Researchers at a California-based company announced the initial results of the three-year trial.
The FDA has said it would consider approving the vaccine if it were 30 percent effective, but the reduction of infection among the entire sample of volunteers, including all racial groups, was 3.8 percent.
A principal investigator in the study, said the study "did not show a statistically significant reduction of HIV infection within the study population as a whole, which was the primary endpoint of the trial."
The researchers studied the vaccine in about 5,400 people -- 5,108 of them were men who had sex with men; 309 were women. All of them had lifestyles that put them at high risk of HIV infection, but none were infected at the start of the trial, researchers said. What researchers see as promising about the study is that it "did show a statistically significant reduction of HIV infection in certain vaccinated groups."
"Trial data indicate that black and Asian volunteers appeared to produce higher levels of antibodies against HIV. White and Hispanic volunteers appeared to develop consistently lower levels of protective antibodies following vaccination."
In blacks, the researchers said the vaccine was 78.3 percent effective, but of all the people in the trial there were only 314 blacks. Blacks account for half of all new HIV infections in the United States, according to government figures.
Among Asians, the vaccine showed 68 percent effectiveness, but that figure may not turn out to be scientifically accurate because only 77 Asians took part in the trial. Researchers said a larger sample of Asians could produce different results.
Why are some protected, others not?
"This is the first time we have specific numbers to suggest a vaccine has prevented HIV infection in humans," said Phillip Berman, inventor of the vaccine called AIDSVAX. "We're not sure yet why certain groups have a better immune response."
If the numbers are certified by independent experts, it could be significant for Africa, where the epidemic is skyrocketing. But the vaccine would require some additional work.
In Africa, where the strain of HIV is slightly different, it would take time to do simple modifications that could make it work there as well.
The company says it planned to continue developing a vaccine and will examine more closely why it worked better in blacks and Asians than whites and Hispanics, according to The Associated Press.
Researchers have started comparing the blood serum of those who were protected by the vaccine with those who weren't to try to find out what happened in the body to protect them.
If the test results bear out, it could prove to be the basis for developing an HIV vaccine.
Researchers counseled patients in the experiments to practice safe sex because the vaccine may not work and because one-third of the volunteers received the placebo.
Experts think a vaccine is the only way to stop the worldwide AIDS epidemic, which already has killed 20 million people and infected 40 million more.
Until now, researchers have had nothing in the lab to use as an example of how to fight the virus that causes AIDS. If Researchers are able to isolate the mechanism that protected people from HIV, they might finally have a guide.
The FDA will have the final decision on whether the vaccine is licensed for certain populations.