The first case of HIV contraction in a person taking PrEP (pre-exposure prophylaxis, or a antiretroviral drug cocktail used to protect people from contracting HIV) daily has been documented and presented by David Knox, MD, an HIV specialist at the Maple Leaf Medical Clinic, at the 2016 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston, according to Benjamin Ryan at Poz. Ryan reports:

Evidence suggests that the individual in question, a 43-year-old man who has sex with men, adhered well to PrEP over the long-term. Nevertheless, after 24 months on Truvada he tested positive for HIV. Initial tests indicated that he was acutely (very recently) infected: He tested positive for the p24 antigen, which appears within about three weeks of HIV infection and disappears a few weeks afterward; and at that time he tested negative for HIV antibodies, which typically appear two to eight weeks after infection.

And furthermore:

“After 32 years of experience with HIV research, I have learned never to say ‘never’,” said Robert M. Grant, MD, MPH, a professor at the University of California, San Francisco, who was the head of the iPrEx trial that first proved PrEP’s effectiveness among MSM and transgender women in 2010. “Yet I also think that gay men benefit from feeling safer during sex, and I am grateful that PrEP affords that feeling.” Pharmacy records indicated that the man in the case study had consistently filled his Truvada prescription on schedule. Dried blood-spot testing on a sample taken 16 days after he tested positive for HIV indicated that he had adhered well to Truvada during the previous one to two months, a period that overlapped with the estimated time when he contracted the virus. “This person claims he was taking PrEP every day and I believe him,” said Grant.

The man tested positive for a strain of the virus that was resistant to multiple drugs, including emtricitabine and tenofovir, which make up Gilead’s Truvada, the current antiretroviral cocktail on the market that’s used as PrEP. Poz adds, “Despite all these resistance mutations, the man in the case study is currently taking HIV treatment and has a fully suppressed viral load.” And also:

What is more rare is a virus that is highly resistant to both tenofovir and emtricitabine, as in this new case report. Indeed, according to Grant, among more than 9,200 participants in the clinical trials of PrEP, such a virus that was highly resistant to both components of Truvada was never seen.

The takeaway is that PrEP is not 100 percent effective, as many hoped it was. That doesn’t mean that it’s ineffective (its efficacy has been estimated by some to be as high as 99 percent), it just means that like virtually everything, it is not the perfect, absolute solution to the HIV epidemic. Antiretrovirals are incredibly powerful tools in curbing the spread of HIV—a study of hundreds of sero-different couples amounting to tens of thousands of condomless sexual encounters found not one instance of transmission of HIV from the positive partner to the negative.

Also presented at the conference was the CDC’s estimation that the combination of PrEP, alsong with expanded HIV testing and treatment has the potential to reduce new infections in the United States by 70 percent by 2020, according to another report for Poz by Ryan.

And while this PrEP news is sobering, it’s far from the most sobering HIV-related headline of the week. That honor goes to the CDC’s estimation that if rates continue as they are, one in two black men who have sex with men will contract HIV in his lifetime. Fifty percent. For a variety of reasons, that population is particularly underserved when it comes PrEP and HIV treatment as a whole.

[h/t Peter Staley]

Note: An earlier version of this post stated that the CDC announced that PrEP would cause a 70 percent reduction of new HIV cases in the U.S. PrEP was just one listed factor in reducing the number—expanded HIV treatment and testing were the other factors.

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