Two and a half years into a large Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) program at a clinic in San Francisco, none of the more than 650 men who have sex with men (MSM) taking the HIV prevention medication have contracted the virus. The men took PrEP for an average of seven months each.

Considering that, on the whole, these individuals were at very high risk for HIV, the lack of new infections among them appears to present a powerful endorsement of PrEP’s ability to prevent infection with the virus in a real-world setting.

On the flip side, the published study about the group challenges the received wisdom from PrEP clinical trials, namely the idea that HIV-negative MSM do not increase sexual risk-taking after starting Truvada.

A survey of about one-fifth of the men, given six months into their time on Truvada, found that 41 percent reported using condoms less frequently after starting PrEP. During their first year, or partial year, on PrEP, half of all those taking Truvada contracted at least one sexually transmitted infection (STI). Two men contracted hepatitis C virus (HCV), apparently through sex.

The good news, when looking just at preventing HIV, is that those at higher risk of the virus were more likely to start Truvada after their initial consultation with the clinic. What’s more, the quarterly check-ups required for a PrEP prescription mean that those contracting other STIs will likely get tested and treated faster than they would have otherwise, thus reducing the time during which they may pass an infection to others.

Julia Marcus, PhD, MPH, a postdoctoral fellow at Kaiser Permanente Northern California, who was one of the lead researchers studying the health care provider’s San Francisco PrEP program, notes an important caveat: There was no control group in this study. The lack of one “prevents us from making definitive conclusions” about PrEP’s overall effectiveness among this group, she says. (Additionally, it is not known how starting PrEP may have affected their STI rate.) However, by looking at the rate of HIV acquisition seen in the control group of a recent British PrEP study of high-risk MSM, Marcus estimates that the San Francisco group “would have seen quite a few HIV infections” if they had not been on PrEP.