Clearly, nothing is infallible. A second case of someone acquiring HIV despite careful and continuous use of PrEP (pre-exposure prophylaxis) has surfaced. As I wrote in a recent Rewire article, Truvada, a two-drug pill taken daily as PrEP, provides at least 90 percent protection from sexually transmitted HIV when taken consistently, according to the Centers for Disease Control and Prevention. Using mathematical modeling based on clinical trial data, some other studies have shown protection rates as high as 99 percent.

The possibility that a Truvada-resistant strain of HIV could overcome this effectiveness, however, has always existed.

Dr. David Knox, a Toronto researcher, presented the first case of this in February at a Boston Conference on Retroviruses and Opportunistic Infections. A Canadian man seroconverted (acquired HIV) after 24 months of taking PrEP daily. Reviews of his blood samples taken during that time suggested that he wasn’t skipping doses. His partner is living with HIV and, due to treatment, is maintaining viral suppression (the amount of HIV in his blood was below detectable levels). When the PrEP-using partner seroconverted, more in-depth testing showed that his positive partner had a strain of HIV resistant to several anti-retroviral (ARV) drugs, including tenofovir disoproxil fumarate and emtricitabine, the two drugs that make up Truvada.

The second case was reported on October 18 at the HIV Research for Prevention Conference in Chicago. A man similarly consistent in his PrEP-taking acquired HIV from a casual partner. His blood and hair samples showed that he had also been taking PrEP consistently. The partner was also found to have HIV resistant to the Truvada drugs.

Drug-resistant strains of pathogens occur when genetic mistakes, or mutations, happen as the pathogen is replicating itself. Mutant strains are often weaker than the original and are usually harmless to the host. But sometimes the mistakes make the mutant strain resistant to the drug designed to disable it. These drug-resistant strains of HIV present an ongoing challenge in both HIV treatment and prevention.

Researchers estimate that only about 1 percent of the people receiving PrEP are likely to be exposed to an HIV strain resistant to tenofovir. They note, however, that this percentage may increase since tenofovir is increasingly used to treat HIV. Resistance to emtricitabine is also a possibility.

If taking PrEP, you can potentially get drug-resistant HIV in one of two ways. If you have sex with someone who has a drug-resistant strain, this exposure can cause that strain to start replicating in your body because it is not hampered by PrEP drugs. Or, if you are taking PrEP inconsistently, you might get a nonresistant strain of HIV that then may mutate into a PrEP-resistant strain in your body.

Precautionary steps are in place to help prevent these scenarios. Thorough HIV testing is done when PrEP is prescribed to make sure that people receiving PrEP are currently HIV negative. Accidentally starting PrEP when you are in the very early stages of HIV infection could increase your risk of developing a Truvada-resistant strain of HIV, thus limiting your future treatment options.

The second step is universal access to antiretroviral drugs (ARVs), which are currently accessible to only 46 percent of the 36.7 million people living with HIV around the world, and encouraging people living with HIV to take their ARVs consistently, without interruptions. Treatment with appropriate combinations of ARVs is key to keeping viral loads low in the bodies of people living with HIV, thus reducing their risk of generating drug-resistant virus that can interfere with treatment. Periodic resistance-testing to check for the presence of drug-resistant virus is also an important tool and, while far from universally available, access to such testing is increasing.

The bottom line is that PrEP is still a very highly effective HIV prevention tool for the vast majority of people using it. An estimated 100,000 people worldwide are taking PrEP and about 49,000 people have filled PrEP prescriptions in the United States alone. Two cases of PrEP failure out of about 100,000 is not perfect, but it is very close.

No one would argue that it is safer to keep HIV out of your body in the first place than it is to deal with it once it is there. For humans, it is preferable to seek out solutions that are not absolute (like virginity) but provide us with the best relative protection we can find. In a world where people have sex and don’t use condoms every single time, PrEP is an invaluable and highly effective addition to the current range of HIV prevention options.