A British pilot study of PrEP among high-risk men who have sex with men (MSM) and among transgender women showed such high efficacy for Truvada that those in the arm of the study that deferred administering PrEP for a year were invited to take it immediately. The PROUD study includes 545 participants who were all offered regular testing for HIV and sexually transmitted infections as well as condoms and safer-sex support. This is the only way Britons can obtain a PrEP prescription at present.

PROUD’s goal is to determine if it is feasible to conduct a larger study about PrEP’s use among MSM in England, a trial that would address questions about: the intervention’s efficacy and cost effectiveness; MSM’s interest in taking Truvada, their adherence levels and their tendency toward riskier behavior on PrEP; and if PrEP users run the risk of drug resistance should they contract HIV.

The researchers initially calculated that they would eventually need 5,000 study participants to come up with statistically significant results.

Sheena McCormack, MSc, a professor of clinical epidemiology at Imperial College in London and the study’s chief investigator, says her team “never expected to see a result” with this small study. “That is only possible under two conditions. You can only have a level of benefit that makes the data monitoring board stop your study if: a) PrEP is incredibly effective; and b) the rate of HIV is much higher than we thought. So it’s sort of good news and bad news.”