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According to a new study presented at this year's Conference on Retroviruses and Opportunistic Infections, a man who had been on (and correctly adhered to) a steady regimen of Pre-Exposure Prophylaxis (PrEP), has been infected with HIV.

In 2012, Truvada, the brand most commonly associated with PrEP was approved by the CDC to be taken as a medication that could protect a person from contracting HIV. The drug is actually a blend of two anti-retroviral drugs (emtricitabine and tenofovir) that are used to treat people living with HIV/AIDS. Early clinical trials showed that when taken correctly (every day at the same time) a man who had sex with other men lowered his chances of contracting HIV by up to 90%.

Clinical trials have shown that the drug's efficacy varies depending on the regularity and consistency of doses, but in this case, David Knox, MD explained in the study, the man in question never missed a dosage.

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"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," Knox explained. "Nevertheless, after 24 months on Truvada he tested positive for HIV."

The specific reasons as to why and how the man contracted the virus are unclear, but the specific strain that has been detected in his bloodstream may hold the answer. The strain of HIV detected in the man's blood happened to be resistant to both drugs in Truvada, something that Poz magazine's Benjamin Ryan described as rare.

"Among more than 9,200 participants in the clinical trials of PrEP," Ryan explained. "Such a virus that was highly resistant to both components of Truvada was never seen."

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While this particular study might seem like cause for alarm, it's beyond important to take into account the many factors that led to its findings.

Compared to other viruses, HIV has an incredibly high genetic variability, which means that it has the ability to mutate very quickly. Though we often speak about HIV as a single virus, there are actually multiple strains, types, and subtypes that can behave and respond to drugs differently. It's for this reason that no study has ever claimed that PrEP is 100% effective at preventing HIV infection.

That being said, as Rich Juzwiack points out for Gawker, this does not mean that PrEP doesn't work, it means that it's imperfect, the same way that nearly all drugs are. Estimates about the drug's efficacy have gone as high as 99% in some cases. The takeaway from this particular incident is that, as always, more science and research is necessary.