WASHINGTON (Reuters) - An AIDS vaccine that appears to have worked at least partly in Thailand may only temporarily protect patients, with the effects starting to wane after a year or so, researchers reported on Thursday.

HIV-positive women are seen in a social center in Ba Vi district, outside Hanoi, November 30, 2009. REUTERS/Kham

That may explain why results of the experimental vaccine have been so difficult to interpret, said Dr. Nelson Michael, a colonel at the Walter Reed Army Research Institute of Research in Maryland, who helped lead the trial,

Michael’s team is trying to find out how or why it might have worked. They surprised the world last September when they showed the experimental vaccine cut the risk of infection by 31 percent over three years.

“It is very likely that this vaccine only worked for a short period of time,” Michael said in a telephone interview.

“It is a weak, a modest effect but something that we can build on.”

The vaccine is a combination of Sanofi-Pasteur’s ALVAC canarypox/HIV vaccine and the HIV vaccine AIDSVAX, made by a San Francisco company called VaxGen and now owned by the nonprofit Global Solutions for Infectious Diseases.

Michael told the Conference on Retroviruses and Opportunistic Infections in San Francisco that it may be possible to design a trial that will show better whether the vaccine can really help people.

Part of the problem, he said, was that the 16,000 Thai volunteers who tested the vaccine were not at especially high risk of AIDS infection.

He said he would work with Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, to design trials in Asia or Africa.

According to the United Nations, more than 33 million people are infected with the fatal and incurable virus, with 2.7 million new infections every year.

Even a vaccine that protected for just a year would be useful, Michael said.

“Is that ideal?” No,” Michael said. “But it is true there are vaccines like the flu vaccine where you have to get them every year.”

Within the next few weeks, Michael said studies will also start to try to find clues from the blood of the vaccinated volunteers.

“Everyone wants to know why this worked and what lab measurements we could take that could predict this,” he said.

Results will take roughly a year, he said, but labs all over the world will be looking for so-called correlates -- measurements such as antibody levels that will show whether a vaccine has affected the immune system in the desired way.

“It is what I call an all-hands-on-deck exercise,” he said.

The AIDS virus has killed 25 million people since it was identified in the 1980s. Cocktails of drugs can control HIV but there is no cure. In 2007, Merck & Co ended a trial of its vaccine after it was found not to work, and in 2003, AIDSVAX used alone was found to offer no protection, either.