PrEP use contributes to decline in STI incidence among MSM

SEATTLE — Results from a modeling study presented at the Conference on Retroviruses and Opportunistic Infections (CROI) 2017 indicated that the use of pre-exposure prophylaxis for HIV prevention in addition to routine screening and treatment of sexually transmitted infections, or STIs, could reduce the incidence of STIs among men who have sex with men, even when condom use was reduced.

“[Men who have sex with men (MSM)] who are at substantial risk for HIV, and therefore indicated for [pre-exposure prophylaxis (PrEP)] are also at risk for STIs through the same sexual partnership networks and behaviors,” Samuel Jenness, PhD, MPH, assistant professor of epidemiology at Emory’s Rollins School of Public Health, and colleagues wrote in their study. “Our study highlights the design of PrEP not only as daily antiretroviral medication, but as a combination HIV/STI prevention package incorporating STI screening and treatment.”

The CDC estimates that approximately 492,000 sexually active MSM in the United States are at a high risk for acquiring HIV and may benefit from PrEP. However, there is evidence that PrEP use may be linked to a higher incidence of bacterial STIs, according to the researchers. They suggested that this may either be due to causal effects of PrEP or noncausal effects due to study biases. One causal hypothesis is that PrEP use increases behavioral risk compensation (RC), where MSM may reduce adherence to condom use after initiating PrEP.

The CDC recommends that PrEP users are tested for STIs at least every 6 months, including those without symptoms. Jenness and colleagues at Emory University and the CDC used a model to determine how many new cases of gonorrhea and chlamydia could be prevented if 10% to 90% of MSM initiated PrEP and underwent biannual bacterial STI testing and treatment. They further examined how reduced condom use may affect STI incidence in this population.

The researchers estimated that more than 40% of chlamydia infections and 42% of gonorrhea infections could be prevented over the next decade if 40% of PrEP–eligible MSM initiated PrEP and underwent biannual STI screening. This finding would occur even with a 40% reduction in condom use while receiving PrEP. STI screening in accordance with PrEP guidelines increased the detection of asymptomatic STI cases by 17%, and rectal cases by 24%.

Jenness and colleagues reported that STI screening at quarterly rather than biannual intervals would further reduce STI incidence. According to a CDC press release, increasing STI testing frequency to four times a year reduced new cases by an additional 5%. The release also stated that STI cases increased when infections were left untreated in more than half of PrEP users.

“Screening interval was strongly associated with projected STI incidence reductions, but even yearly screening and treatment would reduce STI incidence,” Jenness and colleagues wrote. “Clinicians have a critical role to perform the recommended STI screening and treatment, as incidence could increase if PrEP is delivered without those services.”

The models were adjusted to reflect STI incidence in non-PrEP cohorts. The researchers said these findings did not support the causal hypothesis that increased STI incidence among PrEP users is related to RC.

“Our models, calibrated to the non-PrEP cohorts, were unable to reproduce incidence rates close to the PrEP cohorts, even under extreme levels of RC,” they wrote. “[The findings] suggest higher incidence observed in PrEP cohorts more likely resulting from biased comparisons between the cohorts (e.g., selection bias) than causal from RC.” – by Stephanie Viguers

Reference:

Jenness S, et al. Abstract 1034. Presented at: Conference on Retroviruses and Opportunistic Infections; Feb. 13-16, 2017; Seattle.

Disclosure: Jenness reports no relevant financial disclosures.