This is the VOA Special English Health Report.

AIDS researchers say they still have much work to do on a vaccine against H.I.V. But the first reports of some success have raised hopes. Scientists say an experimental vaccine reduced the risk of infection in humans by thirty-one percent and was safe.

A lab technician works on blood samples at the research center in Bangkok, Thailand, where the AIDS vaccine tests took place

The study was designed to test for two abilities. One was the ability of the vaccine to prevent H.I.V. infections. The other was its ability to reduce the amount of virus in the blood of people who became infected during the study.

Volunteers received vaccinations over a period of six months and were tested for H.I.V. for an additional three years. The study began in two thousand three. It was the largest AIDS vaccine trial yet. It involved more than sixteen thousand adults in Thailand.

Half received the vaccine. The other half received a placebo, an inactive substance. The volunteers did not know which they were getting.

Seventy-four people in the placebo group became infected during the study. The researchers say that was compared with only fifty-one of those who received the vaccine.

Supachai Reks-Ngarm

Doctor Supachai Rerks-Ngarm, who led the study for the Thai Ministry of Public Health, called it a scientific breakthrough.

The Surgeon General of the United States Army sponsored the study and released the final results last week.

The National Institutes of Health also took part. Doctor Anthony Fauci at N.I.H. called the findings an important step forward. He said it represents the first time an investigational H.I.V. vaccine has shown some ability to prevent infection. But he also said additional research is needed to better understand how the vaccine reduced the risk in those individuals.

The vaccine did not lower the amount of virus in the blood of volunteers who became infected during the study.

The study was based on versions of H.I.V. commonly found in Thailand. The volunteers received a combination of two vaccines. The first, or prime, vaccine came from the Sanofi Pasteur company. The second, or booster, vaccine was developed by VaxGen. The nonprofit group Global Solutions for Infectious Diseases now has rights to it.

Neither vaccine had been successful by itself when tested earlier. More detailed results of the study are expected to be presented at an AIDS vaccine conference in Paris next month.

And that's the VOA Special English Health Report, written by Caty Weaver. For more health news, go to voaspecialenglish.com. I'm Steve Ember.