A gay man in San Francisco has been diagnosed with HIV, despite adhering to his daily PrEP regimen.

Doctors announced the diagnosis during IDWeek, an Infectious Diseases Society for America conference, Queerty reports.

The man adhered to his once-daily pill regimen — began in 2016 — as well as submitting to regular STD testing and quarterly blood tests, doctors said. However, he was diagnosed earlier this year with a rare, drug-resistant strain of HIV.

Despite PrEP, also known by its brand name Truvada, being 99% effective against HIV, it is insufficient to combat this particular strain.

The source of the infection came from a sexual partner who carried the Truvada-resistant strain, and who had stopped taking his antiretroviral medication.

Viral loads in the HIV-positive partner’s system increased sufficiently to pass on the disease to the PrEP-taking partner.

The University of California, San Francisco’s Dr. Robert Grant reiterated PrEP’s effectiveness in combating HIV infection.

“We know PrEP is greater than 99% effective,” he said in a statement. “There are some cases where HIV will break through. We only have a handful of cases now, and next year, we’ll probably have a handful more. Fortunately, these cases are caught early, treated, and suppressed quickly. The person goes from taking one pill a day to one pill a day. The biggest difference is stigma.”

Had the HIV-positive sexual partner maintained his antiretroviral treatment, there would have been no transmission risk to the partner on PrEP. Last year, a study into HIV transmission between gay male couples found that individuals with undetectable levels of the virus do not transmit HIV to their partners.

In the Opposites Attract study, the largest of its kind, 358 gay couples where one partner had HIV and the other did not were monitored.

The University of New South Wales’ Kirby Institute documented over 17,000 instances of anal sex without condoms between the couples, and found that none resulted in HIV transmission.

“Our data [adds] to previous studies which show that there has never been a recorded case of HIV transmission from an HIV-positive person to their HIV-negative sexual partner when the HIV-positive partner had undetectable viral load,” said the Kirby Institute’s Professor Andrew Grulich.

The San Francisco case is one of only a few reported cases of people contracting PrEP-resistant strains of HIV while taking Truvada.

One man, Joe, who spoke to POZ about his experience, called the revelation that he was HIV-positive “a bit of a shock,” given he had adhered to his daily PrEP regimen.

Joe’s viral load was undetectable within three weeks after an antiretroviral regimen, and following the initial treatment his routine was similar to PrEP — a once-a-day pill.

“To be honest, I don’t wake up in the morning and think of myself as an HIV-positive person,” he told POZ. “Sometimes even when I say it, it’s a bit odd because nothing has changed in terms of my life.”

In another case, reported in early 2016, a man who adhered to his PrEP regimen became HIV-positive after being exposed to a “PrEP-resistant, multi-class resistant strain of HIV-1.”

At the time, Whitman-Walker’s director of community research Megan Coleman, said that in that case — which was considered the first of its kind — the form of the virus was quite uncommon.

“In medicine, we’ve learned never to say anything is 100 percent,” Coleman said. “This particular virus, which is very, very rare, had a lot of mutations and a very complicated interplay of mutations that seemed to make it resistant to Truvada.”

Editor’s Note: Updated to clarify transmission risk with undetectable HIV levels.