HIV-negative heterosexuals who adhere closely to pre-exposure prophylaxis (PrEP) therapy succeed in preventing transmission of the virus from an HIV-positive partner with whom they are in a stable relationship. Publishing their findings in PLOS Medicine, researchers studied 1,147 HIV-negative participants drawn from the three Ugandan sites of the Partners PrEP Study, which gives participants either Viread (tenofovir) or Truvada (tenofovir/emtricitabine) in order to prevent the acquisition of HIV.

The participants were selected for the study because they had at least six months of follow-up remaining in the Partners trial. The group had a median age of 34, was 53 percent male and had an average relationship length of 8.5 years.

The researchers tracked adherence to PrEP through electronic pill bottle monitoring and through unannounced home visits to count pills. If participants' adherence dropped below 80 percent, they received counseling to support their proper adherence.

Past research has shown that the daily regimen of PrEP decreases in efficacy when it is not taken properly.

Adherence was nearly perfect in the study, with an average of 99 percent in the unannounced home pill counts and 97 percent in the electronic monitoring.  

Out of 404 participants receiving a placebo, who between them contributed 333 person-years of follow-up, 14 contracted HIV. Of the 750 participants receiving PrEP, who contributed 750 person-years of follow-up, none became infected.

“Our study shows that PrEP can be extremely effective in preventing HIV infection when adherence to daily dosing is high,” the paper's lead author, Jessica Haberer, MD, MS, of the Center for Global Health at Massachusetts General Hospital at Harvard Medical School, said in a release. “Previous studies of PrEP have found considerable variance in efficacy, ranging from as high as 75 percent to no effect at all. We think that the different levels of adherence in those trials explain the differences in their findings, a hypothesis supported by this new study.”

The study was limited by the fact that its participants were not randomized.   

To read the study, click here.

To read a release about the study, click here.