-exposure prophylaxis (Doxy-PEP) are increasingly adopted by gay and bisexual men worldwide, but real-world evidence on their population impact remains limited, particularly in settings with low HIV incidence and restricted programmatic support. Because Doxy-PEP does not prevent HIV, its use without PrEP is a concern. We analyzed a single-center cohort of HIV-negative men who have sex with men (MSM) in Tokyo (2017–June 2025) to describe: (1) half-year STI incidence trends, (2) annual HIV incidence, and (3) contemporaneous uptake of PrEP and Doxy-PEP, including Doxy-PEP–only use.
Methods
The Sexual Health Clinic (SHC) cohort, established in Tokyo in 2017, offers free quarterly HIV and STI (syphilis, chlamydia, gonorrhea) testing for MSM. We estimated annual HIV incidence and half-yearly STI incidence. STI trends were modeled with negative binomial regression using splines for calendar half-year, log(person-time) offsets, participant-clustered robust errors, and adjustment for age and entry period. Incidence rate ratios (IRRs) were estimated, stratified by ever-PrEP and ever-Doxy-PEP exposure. Prevention uptake was summarized each half-year as the proportion in four groups: neither, PrEP only, Doxy-PEP only, or both.
Results
Among 2,355 participants, STI incidence showed no significant monotonic trend: IRRs (per half-year) were 1.015 (95% CI: 0.993–1.036) for ever-Doxy-PEP and 1.009 (0.992–1.025) for ever-PrEP. HIV incidence declined from 2.8 per 100 person-years in 2018 to 0.2 in 2022 and remained low through 2025. By June 2025, 55% of participants were using PrEP and 30% Doxy-PEP; 23% used both, while 8% used Doxy-PEP without PrEP. Uptake occurred despite limited formal access, with Doxy-PEP emerging in 2023 largely outside supervised programs.
Conclusions
In this cohort, STI incidence remained stable while HIV incidence fell below 1 per 100 person-years in parallel with rising PrEP uptake. These findings demonstrate the prevention potential of PrEP scale-up even in no-coverage settings. The recent emergence of Doxy-PEP, including use without PrEP, highlights the need for proactive policies and guidance to ensure integrated delivery of biomedical prevention worldwide.
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