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Merck got bad news this week on another front. After years of research, one of the most promising AIDS vaccines on the horizon was judged a failure. And it not only failed, data presented at a meeting of AIDS researchers in Seattle showed the vaccine may have increased the risk of HIV. The results are from the South African arm of a multinational study. It may take years to sort out what went wrong.

And as NPR's Brenda Wilson reports, the researchers very gingerly prepared the government and the community there for the news.

BRENDA WILSON: In South Africa, the clinical trial of the Merck vaccine is called Phambili, which means moving forward. But in fact, it has come to a halt.

Dr. Glenda Gray, one of the leaders in the research, says that they had just gotten started this year. Most volunteers had not gotten all three doses. She said she didn't know what to think when she was told that people in the study in North America have become infected after receiving the vaccine, that it appeared that the vaccine may increase the risk of HIV.

Dr. GLENDA GRAY (Chief Investigator, University of the Witwatersrand Perinatal HIV Research Unit, South Africa): We were stressed and disturbed by that information. And the oversight committees were clutchy on the recommendations. We were instructed to, you know, permanently abort study, seize all immunizations and let the participants know that if they're wearing the vaccine on, there was a possibility that there was increased susceptibility to HIV infection.

WILSON: This, for a vaccine that held out great promise when the studies began in North America three years ago, according to Dr. Margaret Johnston, U.S. director of the vaccine study of the National Institutes of Health.

Dr. MARGARET JOHNSTON (Director, U.S. National Institutes of Health): While we had promising test tube results, when we actually went into humans, the vaccine did not perform as we had hoped.

WILSON: In the laboratory and test tubes, it looked as if the blood cells injected with this vaccine would attack HIV-infected cells, suggesting that the vaccine did stimulate an immune response. But in humans, that did not happen.

Dr. JOHNSTON: The other surprising aspect of the trial, and one that's even more perplexing to us, is that more people who received the vaccine got infected than those who received the blank or placebo injection. Now, this could be because of the vaccine or it could because of other factors that we haven't figured out yet such as differences in behavior among the vaccinees and the placebo recipients.

WILSON: The people who got the vaccine instead of the placebo may have figured that out and may have engaged in risky behaviors. Even more surprising is that it appears that those who had a high immunity to a particular cold virus seemed more susceptible to infection. The vaccine is made by combining elements of the cold virus with fragments of HIV. That could just be a coincidence or it could have affected how the immune system worked.

In South Africa, they haven't begun sifting through the data from the 58 participants in the study who did get all three shots. Four of them have become infected and they did receive the vaccine. In South Africa, it could stir up a debate about the scientific consensus on HIV and AIDS that has repeatedly been challenged by the government.

Dr. Gray was very circumspect.

Dr. GRAY: You know, the country is a manic-depressive country and, you know, every day, you know, in the context of, you know, of living in an intermediate developing country setting, you know, we have - we face these problems. And sometimes in terms of health and politics in South Africa, you know, sometimes there are good stories and sometimes there are bad stories.

WILSON: But Gray says the researchers went all out to inform the country in newspapers and on radio. In 16 days, all 801 volunteers had been told whether they got the vaccine or not.

Dr. GRAY: If a participant was told that they had received a placebo, they were relieved. They were elated and they were excited. The other participants that had received the vaccination, we spent a lot of time about telling them how to protect themselves. And some were disappointed, but we had no negative encounters.

WILSON: In the U.S. Merck and the NIH are still considering whether to tell participants here whether they got the vaccine or placebo. A decision is expected with the next 10 days.

Brenda Wilson, NPR News.

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