Taking a daily pill containing either one or two anti-HIV drugs can reduce transmission of the virus by as much as three-quarters among heterosexual couples, two studies in Africa have shown — a breakthrough finding that promises to intensify a new focus on AIDS prevention.

The results were so compelling that the larger study was halted early and the drugs given to all the participants, researchers said Wednesday.

In the absence of a vaccine to protect against the virus, this new approach, termed pre-exposure prophylaxis, may be the best hope for slowing or even halting the spread of the deadly plague throughout the developing world. U.S. health officials are beginning to prepare guidelines for how the drugs could be used in this country to prevent new infections.

The findings “are two more nails in the coffin of HIV,” said Mitchell Warren, executive director of the New York-based AIDS Vaccine Advocacy Coalition. “We are seeing similar results in different populations, and that gives us more certainty that these results are real.”


A study of gay men reported in November showed that one of the drugs in the new trial could reduce the spread of HIV by as much as 70% when taken regularly by uninfected individuals. But a study released this year found that the drugs did not show a similar benefit among uninfected heterosexual women.

The strength of the new findings suggests that the study involving women may have been flawed.

“Our results provide clear evidence that this works in heterosexuals,” said Dr. Jared Baeten of the University of Washington, co-chair of the new study.

The last year has brought several breakthroughs in AIDS prevention research, said Kevin Frost, chief executive of amfAR, the Foundation for AIDS Research. In addition to this latest finding and the study involving gay men, a study released last July found that microbicides could sharply reduce HIV transmission in women and a study in HIV-positive people showed that treating the infected person intensively could reduce transmission by as much as 96%.


Given those and other developments, “we find ourselves in a place where we have an extraordinary opportunity to radically alter the trajectory of the epidemic,” Frost said. “The science is in place. We could do it with the tools we have available. It’s no longer a question of, can we do this? The question is, will we do it?”

The new results are scheduled to be presented next week at the International AIDS Society Conference on HIV Pathogenesis, Treatment and Prevention in Rome, but were released early.

One trial enrolled 4,758 couples in Kenya and Uganda in which one partner was HIV-positive and the other was not. A third of the uninfected participants received a daily pill containing tenofovir, a third received a daily pill containing tenofovir in combination with emtricitabine, and a third received a placebo.

Tenofovir is marketed as Viread and the two-drug combination as Truvada by Gilead Sciences Inc. of Foster City, Calif. They are available generically in many countries for as little as 25 cents per pill, according to the World Health Organization


All couples also received condoms and counseling about how to prevent infection.

By the end of May, researchers had identified 18 new infections among the group receiving Viread, 13 among those receiving Truvada and 47 among those receiving the placebo. That corresponds to a 62% reduction in transmission among those receiving Viread and a 73% reduction among those receiving Truvada.

The second trial, sponsored by the U.S. Centers for Disease Control and Prevention, studied 1,200 healthy, sexually active males and females in Botswana. About half received Truvada and half a placebo. Among the 601 participants who took Truvada, there were nine new infections, compared with 24 among the 599 who received placebo. That amounts to a 62.6% reduction in new infections.

Among those participants who took the drugs regularly, researchers observed an even greater reduction — 77.9% — in new infections.


No significant side effects were observed in either trial. “The perception is that these drugs are really toxic,” said Dr. Thomas J. Coates, an infectious diseases specialist at UCLA’s Geffen School of Medicine. “They are not. The current generation is really quite safe.”

Dr. Jonathan Mermin, director of the CDC’s Division of HIV/AIDS Prevention, said the agency would immediately begin working with other public health groups to establish guidelines for using the drugs prophylactically in this country. Physicians should await those guidelines before prescribing the drugs, he said, but if they believe it is imperative to do it, they should adhere to the guidelines previously announced for using them in gay men.

Dr. Robert M. Grant of UC San Francisco’s Gladstone Institute of Virology and Immunology speculated that the drugs might work even better in the United States than they did in Africa. People in this country “are more accustomed to using pills for prevention,” he noted, and thus more likely to take the drugs regularly.

thomas.maugh@latimes.com