Incidence and risk factors of C. trachomatis, N. gonorrhoeae and syphilis among a cohort of urban Canadian gay, bisexual and other men who have sex with men, 2017-2023: informing the potential impact of doxycycline prophylaxis

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.