Increased use of Truvada (tenofovir disoproxil fumarate/emtricitabine) as pre-exposure prophylaxis (PrEP) among men who have sex with men (MSM) could lead to a decline in the rates of gonorrhea and chlamydia among them, even if starting PrEP leads them to use condoms less frequently.

That is according to a recent mathematical modeling study that projected how the twice-annual sexually transmitted infection (STI) screening and quarterly HIV screening that the Centers for Disease Control and Prevention (CDC) recommends for PrEP users could affect STI rates over time.

Publishing their findings in Clinical Infectious Diseases, Samuel M. Jenness, PhD, MPH, an epidemiologist at Emory University in Atlanta, and colleagues used a network-based mathematical model of the dynamics of HIV, gonorrhea and chlamydia transmission within a theoretical population of 10,000 MSM in the United States. Relying on data from previous studies, they simulated three changing variables: the proportion of MSM at risk for HIV on PrEP, the change in their condom-use rate after starting PrEP and their STI testing rate.

In the reference scenario, MSM tested for STIs twice annually, 40% of those at risk were on PrEP, and starting Truvada for HIV prevention led to a 40% decline in the frequency of men’s condom use. Over a 10-year period, this would avert a projected 42% of gonorrhea and 40% of chlamydia cases. Doubling the reduction in condom use to 80% would still yield a reduction in STIs relative to a total lack of PrEP use. This benefit to STI prevention was driven by the 17% and 16% respective absolute increase in the treatment of asymptomatic and rectal cases of STIs.

Upping the frequency of STI testing to quarterly instead of twice annually would reduce the rate of new cases of gonorrhea and chlamydia by an additional projected 50%.

“Implementation of the CDC PrEP guidelines while scaling up PrEP coverage could result in a significant decline in STI incidence among MSM,” the study authors concluded. “Our study highlights the design of PrEP not only as antiretroviral medication but as combination HIV/STI prevention incorporating STI screening.”

To read the study, click here.