A federally financed study has found that the drug AZT dramatically reduces transmission of HIV, the virus that causes AIDS, from infected mothers to their newborns, government health officials said Sunday.

The findings were considered so significant that the study, which began in April 1991, was ordered stopped on Friday, and officials are spending the holiday weekend notifying the 59 medical centers in the United States and France participating in the study to offer AZT to the pregnant women who had just been receiving a placebo.In addition, said Dr. Anthony S. Fauci, the head of the National Institute of Allergy and Infectious Diseases, the data from the study were being distributed as a "clinic alert" through the National Library of Medicine, which has a computer network available to health-care workers around the world.

Dr. Harold W. Jaffe, an epidemiologist and the top scientist on HIV at the Centers for Disease Control and Prevention in Atlanta, said in an interview Sunday that the finding was one "of major public health importance."

"It is the first indication that mother-to-child transmission of HIV can be at least decreased, if not prevented," he said, "and it will provide a real impetus for identifying more HIV-infected women during pregnancies so that they could consider the benefit of AZT treatment to themselves and their children," he said.

About 4 million women give birth in the United States each year, and the disease centers estimate that 6,000 to 7,000 of the women are HIV-infected. About 1,500 to 2,000 of their babies later become HIV-infected.

The transmission of the virus to newborns is a much bigger public health problem in developing countries in Africa, Asia and South America, where millions of people are infected and where infection rates among childbearing women can reach 10 percent to 30 percent in some areas, said Dr. James Curran, coordinator of all HIV activities at the disease centers. He added that in some areas of the United States, including some urban areas in the Northeast, the comparable figure is as high as 5 percent.

On average, about 25 percent of pregnant women who are HIV-infected pass along the virus to their babies. The researchers had confidence in the study because it found that 26 percent of newborns born to mothers who received a placebo during pregnancy were infected. But the infection rate was only 8 percent for those whose mothers received AZT, officials said.

The officials said that they could find no difference in the number and type of birth defects in babies whose mothers received AZT or the placebo. They added that there has been no significant health hazard among the children during the first 18-month follow-up period.

"This is a real breakthrough and it has worldwide implications," Cur-ran said.

The development is another strange twist in the up-and-down reputation of AZT.

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The drug has been shown to delay the progression from HIV infection to full-scale AIDS, although experience has shown that the benefit generally wanes after a year or two. But Fauci said that the finite period of pregnancy may allow AZT to reduce the risk of transmission to newborns.

The start of the new study was delayed by debates over the ethics. Among the questions raised by critics was whether many uninfected fetuses would be subject to possible hazards from AZT.

The new findings raise major practical and ethical questions. Until now, testing for HIV infection in the United States has been recommended for those who consider themselves at risk. But testing is not mandatory, and there is no general recommendation to test all pregnant women. The American Academy of Pediatrics recommends testing of pregnant women in areas where the prevalence of HIV is high. But compliance among health officials varies.

The new study offers impetus for more aggressive testing programs for pregnant women because of the proven benefits of the AZT treatment. But any calls for mandatory tests would raise the issue of a woman's right to privacy. In addition, developing countries will face the issue of paying for the drug.

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