PrEP use tied to ‘stable’ STI incidence, sexual behaviour among MSM

The incidence of sexually transmitted infection (STI) over the first 4 years of pre-exposure prophylaxis (PrEP) use remains high and stable, but the incidence of chlamydia and gonorrhea has slightly decreased among daily PrEP users, as shown in a study presented at the 30th Conference on Retroviruses and Opportunistic Infections (CROI).

In addition, a decrease in the number of sex partners has been observed among daily and event-driven PrEP users, while that of condomless anal sex (CAS) acts with casual partners seems to have increased initially before stabilizing.

“Notably, this did not result in increased incidence of STIs,” said the researchers, led by Mark van den Elshout from the Public Health Service in Amsterdam, Netherlands.

Van den Elshout and his colleagues recruited participants to AMPrep, a prospective demonstration project in Amsterdam that provided PrEP to individuals at high risk of HIV in 2015 to 2020. At baseline, AMPrEP participants chose between oral PrEP daily or event-driven and were given the choice to switch regimens at each 3-monthly study visit. They were then tested for STIs at or in between these visits.

Follow-up started at PrEP initiation and either continued up to 48 months or was censored on 15 March 2020 (start of COVID-19 pandemic), whichever occurred first.

The researchers used Poisson regression to evaluate changes over time in incidence rates (IR) of chlamydia, gonorrhea, and infectious syphilis. They also estimated the IR of hepatitis C (HCV) diagnoses per consecutive year. Finally, the team reported the number of HIV diagnoses, as well as sexual behaviour (ie, number of sex partners, CAS acts with casual partners).

A total of 367 participants initiated PrEP, contributing 1,249 person-years (PY) of observation. Of these, 365 were men who have sex with men (MSM). [CROI 2023, abstract 971]

STIs had an IR of 87 per 100 PY (95 percent confidence interval [CI], 82‒93). No change was seen in the IR of any STI and infectious syphilis over time on PrEP users. On the other hand, incident chlamydia and gonorrhea in daily PrEP users slightly declined.

Interestingly, two incident cases of HIV were diagnosed in the first year of follow-up. IRs for HCV were 1.5, 2.5, 0.7, and 0.4 per 100 PY for every consecutive year on PrEP.

In terms of sex partners, the median number per 3-month period among daily and event-drive PrEP users decreased from 16 and 12 at baseline, 15 and eight at 24 months, and 12 and five at 48 months, respectively. Additionally, the median number of CAS acts with casual partners were seven and four at baseline, 14 and four at 24 months, and 12 and four at 48 months, respectively.

“Regular testing and treatment of STIs remain a priority among PrEP users,” the researchers said. “Biomedical prevention of STIs can be examined in this context.”

In another study presented at CROI 2023, use of doxycycline as postexposure prophylaxis significantly reduced the incidence of C. trachomatis and syphilis and showed a substantial effect on the incidence of gonorrhea among MSM on PrEP. [CROI 2023, abstract 119]