A study published online in the journal Clinical Infectious Diseases reviewed the medical records of patients in the Kaiser Permanente system in San Francisco who had been referred for a pre-exposure prophylaxis (PrEP) evaluation between 2012 (when the drug became available) and early 2015. The majority of these patients (82 percent) decided to start taking PrEP to prevent HIV and in the almost three years of the study, not one of them contracted HIV.

These results are better than what would have been expected given clinical trials, and experts are excited that this new prevention method is working so well. But some are worried because condom use among these patients has dropped, and more than half of them were diagnosed with at least one sexually transmitted infection (STI) during the course of the study.

PrEP is a combination of two antiretroviral drugs—tenofovir and emtricitabine—used to treat people who have HIV, as Rewire has reported. When used daily by people who are HIV-negative, these drugs have been shown to prevent transmission of the virus. In May 2014, the Centers for Disease Control and Prevention (CDC) released guidelines that recommended HIV-negative individuals who are at “substantial risk for HIV infection” consider taking the drug.

The agency defined those at substantial risk as: anyone in an ongoing relationship with an HIV-infected partner; gay or bisexual men who are not in a mutually monogamous relationship with an HIV-negative partner and who have had sex without a condom or been diagnosed with a sexually transmitted infection within the past six months; heterosexual men or women who are not in a mutually monogamous relationship with an HIV-negative partner and do not regularly use condoms when having sex with partners known to be at risk for HIV (such as injecting drug users or bisexual male partners of unknown HIV status); or anyone who has injected illicit drugs and shared equipment or been in a treatment program for injection drug use within the past six months.

The majority of the patients in the Kaiser Permanente study were men who had sex with men, though the study included three heterosexual women and one transgender man who was having sex with men.

The results were better than expected. The researchers followed 657 patients who took PrEP for a period of time that amounted to 388 person years. None of the patients contracted HIV.

“This is very reassuring data. It tells us the PrEP works even in high risk populations,” the study’s lead author, Dr. John Volk, told the New York Times.

Anthony Fauci, the director of the National Institute for Allergies and Infectious Diseases, agreed. “This shows the effectiveness of PrEP is really strikingly high,” he told the Times. “And this study takes it out of the realm of clinical trials and into the real world.”

The researchers noted that the study should allay fears that PrEP use would lead to less condom use and therefore more HIV. In the article, they write: “Our data suggest that fears about risk compensation resulting in increased HIV acquisition among PrEP users may be unfounded.”

Some public health experts are still concerned because while none of these patients contracted HIV, many of them contracted other STIs. A year after starting PrEP, 50 percent of patients had been diagnosed with an STI.

One-third of those patients had contracted chlamydia, 28 percent of patients had gonorrhea, and 5.5 percent had syphilis. Many of these infections were rectal.

Since there was no control group in this study, it is impossible to know whether these rates of STIs are higher than they would have been if the patients had not taken PrEP. Patients on PrEP are required to have frequent screenings for STIs. The high rates of STIs among this group, therefore, may reflect better testing rather than any real increase in the number of infections.

Still, there seems to be some behavior changes among patients on PrEP. Researchers surveyed 143 of the patients about their behavior. Seventy-four percent said the number of sexual partners they had did not change while on the drug, 15 percent said they had fewer partners, and 11 percent said they had more. Fifty-six percent said they did not change their condom use habits, 41 percent said they used condoms less often, and 3 percent said they used them more.

Though the patients enrolled in the study were engaging in high risk behaviors to begin with and some did not change their behavior because of PrEP, the STI rates among them are very concerning to some public health experts.

“We’re thrilled that there were no cases of HIV in the Kaiser PrEP study,” Deborah Arrindell, vice president of health policy at the American Sexual Health Association, told Rewire. “We’ve all been waiting for a study with these positive results. But there are thorns on the rose. Some participants did not follow the FDA guidelines to continue condom use. About half of those in the study contracted a sexually transmitted infection and that is worrisome.”

“More than one in four got gonorrhea—an infection for which we’re running out of treatment options,” Arrindell added. “That is cause for concern, even as we celebrate.​”