TUESDAY, Oct. 20 (HealthDay News) -- The full results of a landmark trial of an AIDS vaccine show that the shot did have limited success in protecting recipients from HIV.

But the study, published Tuesday in the New England Journal of Medicine, also suggests that this effect fades with time and may not work well for those at highest risk from HIV, such as people who engage in risky sexual practices or abuse intravenous drugs.

"Although our study provided preliminary evidence that an HIV vaccine regimen has the potential to prevent infection, it did not have the power to address two intriguing questions: vaccine efficacy may have decreased over the first year after vaccination, and vaccine efficacy may have been greater in persons at lower risk of infection," wrote the researchers who tested the vaccine on more than 16,000 young adult volunteers in Thailand.

Overall, the vaccine cut the risk of infection by almost a third, the researchers first reported in September. The full results were also described Tuesday at the AIDS Vaccine 2009 conference in Paris.

The researchers acknowledged that the protection offered by the vaccine was relatively modest and did not represent a breakthrough. But the trial results marked a significant gain in the so-far frustrating fight against AIDS, which has killed an estimated 32 million people worldwide since it struck more than a quarter century ago.

Speaking when preliminary results were first described last month, experts said the trial should give scientists important insights into HIV, the virus that causes AIDS, and how it attacks the body's immune system, with the ultimate goal of producing a more effective vaccine.

"I don't want to use a word like 'breakthrough,' but I don't think there's any doubt that this is a very important result," Dr. Anthony S. Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, one of the trial's sponsors, told The New York Times last month.

"For more than 20 years now, vaccine trials have essentially been failures," he said. "Now it's like we were groping down an unlit path, and a door has been opened. We can start asking some very important questions."

The World Health Organization and the U.N. agency UNAIDS said in September that the results "instilled new hope" in the field of HIV vaccine research.

The vaccine is a combination of two vaccines that had previously been unsuccessful in clinical trials. When the Thai clinical trial began in 2006, many scientists thought it would also fail.

The study, which used strains of HIV common in Thailand, tested the two-vaccine combination in what's called a "prime-boost" approach. The first vaccine primes the immune system to attack HIV and the second one strengthens the response.

The two vaccines are called ALVAC and AIDSVAX. ALVAC contains canarypox -- a bird virus that has been genetically altered so it can't cause disease in humans -- to transport synthetic versions of three HIV genes into the body. AIDSVAX contains a genetically engineered version of a protein on HIV's surface. Because the vaccines aren't made from a whole virus -- either dead or alive -- they can't cause AIDS, according to the Associated Press.

The study was done in Thailand because U.S. Army scientists did key research in that country when the AIDS epidemic emerged there, isolating virus strains and providing genetic information on them to vaccine makers. The Thai government also strongly supported the idea of doing the study, the AP reported.

For the trial, half of the 16,402 volunteers were given six doses of the two vaccines in 2006 and half were given placebos. They then got regular HIV tests for three years. Fifty-one of those who got the vaccines became infected compared to 74 who were given placebos, the Times said.

Although the 31 percent reduction in rates of infection was modest, Col. Jerome H. Kim, a doctor who manages the U.S. Army's HIV vaccine program, called the finding statistically significant. And, he added, it's "first evidence that we could have a safe and effective preventive vaccine," AP reported.

The Thais chosen for the study were a cross-section of that country's young adult population, not just high-risk groups such as intravenous drug users or sex workers, Kim added.

According to the study, there was a non-significant trend that suggested that low-risk individuals gained more protective benefit from the vaccine than those at higher risk, such as intravenous drug users who shared needles, or people who had contact with prostitutes.

"Perhaps the requirements for protection against transmission in low-risk, heterosexual persons are considerably different or less stringent than those in high-risk subjects," Dr. Raphael Dolin, of Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, wrote Tuesday in an accompanying editorial in the journal.

Dorin also noted that although the trial lasted three years, "there is a suggestion that more of the effect might have occurred during the first year." More study needs to be done to determine if, in fact, the shot's benefits fade with time, he said.

The chief usefulness of the ALVAC-AIDSVAX vaccine will probably be what it can teach infectious-disease researchers about what is happening in the immune system when a person is even somewhat protected against HIV.

More information

To learn more about HIV and AIDS, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Oct. 20, 2009, New England Journal of Medicine; Associated Press; The New York Times

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