Preexposure prophylaxis (PrEP) is highly effective for preventing HIV, but modest levels of risk compensation (RC) – such as reduced condom use – among men who have sex with men (MSM) have raised concerns about increased incidence of sexually transmitted infections (STIs). In contrast, CDC’s PrEP guidelines recommend biannual STI screening, which may reduce STI incidence by treating STIs (e.g., asymptomatic rectal infections) that often remain undiagnosed. We used modeling to estimate the effect of these two potentially counteracting phenomena.
We expanded our network-based mathematical model of HIV among MSM to include transmission of rectal and urethral Neisseria gonorrhoeae (GC) and Chlamydia trachomatis (CT). PrEP use was simulated following the behavioral indications and ongoing HIV/STI screening recommendations in the CDC PrEP guidelines. Model scenarios varied PrEP coverage (the proportion of MSM indicated for PrEP who received it), the STI screening interval, and RC level (the reduction in the probability of condom use within partnerships).
Under 20% PrEP coverage, the recommended biannual STI screening, and no risk compensation, an estimated 27.5% of GC infections and 31.2% of CT infections would be averted over the next decade. This occurred because PrEP-related STI screening resulted in a 2.1-fold and 2.8-fold increase in the treatment of rectal GC and CT, respectively. Screening at every 3 months (vs 6 months) would avert 33.9% of GC and 35.9% of CT. Doubling PrEP coverage to 40% (with biannual screening) would avert about half of GC and CT infections. At 50% RC (with 20% PrEP coverage & biannual STI screening), the incidence rates of GC and CT would be 1.6 and 1.5 times higher compared to 0% RC.
Implementation of the CDC clinical practice guidelines for PrEP may serve as a high-impact STI prevention intervention due to the salutary effects of the recommended STI screening schedule. The bio-behavioral indications for PrEP could also be well-suited to identify MSM at substantial risk of STI infections that would otherwise remain undiagnosed, including asymptomatic rectal GC and CT. Reducing the STI screening interval to quarterly may not be efficient, as this doubling of STI testing and treatment only reduces population-level STI incidence by a further 5–6%. RC increases STI incidence, but is unlikely to offset the STI prevention benefits of adherence to PrEP-related STI screening recommendations.