The experimental vaccine is a combination of ALVAC, from Sanofi Pasteur, the vaccine division of Sanofi-Aventis (SNY), and AIDSVAX, originally developed by VaxGen (VXGN) and now held by the nonprofit Global Solutions for Infectious Diseases. Neither vaccine worked individually in previous trials, but the combination, called RV144, showed modest results. (Neither vaccine causes HIV, separately or together.) The trial was a collaborative effort among the U.S. Army, the Thai Ministry of Public Health, the National Institute of Allergy and Infectious Diseases, the National Institutes of Health, Sanofi Pasteur, and GSID.

16,402 volunteers in Thailand participated in the trial, which opened in 2003. Half received the vaccine, and half placebos; all were counseled on HIV prevention. Of the 8,198 people injected with the placebo, 74 contracted HIV; of the 8,197 who got the vaccine, 51 got the virus -- a difference the collaborative effort calls statistically significant. More details will be given at the AIDS Vaccine Conference in Paris in October.

Researchers seemed surprised. "These results show that development of a safe and effective preventive HIV vaccine is possible," said Col. Nelson Michael, director of the U.S. Military HIV Research Program, in a statement. "While these results are very encouraging, we recognize that further study is required to build upon these findings."

The U.S. HIV/AIDS epidemic began in 1981, according to the NIAID; 565,927 people in the U.S. have died of AIDS. Globally, 33 million have lived with HIV/AIDS, and 2 million died of related illnesses in 2007.

But many challenges lie ahead. Different strains of HIV exist throughout the world; those common in Thailand unusual in the U.S., Africa, and elsewhere. Scientists will need to determine how long the protection lasts, whether booster shots will be needed, and so on. And some in the scientific community are skeptical; Jon Cohen at Science Insider explains that many AIDS vaccine researchers had predicted that the study would fail. They are now "dumbfounded -- and circumspect": concerned that the results came by chance.

This is the third big vaccine trial since 1983, when HIV was identified as the cause of AIDS, the AP reports. As recently as 2007, Merck & Co. (MRK) halted a study of its experimental vaccine after seeing that it actually increased the risk of infection. Before that, in 2003, AIDSVAX also failed in trials.

Vaccine makers might try to license the two-vaccine combo in Thailand. The U.S. Food and Drug Administration will need more studies before the vaccine can be considered for U.S. licensing, and its effectiveness will most likely need to be higher before it gets approval.

While Sanofi shares are down today, VXGN's are up over 10 percent.