Men and transgender women who have sex with men and who took Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) in a recent trial were totally protected against HIV if they adhered four or more days a week to the daily regimen. However, participants in the iPrEx trial’s open-label extension phase actually adhered that well just 33 percent of the time. Daily adherence occurred just 12 percent of the time. When considering all who received Truvada, regardless of adherence, the study showed that PrEP lowered the HIV infection rate by about half.

Those at greater risk of HIV appeared more likely to take PrEP, and the higher-risk participants also tended to adhere better. There was no evidence of increased sexual risk taking.

No one contracted HIV while taking Truvada four or more days a week. The researchers projected that PrEP reduces HIV risk by 100 percent at this general level of adherence, but that the actual risk reduction could be as low as 86 percent, as per the statistical calculation’s estimate range.

“There’s all of this opposition to PrEP, and people up in arms: ‘Oh, it’s going to replace everything else,’” reflects Tom Coates, PhD, director of the University of California, Los Angeles Program in Global Health. “No one is saying that this is the ultimate answer. It’s one more tool in the HIV prevention toolbox—meant to supplement, but not replace, other strategies. And even at that, PrEP is at a beginning stage. It does require a pill every day. There’s work going on to identify longer-acting agents that might be administered monthly or quarterly, and those will be much easier to take.”