If you follow HIV news or your Facebook feed, you may have already seen the report that a 43-year-old man who was taking Truvada as PrEP has been documented to have become HIV-positive, and it appears he was taking the drug correctly and had levels of it in his blood that are usually effective at preventing HIV infection.

What that means is that, in this one case, PrEP didn’t work. This first-ever event is important news when it comes to communicating about prevention because it means we can no longer say “there has never been a documented case in which PrEP failed to prevent an HIV infection” through sexual contact. Now there is one documented case. But we still know from extensive empirical studies that Truvada taken daily as PrEP prevents a lot of HIV infections—a 92 to 99 percent reduction in risk of infection. It remains a highly-effective prevention tool for those at risk of exposure to HIV. That hasn’t changed.

We also know that this individual was infected by a strain of HIV that was already resistant to the drugs in Truvada before he was exposed to it. Indeed, we know from other studies that about 1 percent of new HIV infections in circulation are resistant to tenofovir, one of the drugs in Truvada, and that a higher number of infections are resistant to the other drug, emtricitabine. This is the first-ever incident in which a single infection was found to be resistant to both at the same time.

Fortunately, because those on PrEP continue to receive HIV tests as they refill the drug, an HIV infection like this will be caught and treated early to reduce the chance of it spreading to someone else. This case certainly reiterates the importance of ongoing testing while on PrEP. Another bit of good news in this case is that the individual’s HIV infection is now fully suppressed with an undetectable viral load, thanks to some of the many other HIV drugs available.

PrEP remains an excellent tool for lowering your risk of acquiring HIV. When considering PrEP or any other HIV prevention method, it’s up to every individual to decide the level of risk they can live with. PrEP is more effective at preventing HIV infections than condoms alone, and extremely effective when used in combination with condoms. That said, PrEP is a risk reduction tool, not a risk elimination tool. No prevention method (aside from abstaining from all risk) is known to be 100 percent effective with 100 percent certainty, and PrEP does not protect against other sexually-transmitted infections such as chlamydia, gonorrhea, herpes or syphilis.

We now know that HIV transmission can happen while on PrEP. What we don’t know from this is just how likely it is; this is the only known transmission to occur in someone who was taking Truvada daily from among the more than 40,000 people who have used PrEP. So although it is unfortunate that this kind of incident can happen, we can still be confident that it is very rare.

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