Incidence of Gonorrhea and Chlamydia Following Human Immunodeficiency Virus Preexposure Prophylaxis Among Men Who Have Sex With Men: A Modeling Study

Citation: Jenness SM, Weiss KM, Goodreau SM, Gift T, Chesson H, Hoover KW, Smith DK, Liu AY, Sullivan PS, Rosenberg ES. Incidence of Gonorrhea and Chlamydia Following Human Immunodeficiency Virus Preexposure Prophylaxis Among Men Who Have Sex With Men: A Modeling Study. Clin Infect Dis. 2017;65(5):712-8.

Preexposure prophylaxis (PrEP) is highly effective (>95%) for preventing human immunodeficiency virus (HIV) infection among men who have sex with men (MSM). The Centers for Disease Control and Prevention (CDC) clinical practice guidelines for PrEP recommend at least biannual (i.e., every six months) STI screening after PrEP initiation. Screening for STIs is an important part of PrEP clinical follow-up and monitoring because it allows for the timely detection and treatment of STIs that would otherwise go undetected such as asymptomatic rectal chlamydia and gonorrhea, thus preventing onward transmission. However, increased PrEP use raises the concern of risk compensation (e.g., reduced condom use) that could lead to higher STI incidence. In an open-label clinical cohort study in California, no incident HIV infections were reported but 50% of the men on PrEP were diagnosed with a STI within 12 months of starting medication. High levels of STI incidence were also observed in the PrEP Demo Project, where the overall STI incidence rate was 90 per 100 person-years.

While there are several non-causal explanations for higher STI incidence among MSM taking PrEP (e.g., increased screening), the primary causal mechanism would be behavioral risk compensation––MSM who initiate PrEP may reduce their use of STI prevention strategies. Estimates of whether risk compensation occurs among MSM on PrEP, and by how much, have been mixed. The PrEP Demo Project saw no increase in condomless anal intercourse, consistent with the iPrEx trial, whereas 41% of PrEP users in the California clinic-based cohort reduced condom use. In the PROUD trial that tested immediate vs deferred PrEP initiation, there were increases in reported risk behavior for those on PrEP but no differences in the STI incidence rates between the arms. In that population, STI rates were high at baseline and continued to rise during the study in both arms.

The goal of this NEEMA project was to investigate the counteracting phenomena of increased STI screening and increased risk compensation as PrEP coverage increases. Mathematical modeling was used to estimate the effect of varying levels of PrEP coverage, risk compensation, and STI screening frequency on the incidence of rectal and urogenital gonorrhea and chlamydia infections among MSM.

Project Highlights
  • PrEP is highly effective in preventing HIV infection. As part of clinical follow-up and monitoring, the CDC guidelines for PrEP recommend at least biannual (i.e., every 6 months) STI screening after PrEP initiation. Additionally, quarterly STI testing is recommended for sexually active persons with signs or symptoms of infection and screening for asymptomatic MSM at high risk for recurrent bacterial STIs (e.g., those with syphilis, gonorrhea, or chlamydia at prior visits or multiple sex partners).
  • Increased screening and treatment for STIs could lead to reduced onward transmission and fewer STIs. However, risk compensation (e.g., reduced condom use) could counteract the benefits of increased screening leading to increased STIs.
  • This NEEMA project used mathematical modeling to investigate these 2 counteracting phenomena by estimating the effect of various levels of PrEP coverage, risk compensation, and STI screening frequency on the incidence of rectal and urogenital gonorrhea and chlamydia infections among MSM.
  • In a hypothetical scenario of 40% PrEP coverage and 40% reduction in condom use with biannual STI screening frequency, 42% of gonorrhea and 40% of chlamydia infections would be averted over the next decade, and increasing screening and timely treatment to quarterly vs biannual intervals would reduce STI incidence by an additional 50%.
  • This study demonstrated that implementation of the CDC PrEP guidelines, particularly the update to screen MSM at high risk for recurrent bacterial STIs every 3 months, while scaling up PrEP coverage could result in a substantial decline in STI incidence among MSM.
Key Findings
  • In a hypothetical scenario of 40% PrEP coverage and 40% reduction in condom use (i.e., risk compensation) with biannual screening frequency, 42% of gonorrhea and 40% of chlamydia infections would be averted over the next decade.
  • Doubling risk compensation to 80% would still result in fewer STIs compared to no PrEP coverage. STIs declined because PrEP-related STI screening resulted in a 17% and 16% absolute increase in the treatment of asymptomatic and rectal STIs, respectively.
  • Increasing screening frequency and timely treatment to quarterly reduced STI incidence by an additional 50% when compared to biannual intervals.

This NEEMA study shows that implementation of the CDC PrEP guidelines while scaling up PrEP coverage could result in a significant decline in STI incidence among MSM. The study highlights the design of PrEP not only as HIV prevention but also as combination HIV/STI prevention because it incorporates STI screening and treatment. Additionally, it demonstrates evidence that increasing the screening frequency (and timely treatment) to quarterly rather than biannual would reduce STI incidence further.

Page last reviewed: February 6, 2019