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In 2012, the FDA approved Truvada as pre-exposure prophylaxis (PrEP) for HIV. An antiretroviral drug used to treat HIV (in combination with other drugs) to the point of suppressing viral loads to undetectable levels in the bloodstream of HIV-positive individuals, Truvada was discovered to be effective in preventing transmission in non-infected individuals if taken everyday. From then until last year (and in the trials that preceded its FDA approval), no known transmissions of HIV had occurred in people who were adhering to a daily PrEP regimen. Many experts warned against deeming the drug 100 percent effective in preventing HIV, but it nonetheless seemed to be the case.

That changed with the announcement in February 2016 that a Canadian man adhering to a daily regimen (per his prescription pill count and a dried-blood spot test that gauges the level of the drug in one’s system) had nonetheless contracted a strain of the virus known to be resistant to emtricitabine and tenofovir, Truvada’s antiretroviral cocktail. A second, similar case of a drug-resistant strain infecting a PrEP-adherent man was made public in October.

And now, there is word of a third case, but this time it’s a bit of a mystery as to how transmission occurred, as the strain contracted by a 50-year-old Dutch man is not known to be resistant to the drugs. POZ’s Benjamin Ryan reports that Elske Hoornenborg, MD, an infectious disease specialist the Public Health Service Amsterdam shared these findings at the Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle. The sexual history of the man was scrutinized as it may be key to what, at this point, appears to be an aberrant transmission. Writes Ryan:

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He kept a diary of his sexual activity on a mobile phone application. During months one through eight of his time on PrEP, he reported a respective 75, 56, 56, 50, 38, 49, 66 and 12 anal sex partners. (The eighth month constituted only 20 days.) During each of these months, he reported having condomless anal sex on a respective 21, 12, 13, 15, 15, 19, 17 and 3 days. According to his reports, during the days he had condomless anal sex, the median number of partners he had on those days was 3 during the first month, 4.5 during the second, and 4, 4, 2, 3, 5 and 5 for each following month, respectively.

After eight months, the man developed a fever and dysuria (pain while urinating), and an HIV test suggested he had contracted the virus (although the results of another HIV test suggested he hadn’t, adding to the uniqueness of his situation). He was taken off PrEP in fear that he’d develop a Truvada-resistant strain on HIV if he were infected. He turned out to be infected, has been treated, and is currently undetectable.

As to why this man’s sexual activity is particularly relevant, Ryan writes:

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Hoornenborg theorized that it is possible that the man was repeatedly exposed to HIV, which, kept relatively in check by Truvada, remained in a localized infection in his rectum for a time without spreading throughout the body. Then perhaps a drop in Truvada concentration in the rectum, maybe occurring without a corresponding drop in blood levels, allowed a window for HIV to thrive and establish an infection throughout the body. Because there are only two HIV drugs in Truvada as opposed to the three or four needed to successfully treat the virus, PrEP may be unable to thwart such a systemic infection.

The man also reported using crystal meth, cocaine, GHB, mephedrone, and ketamine during sex.

For many men who have sex with men, a population whose members that can access it have embraced PrEP, the antiretroviral cocktail has helped prompt a new age of sexual liberation. This man’s sex partner count may look astounding on the outside, but he’s far from the only one racking up such numbers. (I also have to wonder how accurately he could have made such a count if he was using the drugs that he did.) The official talking point from sexual health experts has always been that PrEP should be used in conjunction with condoms for maximum effectiveness. That’s not always realistic given a host of factors including the way desire can supersede rational decision-making and the fact that many people really hate condoms, but this case should at least remind people that PrEP isn’t a license to abandon all thought regarding sexual health, figuring that one pill will take care of everything.

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It’s also important to keep things in perspective. We now know of three PrEP-adherent people who contracted HIV. That’s an infinitesimal percentage of the number of people who are on PrEP—in July of 2016 it was reported that almost 80,000 people had started it in the U.S. alone. At CROI, Robert M. Grant, MD, MPH, who had led iPrEx, the study that proved PrEP’s effectiveness and led to its FDA approval noted, “PrEP has prevented thousands of infections.”