The recent upswing in use of Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) among men who have sex with men (MSM) in San Francisco coincided with an acceleration in the already increasing rates of sexual risk taking and sexually transmitted infections (STIs) among this population. Whether PrEP’s uptake among local men has in fact influenced the change in these rates, however, remains unclear.

Publishing their findings in AIDS and Behavior, researchers analyzed data on HIV-negative men who have sex with men (MSM) in San Francisco from the National HIV Behavioral Surveillance (NHBS) study, which is conducted every three years in major U.S. cities. The study found that, overall, condom use has fallen in recent years among MSM across the country, but that use of condoms varies based on context.

NHBS recruited a respective 1,211, 383, 373 and 268 HIV-negative MSM in San Francisco in 2004, 2008, 2011 and 2014.

None of the men reported any PrEP use until 2014, when 9.6 percent reported using Truvada for prevention.

The proportion of men who reported consistent condom use declined by more than half between 2004 and 2014, with an accelerated decline—a 40 percent drop—between 2011 and 2014, a period that overlaps with the uptake of PrEP among local MSM. The rates of consistent condom use were a respective 36.8 percent, 30.5 percent, 30.5 percent and 18.3 percent in 2004, 2008, 2011 and 2014. The proportion of men reporting no condomless anal sex partners was a respective 60.6 percent, 58.2 percent, 54.2 percent and 40.2 percent in those years.

The proportion of those who reported practicing what is known as pure serosorting, or having condomless anal sex with partners they presume are HIV-negative, increased sharply between 2011 and 2014.

The rate of those reporting multiple condomless anal sex partners more than doubled from 2004 to 2014.

San Francisco surveillance data shows that STI rates have increased among local MSM since 2004, and that the rates saw sharp increases in recent years.

San Francisco men at higher risk for HIV appear more likely to use PrEP. While 25.2 percent of non-PrEP-using men in 2014 said they were consistent condom users, only 10 percent of PrEP users said they always used condoms.

In 2014, 28.6 percent of the men with no harm reduction strategy for averting HIV were taking PrEP. Such strategies include not having anal intercourse; using condoms consistently; pure serosorting; condom serosorting, which means having at least one partner of unknown HIV status or who is HIV positive, while only having condomless sex with men presumed to be HIV negative; and seropositioning, in which men refrain from being the bottom, or receptive partner, with men in certain scenarios. The researchers argue that ideally all men with no prevention strategy should be taking PrEP, since they are likely at the highest risk for contracting the virus.

The researchers theorized that PrEP’s introduction may affect condom use rates among those not taking Truvada: Some MSM may have stopped using condoms consistently under the assumption that other men are taking PrEP.