STI incidence among participants in the HIV Pre-exposure Prophylaxis (PrEP) impact trial in England


BACKGROUND: Pre-exposure prophylaxis (PrEP) is effective at reducing risk of HIV acquisition. However, there are concerns that widespread PrEP-use may lead to changes in sexual behaviour that increase the transmission of sexually transmitted infections (STIs). We describe STI incidence among participants enrolled in a non-interventional, non-randomised trial of PrEP implementation at sexual health clinics (SHCs) across England.
METHODS: Participants were enrolled between 13/10/2017 and 12/07/2020. Demographic, clinical and prescribing data were collected via electronic case report forms and routine STI surveillance. For this analysis, data were included up to 29/02/2020. We compared incidence of chlamydia, gonorrhoea and syphilis across subgroups using univariate and multivariate zero-inflated negative binomial regression models, adjusted for individual follow-up time and testing frequency.
RESULTS: This analysis included 18,358 participants who had at least one post-enrolment visit; median follow-up was 11.9 months [IQR 4.7-20.9].
19,419 STIs were diagnosed among 8,712 participants. Multiple infections were observed in 4,580 (25.4%) participants. Mean incidence of any STI during follow-up was 101.2 (95%CI 99.9-102.7) per 100 person-years: 43.6 (95%CI 42.6' 44.5) for chlamydia, 50.5 (95%CI 49.5-51.5) for gonorrhoea, and 7.2 (95%CI 6.8-7.6) for syphilis.
STI incidence was highest among 16-24 year olds (IRRs 0.93, 95%CI 0.87-0.98 for 25-34yo, 0.79, 95%CI 0.74-0.84 for 35-44yo, 0.66, 95%CI 0.61-0.71 for 45-54yo and 0.59, 95%CI 0.53-0.66 for 55+yo) and in London (IRR 0.86, 95%CI 0.79-0.94 for Midlands and East, 0.84, 95%CI 0.78-0.90 for North and 0.80, 95%CI 0.74-0.89 for South). An STI diagnosis in the year before enrolment (IRR 1.51, 95%CI 1.44-1.58) and being born outside the UK (IRR 1.22, 95%CI 1.16-1.29 for Europe and 1.15, 95%CI 1.09-1.22 for elsewhere) were associated with increased STI incidence, while being on an event-based PrEP instead of daily was associated with lower incidence (IRR 0.91, 95%CI 0.86-0.96). Lower number of tests and STI diagnosis before enrolment, shorter follow-up, and region of residence successfully predicted the chance of having zero STI diagnoses.
CONCLUSIONS: There are considerable differences in STI incidence among PrEP users, even after accounting for different attendance and testing frequency. Efforts in prevention should be focussed on the youngest living in London with history of STIs.