Gilles Lambert

Objectives: Doxycycline as post-exposure prophylaxis (doxy-PEP) has emerged as an efficacious strategy to reduce Chlamydia trachomatis (C. trachomatis) , Neisseria gonorrhoeae (N. gonorrhoeae) and syphilis (sexually transmitted infections (STIs)) among gay, bisexual and other men who have sex with men (GBM). There is a need to identify prescribing criteria that maximise the number of STIs averted while minimising excessive use. Design: In this prospective longitudinal cohort study with repeated measures and biobehavioural data collection, participants completed a questionnaire and tested for STIs at each visit. Setting: Community-based, population-level study conducted in three large Canadian cities between February 2017 and July 2023. Participants: 2449 GBM were recruited through respondent-driven sampling (RDS); 1998 had ≥1 follow-up visit, contributing 7551 person-years of observation. Eligible participants were aged ≥16 years, cis- or transgender men, reported sex with another man in the past 6 months and resided in Montreal, Toronto or Vancouver. Primary and secondary outcome measures: Adjusted rate ratios (aRR) of STIs, accounting for RDS recruitment, loss to follow-up and confounding were estimated using generalised estimating equations (GEE) Poisson regression. For identified STI risk factors, the proportions of STIs averted through doxy-PEP prescription (based on the efficacy of doxy-PEP for each bacterial STI) and the number needed to treat (NNT) for 1 year to avert one STI, assuming 100% adherence, were calculated. Results: Among 1998 participants, the combined incidence rate of any C. trachomatis , N. gonorrhoeae and syphilis infection was 29.5 (95%CI 27.3 to 31.9) per 100 person-years. STI risk factors that had the most impact as doxy-PEP criteria were history of any of the three STIs in the past 12 months (P12M) (aRR=2.0, 95% CI 1.8 to 2.2, 36% STI averted, NNT=2.1); ≥10 male sexual partners in the past 6 months (P6M) (aRR=3.8, 95% CI 3.0 to 4.9, 41% STI averted, NNT=2.4); HIV-pre-exposure prophylaxis (PrEP) use P6M (aRR=1.7, 95% CI 1.5 to 2.0, 29% STI averted, NNT=2.5); use of any chemsex-related substance P6M (aRR=1.2, 95% CI 1.1 to 1.4, 28% STI averted, NNT=2.6); and group sex event attendance P6M (aRR=1.2, 95% CI 1.1 to 1.3, 27% STI averted, NNT=2.3). Reporting≥10 male sex partners P6M represented the most useful criterion for syphilis prevention (52% syphilis infections averted, NNT=20). Prescribing doxy-PEP to GBM having any of the following STI risk factors, namely, ≥1 bacterial STI P12M, ≥10 male sex partners P6M, or HIV-PrEP use P6M, would substantially increase the proportion of all STI diagnoses potentially averted (60%) with minimal increase of the NNT (2.7). Conclusion: This work informs on the impact of various doxy-PEP clinical prescribing criteria and demonstrates the benefit of focusing on any of the following three criteria: ≥1 bacterial STI P12M, ≥10 male sex partners P6M or HIV-PrEP use P6M.