Enrollment in the trial began in June of 2007 and by the end of February 2011 the team used gene-based methods to confirm 28 cases where a previously uninfected partner had contracted the virus from his or her HIV-positive mate.
In only one of the 28 cases, the transmission had occurred in a couple randomized to the early-treatment group.
"The sustained suppression of HIV-1 in genital secretions resulting from antiretroviral therapy is the most likely mechanism for the prevention of HIV-1 transmission that we observed," Cohen's team wrote.
They also found early treatment to be associated with a 41 percent drop in HIV-linked "clinical events" -- sickness and/or death -- in HIV-infected patients, compared with those whose treatment was delayed.
The findings echo those reported by the CDC last week. Both of the CDC trials were done in Africa. In one, a daily dose of Truvada, a combination pill that includes tenofovir disoproxil fumarate and emtricitabine, reduced the risk of contracting HIV from infected partners by about 63 percent.
The other study found that two different regimens -- tenofovir, sold as Viread, and Truvada -- also reduced the risk of transmission through heterosexual sex.
No significant safety concerns were associated with Truvada, the study said, although people taking it were more likely to report nausea, vomiting and dizziness than those taking placebo.
Using antiretrovirals in this way is called pre-exposure prophylaxis, or PrEP.
Earlier research has found that PrEP reduced HIV transmission among gay and bisexual men, but whether it could prevent HIV infection among heterosexuals had remained unknown.
"Now, more than ever, the priority for HIV prevention research must be on how to deliver successful p
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