WEDNESDAY, July 13 (HealthDay News) -- Giving antiretroviral drugs to heterosexuals at high risk of HIV infection can significantly reduce the chance they will develop the AIDS-causing virus, two new studies suggest.

"This is an extremely exciting finding for the field of HIV prevention," said Dr. Jared Baeten, co-chair of one study and a University of Washington associate professor of global health.

Both 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 getting 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.

Using antiretrovirals in this way is called pre-exposure prophylaxis, or PrEP.

Earlier research found that PrEP reduced HIV transmission among gay and bisexual men, but whether it could prevent HIV infection among heterosexuals was unknown.

A subsequent trial, reported in May, that involved heterosexuals found that people with HIV could reduce the risk of infecting their sex partners by more than 90 percent if they started treatment with antiretroviral drugs when their immune system was still relatively healthy.

The latest research includes a trial conducted by the U.S. Centers for Disease Control and Prevention (CDC) and the Botswana Ministry of Health. For that study, researchers assigned 1,219 HIV-negative men and women to a daily dose of Truvada or a dummy pill. All the participants also received HIV prevention services, including condoms, risk-reduction counseling and testing and treatment for sexually transmitted diseases, according to the CDC.

Nine of those taking Truvada became HIV-positive, compared with 24 of those taking the placebo. That is a 62.6 percent reduced risk for those on Truvada, the researchers said.

Among those who continued taking the pill, the risk reduction was greater -- 77.9 percent.

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.

The other new trial, called the Partners PrEP study, was headed up by the University of Washington and funded by the Bill & Melinda Gates Foundation. The placebo portion of the study was halted sooner than expected because early findings so strongly indicated that the pill prevented the spread of HIV. That led to the CDC releasing the results of its study early as well, on Wednesday.

The Partners PrEP trial, done in Kenya and Uganda, included 4,758 couples with one partner who was HIV-positive. Individuals without HIV were randomly assigned to a single drug (Viread), a drug combination (Truvada) or a placebo.

As of late May, 78 HIV infections had occurred; 18 of them in the Viread group, 13 taking Truvada, and 47 who took the dummy pill.

For those getting Viread, the single drug, the risk of developing HIV was reduced 62 percent, while the two-drug combination reduced the risk 73 percent compared with placebo, the researchers said.

"Now, more than ever, the priority for HIV prevention research must be on how to deliver successful prevention strategies, like PrEP, to populations in greatest need," said Baeten in the statement.

Truvada is approved by the U.S. Food and Drug Administration for use in combination with other antiretroviral agents to treat HIV-1 infection in adults and children 12 and older. It has not been approved for PrEP.

Based on the new study results, the CDC will start working to develop guidance on the use of PrEP among heterosexuals in the United States, the agency said.

"To use PrEP in the United States we would use basically the same guidelines -- someone who is truly at risk for HIV," said Dr. Margaret A. Fischl, professor of medicine and director of the AIDS Clinical Research Unit and co-director of the University of Miami Developmental Center for AIDS Research, commenting on the study.

Those at high-risk have multiple sex partners, use intravenous drugs or have multiple sexually transmitted diseases, Fischl said. "You are talking about a group that is at risk for multiple sexually transmitted diseases including HIV," she said.

However, it might be difficult to get the drugs to the people who need them, Fischl said.

"In addition, we need to identify people with HIV and get them into care, because in doing that we know that we decrease the transmission of HIV," Fischl said.

More information

For more information on HIV/AIDS, visit AIDS.gov.