(The New York Times News Service) -- As Bay Area scientists celebrated the first promising results from the largest-ever AIDS vaccine trial, they cautioned that much more research is needed before a vaccine could be available to the public.

The news that an experimental AIDS vaccine tested on 16,000 heterosexual volunteers in Thailand had been shown to be safe and modestly effective surprised researchers, who had become used to failure in the decades-long effort to find a vaccine to protect against HIV infection.

The elusive search for an AIDS vaccine has its roots in the Bay Area, where one of the two genetically engineered vaccines used in the Thai trial was developed.

The trial relied on a combination of a modified canary-pox vaccine from Sanofi Pasteur and a drug made from an engineered version of a protein found on the AIDS virus, which was made by a Brisbane biotechnology company called VaxGen Inc. The patent for the VaxGen vaccine is now owned by Global Solutions for Infectious Diseases, a nonprofit based in South San Francisco.

"At this point, especially after all the failures, any promising information is exciting," said Dr. Phillip Berman, a VaxGen co-founder and inventor of the vaccine, who woke up to the news Thursday morning. "But it's still a long way to having an approved product and figuring out how to deliver the vaccine to the people who need it the most."

The watershed results announced Thursday in Bangkok came just over a year after the National Institutes of Health dropped plans for a large-scale test of its AIDS vaccine. Two years ago, Merck & Co. halted its vaccine study after results showed the drug may have actually increased some participants' susceptibility to the virus.

The vaccine, known as RV 144, employed a two-pronged or "prime-boost" approach in the phase 3 trial. The drug made by Sanofi Pasteur "primed" the body's immune system to attack HIV and the drug developed by VaxGen, "boosted" or strengthened the body's response.

According to the Thai Ministry of Health and the study's other backers, the vaccine regimen was safe and 31 percent effective in preventing HIV infection compared to the placebo. Researchers described that result as significant and promising for the future, but not enough to make such a product available to the public in the near term.

"We are still many years away from a vaccine that will be used universally," said Dr. Jay Levy , professor of medicine at the AIDS Research Institute at UC San Francisco and one of the first discoverers of HIV. "But this is encouraging because it says maybe we can derive things from this study and not have to go into something totally different."

Half of the more than 16,000 study participants were given six doses of the vaccines in 2006, and half received placebos. All received condoms, counseling, regular HIV testing and treatment for any sexually transmitted diseases. Of the 8,197 volunteers who were given the vaccine, new infections occurred in 51 people. New infections occurred in 74 of 8,198 participants who received the placebo shot.

The most confusing aspect of the study was that the people who received the vaccine and still got infected had no lower levels of the virus in the blood than those who took the placebo and got infected. That indicates the vaccine may be better at preventing the virus than controlling it once it got into the body.

"That would suggest the immune stimulus isn't very strong," Levy explained. "With that said, it still was enough to block low levels of the virus from coming into the body. We would want a vaccine that would also control the virus if it infects a person."

In addition, the trial focused on two subsets of the HIV strain that are common in Thailand and Southeast Asia, only one of which is commonly found in the United States. So it's unclear whether the results of the vaccine combination could be replicated in a different population.

Researchers said many aspects of the trial results are unclear.

For example, several scientists said it is uncertain how the drug combination worked and whether one part was more effective than the other.

"We don't really know why and how this vaccine worked and did what it did," said Dr. Alan Bernstein, executive director of the Global HIV Vaccine Enterprise, an alliance of AIDS scientists, governments and donors. "This trial is raising more questions almost than it's answering," he said. "It's opened the door and it's opened up a whole lot of questions that are answerable and will be answered over the next months and years to come."

Because of the long time frame, health advocates warned that people should not count on a potential vaccine to treat and contain new infections.

"It doesn't have a public health impact perhaps for many, many years," said Mark Cloutier, San Francisco AIDS Foundation. "We need to continue to keep our vigilance about prevention, testing and getting people into care."

Copyright 2009 The New York Times News Service. All rights reserved.