Success Story: Estimating Effects of HIV PrEP on Gonorrhea and Chlamydia Cases

PrEP modeling study

Preexposure prophylaxis (PrEP) is highly effective (over 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 sexually transmitted infection (STI) screening at least every 6 months after starting PrEP.

Screening for STIs is an important part of PrEP clinical follow-up and monitoring. It allows for early detection and treatment of STIs that would otherwise go undetected, such as asymptomatic rectal chlamydia and gonorrhea.

However, increased PrEP use raises concern of risk compensation (e.g., reduced condom use) that could lead to higher STI infections.

Approach

In an open-label clinical cohort study in California, researchers found that within 12 months of starting medication, men on PrEP reported no new HIV infections. However, 50% were diagnosed with an STI.

High STI incidence was also reported in the PrEP Demo Project, where the STI rate was 90 per 100 person-years.

There are several explanations for higher STI incidence among MSM taking PrEP (e.g., increased screening). The main cause, however, would be behavioral risk compensation, or MSM who initiate PrEP but reduce 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. The California clinic-based cohort, however, saw 41% of PrEP users reduce condom use.

The PROUD trial tested immediate vs. postponed PrEP initiation. It found increases in reported risk behavior for those on PrEP but no differences in the STI incidence rates between the groups. In that population, STI rates were high at the baseline and continued to rise during the study.

This NEEMA study investigated STI rates and risk compensation change as PrEP coverage increases. A model examined urogenital gonorrhea and chlamydia infections against varying levels of PrEP coverage, risk compensation, and STI screening frequency.

Results

In a hypothetical scenario of 40% PrEP coverage and 40% reduction of condom use with STI screening every 6 months, 42% of gonorrhea and 40% of chlamydia infections would be prevented over the next decade.

Doubling risk compensation to 80% would still result in fewer STIs compared to no PrEP coverage. STIs decreased 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 treatment to every 4 months reduced STI incidence by an additional 50% when compared to intervals of every 6 months.

Lessons learned

This NEEMA study shows that implementation of the CDC PrEP guidelines while scaling up PrEP coverage could result in a significant decrease in STI incidence among MSM. The study highlights the design of PrEP not only as HIV prevention but also as combination HIV/STI prevention. This is because it incorporates STI screening and treatment.

Additionally, this study demonstrates evidence that increasing the screening frequency (and timely treatment) to every 4 months rather than every 6 months would reduce STI incidence further.