Josh Barro writes for the New York Times:
The sample size (2,500 subjects, half taking a placebo) wasn’t large enough to establish that Truvada is 100 percent effective when taken daily, especially because only 18 percent of subjects who were given Truvada actually had the medication in their blood at levels that were consistent with daily use. But by looking at the handful of infections among people taking their pills less than daily, the iPrEx researchers were able to build a statistical model of how the risk of infection declines as the number of pills taken weekly rises. In 2012, they estimated that actually taking Truvada every day produces a 99 percent reduction in the risk of H.I.V. infection, despite not directly observing any such infections.
In an email, Dr. Robert Grant, a top H.I.V. researcher at the University of California at San Francisco and the lead investigator for the iPrEx study, called the 99 percent figure “our best estimate of the H.I.V. risk reduction when men and transgender women who have sex with men use PrEP daily.” (PrEP, or “pre-exposure prophylaxis,” is the practice of using antiviral medication like Truvada to prevent H.I.V. infection, rather than to treat it.) Dr. Grant noted that lower estimates, such as the 92 percent figure, include results for people who were not taking the drug daily.
Barro concludes: “Putting too much weight on the 99-percent figure may lead some PrEP users to perceive virtually zero H.I.V. risk when they should really be thinking about very low risk.”