WASHINGTON (Reuters) - More than 25 years into the AIDS pandemic, scientists finally have a vaccine that protects some people -- but instead of celebrating, they are going back to the drawing board.

The vaccine, a combination of two older vaccines, only lowered the infection rate by about a third after three years among 16,000 ordinary Thai volunteers. Vaccines need to be at least 50 percent effective, and usually 70 to 80 percent effective, to be useful.

Worse, no one knows why it worked.

"Additional studies are clearly needed to understand how this vaccine regimen reduced the risk of HIV infection," Dr. Eric Schoomaker, surgeon general of the U.S. Army, which helped pay for the study, told reporters.

Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said, "We need to bring the best minds together and map the way forward."

The vaccine is a combination of Sanofi-Pasteur's ALVAC canarypox/HIV vaccine, which includes synthetic versions of three HIV genes, and the failed HIV vaccine AIDSVAX, made by a San Francisco company called VaxGen and now owned by the nonprofit Global Solutions for Infectious Diseases.