Encountering an HIV-positive individual with a Truvada-resistant strain is very rare [note 1]. The medication we call Truvada contains two different drugs: tenofovir (TDF) and emtricitabine (FTC). To break through the protection of PrEP, a strain of HIV would have to be resistant to both these drugs. In addition to both these resistances a person would have to have detectable viral load, in order to pass this strain on to someone else. HIV that is resistant to Truvada also has a lower capacity to replicate, spread, and cause disease. For all these factors to fall into place is possible, but exceedingly rare. In studies of two different cities, only about 0.001% of HIV circulating in the population met these criteria [2], [3].

Can PrEP protect against such rare drug-resistant strains of HIV? The short answer seems to be “not always.” There are now two documented cases (among many tens of thousands of PrEP users) where a patient, not in a research study, contracted a multi-drug-resistant strain of HIV despite adhering to daily use of PrEP [4]. There were no such cases in any of research studies on PrEP, and there are no other documented cases in clinical practice. Cases like this are likely to remain rare.

To quote Bob Grant, who led the iPrEX study (the first study that demonstrated that PrEP works): “The prevalence of this kind of virus among recently infected persons is less than 1%. Maybe much less. If PrEP is not fully effective against viruses that are HIGHLY resistant to both drugs in FTC/TDF PrEP, the efficacy of PrEP when taken may decrease from 99% to 98%. Or from 99.9% to 98.9%. Or from 100% to 99%. The decimal points are not certain.” [5]

No prevention method is 100% effective. Daily PrEP, however, provides a very high level of protection from HIV. It has been shown to reduce the risk of contracting HIV by 92-99%, depending on the study [6]. Verifying your HIV-negative status before starting PrEP, taking the medication as directed, and completing your quarterly medical visits on time are still the keys to using PrEP in the safest and most responsible way possible.