San Francisco’s largest private health insurer said this week that not a single one of their clients that were taking a daily dose of PrEP for more than two years had become infected with HIV.

A study published in Clinical Infectious Diseases and reported in the New York Times, said the participants, who were almost all gay men, used fewer condoms and contracted other venereal diseases, but none got HIV.

Of the 657 participants in the study, all but four were gay men and 84 percent claimed to have more than one sexual partner.  After taking the medicine, half of the group became infected with syphilis, gonorrhea or chlamydia within the next year.

The researchers say that while it is possible that PrEP is contributing to an increase in venereal diseases, the infection rate among gay men had already been rising before the drug was available.

Critics of the so-called pre-exposure prophylaxis, or PrEP, had predicted that HIV infections would rise among the group because of less condom use.

The study’s lead author, Dr. Jonathan E. Volk, an epidemiologist for the insurer, Kaiser Permanente of San Francisco, said, “This is very reassuring data.  “It tells us that PrEP works even in a high-risk population.”

The Food and Drug Administration approved the use of a two-drug combination called Truvada for prevention of HIV in 2012, and the study monitored the group of men engaged in “very risky behavior” through February of this year.

A clinical trial in England in 2014 found that nine of every 100 persons that received a placebo instead of the drug contracted an infection.  That study was aborted when it became clear that the drug was working and researchers felt it was unethical to continue to give the placebo to participants.

The study did not monitor the participants to see if they were taking the prescribed medication regularly, but the researchers felt that since the men in the study had sought the drug from their primary care physician, it was safe to assume they were using it.