In what researchers are calling a major breakthrough, an experimental vaccine being tested in Thailand cut the risk of becoming infected with HIV, the virus that causes AIDS, by nearly a third.

The trial of the new vaccine involved 16,000 adults in Thailand and was sponsored by the Surgeon General of the U.S. Army.

“These new findings represent an important step forward in HIV vaccine research,” says Anthony S. Fauci, M.D., director of  National Institute of Allergy and Infectious Diseases, part of the NIH, which provided major funding and other support for the study.

The Thai Phase III HIV vaccine study, also known as RV144, opened in October 2003. The placebo-controlled trial tested the safety and effectiveness of a prime-boost regimen of two vaccines: ALVAC-HIV vaccine (the primer dose), a modified canarypox vaccine developed by Sanofi Pasteur, based in Lyon, France; and AIDSVAX B/E vaccine (the booster dose), a glycoprotein 120 vaccine developed by Vaxgen Inc., and now licensed to Global Solutions for Infectious Diseases (GSID), based in South San Francisco, Calif. The vaccines are based on the subtype B and E HIV strains that commonly circulate in Thailand. The subtype B HIV strain is the one most commonly found in the United States.

Even a marginally helpful vaccine could have a big impact. Every day, 7,500 people worldwide are newly infected with HIV; 2 million died of AIDS in 2007, the U.N