Previous studies had found that antiretroviral drugs given to animals before exposure to the AIDS virus did protect them against infection.
And antiretrovirals are also used to prevent infection in people who have been exposed to HIV-infected fluid; those regimens need to start within 72 hours of exposure.
The current study, published in the Nov. 25 issue of the New England Journal of Medicine, is the first human trial to look at the effectiveness of this combination in prevention.
For this large study, about 2,500 men or transgender women (men who consider themselves women but are born male) who have sex with men were randomly assigned to receive Truvada or a placebo once daily.
The pills were provided by Gilead Sciences, which makes Truvada, although the study was funded by the U.S. National Institutes of Health and the Gates Foundation.
Both groups also received intensive counseling on safe sex practices and were given condoms (about 500,000 condoms were distributed throughout the course of the trial).
The researchers explain that the trial participants were assumed to be at extremely high risk for HIV infection, having had an average of 18 sexual partners in the past 12 weeks.
In all, 64 people in the placebo group of 1,248 were infected with HIV over the course of the study (average follow-up was 1.2 years) as opposed to 36 among the 1,251 participants receiving the drug.
Although the average risk reduction in the entire study population was 44 percent, it went as high as 72.8 percent among those who took the pill at least 90 percent of the time.
The authors were able to verify compliance by measuring blood levels of the drugs, and levels of the drug in participant's blood seemed to correlate with HIV status 51 percent of HIV-negative people showe
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