Expanded use of Truvada (tenofovir disoproxil fumarate/emtricitabine) as pre-exposure prophylaxis (PrEP) among men who have sex with men (MSM) at high risk for HIV would likely drive down rates of sexually transmitted infections (STIs) among the larger MSM population. This is according to mathematical modeling conducted by researchers at the Centers for Disease Control and Prevention (CDC) who factored into their calculations varying degrees of PrEP-prompted increased sexual risk taking (known as risk compensation) among MSM after starting PrEP.

Research has indicated that rates of STIs are very high among MSM who take PrEP. However, this finding does not necessarily mean that PrEP leads to a rising STI rate among this population. In part, the high rates of STIs among PrEP users is unsurprising since the factors that make MSM good candidates for PrEP in the first place, specifically behavioral patterns that put them at risk for HIV, often raise their risk of STIs as well.

Findings regarding whether PrEP causes risk compensation among MSM have been mixed. Some studies, in particular the early major studies that established PrEP’s efficacy among MSM, have seen no such effect. More recent studies have begun to identify such behavioral shifts among MSM after starting Truvada for prevention. However, in general, research has not shown that beginning PrEP is associated with a rise in STI rates.

Publishing their findings in Clinical Infectious Diseases, CDC researchers behind the new paper on projected STI rates created a mathematical model based on previous studies of sexual behavioral patterns among MSM. They sought to predict how shifts in various factors would likely affect transmission rates of urethral and rectal gonorrhea and chlamydia among a theoretical population of 10,000 MSM. As a base-case scenario, they presumed that: 40 percent of those men who were indicated for PrEP use according to CDC guidelines would take Truvada for prevention; starting PrEP would lead to a 40 percent reduction in the men’s condom use rate for acts of anal intercourse; the men would receive STI testing twice yearly, following CDC guidelines for those on PrEP.

Without the introduction of PrEP, the model presumed that during one year, the population of 10,000 MSM would receive 420 gonorrhea diagnoses and 660 chlamydia diagnoses.

According to the model’s base scenario, the use of PrEP would prevent a projected 42 percent and 40 percent of the expected cases of gonorrhea and chlamydia, respectively, over a 10-year period. This phenomenon would occur because starting PrEP would result in more frequent screening and thus improved treatment of these STIs, curbing their spread.

The model presumed that 100 percent of MSM would complete antibiotic treatment for diagnosed STIs. Under what the paper described as “a more realistic scenario where 75 percent successfully completed treatment,” the authors’ modeling nevertheless projected a 39 percent lower STI rate thanks to PrEP.

If the risk compensation effect among MSM starting PrEP amounted to less than a 40 percent reduction in their condom use rate, Truvada use among HIV-negative MSM would drive down gonorrhea and chlamydia transmissions in the population even further than in the base scenario: to a maximum of a respective 89 percent and 83 percent reduction in the STIs if there were no risk compensation at all (condom use rate stays the same).

If MSM’s condom use rate dropped by a factor greater than 40 percent as a result of their starting PrEP, this would partially offset the predicted reduction in STI rates associated with PrEP use, more so for gonorrhea than chlamydia. Men would have to reduce their condom use rates by nearly 90 percent as a result of PrEP for there to be no reduction in the gonorrhea rate. Specifically, a 100 percent risk compensation rate—no use of condoms whatsoever after starting PrEP—was associated with a predicted 21 percent rise in the gonorrhea rate. No level of risk compensation would lead to an increased risk in chlamydia rates. If 50 percent or more of those indicated for PrEP took it, even the total elimination of condoms among its users would result in an overall reduction in gonorrhea and chlamydia transmissions.

The CDC, which thus far has not heeded researchers’ calls to increase the recommended frequency of STI testing among MSM PrEP users from six months to three months, projected that such a shift would in fact cut STI transmission rate in half. (Clinicians do have the option of ordering STI tests at each of the quarterly visits required to maintain a PrEP prescription. So many MSM on PrEP may indeed test for STIs every three months.)

The study is limited by the fact that it relies on mathematical modeling and its authors did not take into consideration infections of gonorrhea or chlamydia in the throat, which, while less common than urethral or rectal infections, may result in transmissions into those two other parts of sexual partners’ bodies.

To read the study abstract, click here.