Doxycycline, meningo vax reduce STI incidence among MSM on PrEP

Use of doxycycline as postexposure prophylaxis (PEP) results in a significant decrease in the incidence of Chlamydia trachomatis (CT) and syphilis and demonstrates a significant impact on the incidence of Neisseria gonorrhoeae (GC), reports a study presented at the 30th Conference on Retroviruses and Opportunistic Infections (CROI). Meningococcal B vaccine also helps lower the incidence of GC.

“Increased rates of sexually transmitted infections (STI) are reported among men who have sex with men (MSM), in particular those using pre-exposure prophylaxis (PrEP) for HIV,” according to the investigators, led by Jean-Michel Molina from the University of Paris Cité in Paris, France. “Interventions to reduce STI incidence are needed.”

Molina and colleagues randomized MSM on PrEP with a history of STI in the past year in an open-label factorial design trial to receive doxycycline PEP (200 mg within 72 h of condomless sex) or no PEP (2:1) and two shots of meningococcal B vaccine or no vaccine (1:1).

The investigators tested participants centrally at baseline, every 3 months, and when symptomatic for GC and CT using PCR in the throat, anus, and urine. They also performed serologic tests for syphilis every 3 months. A committee blinded to the study arms was assigned to corroborate STI incidence.

An intent-to-treat analysis was conducted in the assessment of the primary endpoints: the incidence of first episode of CT or syphilis for doxycycline PEP and the incidence of a first episode of GC, 1 month after the second injection, for the vaccine intervention. Cox proportional hazard models were used to compare the incidence between PEP and no PEP, adjusted for vaccine intervention and vice versa.

Following external evidence, the investigators carried out a single interim analysis in September 2022 at the request of the Data Safety Monitoring Board who recommended to stop the trial for efficacy. They then presented the results for data obtained until 15 July 2022.

A total of 546 MSM were randomized, of which 502 (median age 39 years) were analysed between 19 January 2021 and 15 July 2022. The median number of sexual partners of the participants in the past 3 months was 10. [CROI 2023, abstract 119]

Over a median follow-up of 9 months, no interaction was observed between the two prevention strategies for the primary endpoints.

The incidence of a first episode of CT or syphilis was 5.6 per 100 person-years (PY) in the doxycycline PEP arm and 35.4 per 100 PY in the no PEP arm (adjusted hazard ratio [aHR], 0.16, 95 percent confidence interval [CI], 0.08‒0.30). For GC, the incidence of a first episode was 20.5 per 100 PY in the doxycycline PEP and 41.3 per 100 PY in the no PEP arm (aHR, 0.49, 95 percent CI, 0.32‒0.76).

In the meningococcal B vaccine and no vaccine arms, the incidence of a first episode of GC was 9.8 and 19.7 per 100 PY, respectively (aHR, 0.49, 95 percent CI, 0.27‒0.88).

Notably, drug-related serious adverse events were not reported.

In another study presented at CROI 2023, STI incidence among MSM is high and stable during the first 4 years of PrEP use, but the incidence of chlamydia and gonorrhea has slightly decreased among daily PrEP users. [CROI 2023, abstract 971]