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ATLANTA - Two new studies found that daily pills prevented infection with the AIDS virus in heterosexual men and women in Africa, bringing new hope for someday offering a medical shield against HIV infection.

"This is good news. This is a good day for HIV prevention," said Dr. Lynn Paxton of the US Centers for Disease Control and Prevention, who has coordinated the agency's research into HIV prevention.

Earlier this year, another study found the same pills did not prevent the AIDS virus among women in Kenya, Tanzania and South Africa. But researchers now say that study may have been flawed based on the success of the two studies announced Wednesday.

The first of the new studies, run by the CDC, involved more than 1,200 men and women in Botswana. About half got a daily pill, Truvada, an HIV treatment made by Gilead Sciences Inc. The other half got a fake pill.

An analysis of people who were believed to be regularly taking the pills found four of those on Truvada became infected with HIV, compared with 19 on the dummy pill. That means the real drug lowered the risk of infection by roughly 78 percent, researchers said.

The second study was funded by the Bill & Melinda Gates Foundation and run by the University of Washington. It involved more than 4,700 heterosexual couples in Kenya and Uganda. In each couple, one partner had HIV and the other did not. The uninfected were given either daily placebos, Truvada pills, or another Gilead treatment, Viread.

The study found 13 HIV infections among those on Truvada, 18 in those on Viread, and 47 of those on dummy pills. So the medications reduced the risk of HIV infection by 62 percent to 73 percent, the researchers said.

An independent review panel on Sunday said the benefit was clear-cut and stopped giving placebos, instead offering the preventive pills. Essentially, they deemed it unethical to withhold the medications from people who had been on placebo, said Dr. Jared Baeten, the University of Washington researcher who co-chaired the study.

"Our results provide clear evidence that this works in heterosexuals," he 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 the CDC and the Washington team made hasty decisions to release the results.

These are the third and fourth widely reported studies of AIDS prevention medications.

The first was announced last year. It was a study of Truvada in gay men in Peru, Ecuador, Brazil, South Africa, Thailand and the United States (San Francisco and Boston). The drug 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 gave advice to doctors on prescribing Truvada 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 the study of 3,900 women in Kenya, Tanzania and South Africa did not show a benefit from taking Truvada.

Scientists are still piecing together why that study pointed to failure and the two latest indicate success. One theory is that the women in the earlier study did not take the medication as often as they should have, Paxton said.

Gilead Sciences of Foster City, California, is a major producer of AIDS drugs. On Tuesday, United Nations health officials announced the company had agreed to allow some of its drugs to be made by generic manufacturers, potentially increasing their availability in poor countries.