Trends in incidence of bacterial sexually transmitted infections among gay and bisexual men using PrEP in Australia

Background: Multiple PrEP studies suggest incidence of bacteria STIs among gay and bisexual men (GBM) increases following PrEP initiation, possibly due to changes in behaviour, sexual networks or testing. Regular, asymptomatic STI testing of PrEP users (Australian guidelines recommend three-monthly) may also help suppress incidence longer-term. Using national sentinel surveillance data, we report trends in STI incidence among GBM PrEP-users. Methods: Retrospective patient data were extracted from 35 clinics, with individuals' records linked between clinics. HIV-negative GBM with evidence of TDF/FTC prescription (excluding PEP) contributed person-time from their first subsequent STI test and were censored at their last test or six months after their last PrEP script. Censored participants could re-enter at PrEP re-initiation. Infection date was imputed as a random date between diagnosis and previous negative test. Using Poisson regression, we estimated half-yearly incidence and assessed trends (July 2016-December 2019) for syphilis, chlamydia and gonorrhoea, overall and stratified by anatomical site and age (</>35 years). To account for selection bias associated with differences in characteristics of GBM initiating PrEP over time, a closed-cohort analysis was conducted among GBM continuously using PrEP across the study period. Results: Among 17,250 PrEP-users contributing 24,823 person-years (median time between tests = 84 days) chlamydia (51.3/100 py to 39.1/100 py; p < 0.001) and gonorrhoea (43.1/100 py to 32.8/100 py; p < 0.001) incidence declined, whereas syphilis incidence increased (6.4/100 py to 9.9/100 py; p > 0.001). Among 3498 continuous-PrEP-users (10,538 person-years), declines were observed for overall chlamydia (54.0/100 py to 42.3/100 py; p < 0.001) and gonorrhoea (43.8/100 py to 34.9/100 py; p < 0.001) incidence; pharyngea chlamydia (p = 0.116) and urogenital gonorrhoea (p = 0.973) were stable, and syphilis incidence increased (6.8/100 py to 11.8/100 py; p < 0.001). For both cohorts, gonorrhoea and chlamydia incidence were greater among GBM < 35 years, with declines most prominent among those [greater than or equal to] 35 years. Syphilis rates and trends did not differ by age. Conclusions: Representing the largest cohort analysis of STI incidence among PrEP-users reported internationally, these data suggest that initial increases in incidence of some STIs following PrEP initiation may be attenuated over time. Frequent testing or changes in the risk profile of PrEP initiates over time may explain some reductions. Risk-mitigation strategies may be needed for younger GBM and to curtail syphilis incidence.
OA16.01 M. Traeger (1); J. Asselin (1); C. El-Hayek (1); J. Dittmer (1); E.Wright (2); A. Carter (3); T. Vickers (3); P. Patel (3); C. Fairley (4); B. Donovan (3); [...]