of those on dummy pills. So the medications reduced the risk of HIV infection by 62 percent to 73 percent, the researchers said.

"Our results provide clear evidence that this works in heterosexuals," said Baeten, who co-chaired the study.

An independent review panel on Sunday said the benefit was clear-cut and stopped handing out placebos, instead offering the preventive drugs. Essentially, they deemed it unethical to withhold the medications from people who had been on placebo, Baeten said.

In both studies, participants also were offered counseling and free condoms, which may help explain the relatively low overall infection rate.

The studies were to be announced at an AIDS conference in Rome next week. But following the recommendation of the review panel to the University of Washington study, both study teams made hastydecisions to release the results.

These are the third and fourth widely reported studies of Gilead's treatments.

The first was announced last year, involving gay men in Peru, Ecuador, Brazil, South Africa, Thailand and the United States (San Francisco and Boston). Truvada lowered the chances of infection by 44 percent, and by 73 percent or more among men who took their pills most faithfully.

Experts celebrated. The CDC advised doctors on prescribing the pill along with other prevention services for gay men, based on those encouraging results.

But momentum seemed to stall in April, when an interim analysis of a study of 3,900 women in Kenya, Tanzania and South Africa did not show a benefit from Truvada. Scientists can't explain the failure in that study but one theory is that the women