A study of “on demand” PrEP abruptly canceled its placebo arm when the protocol showed high efficacy in preventing HIV. The IPERGAY trial involves 400 high-risk MSM in France and Canada who were randomly assigned to receive either a placebo or Truvada, with instructions to take two pills two to 24 hours before they anticipated having sex and two separate single-pill doses a respective 24 hours and 48 hours after sex.

The study’s data safety monitoring board found that those taking PrEP enjoyed a “very significant” reduction in risk for contracting the virus that was “much higher” than the 44 percent found in the iPrEx trial. (iPrEx was published in 2010 and first proved PrEP’s efficacy among MSM.)

“This is not a strategy to be used alone,” cautions Jean-Michel Molina, MD, PhD, the study’s principal investigator, “but it should be delivered with comprehensive care and monitoring.”

Molina says he and his team will present more specifics about the study at the Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle in February.