However, the vaccine might still prove valuable because it seemed to reduce infection levels for black and other non-Hispanic ethnic minorities, company officials said.
The vaccine, called AIDSVAX, contains a form of a protein found in the human immunodeficiency virus. When administered to two-thirds of study participants, it reduced infection by less than 4 percent compared to a group receiving placebo, VaxGen said. But in a non-Hispanic subgroup, researchers reported 67 percent fewer HIV infections and in a group of more than 300 African-Americans, there were 78 percent fewer infections.
Nevertheless, experts said it is too early to determine whether the results from minority subgroups are conclusive.
"There could be a variety of confounding variables" that could skew the percentages, said Chris Collins, executive director of the AIDS Vaccine Advocacy Coalition in New York City. Such variables could be geographic location, behavior, gender and timing of infection, he said.
"It's important to be cautious," Collins said. "I'm concerned that these results are being overplayed. Raising false hopes does not serve the AIDS vaccine research agenda."
Because of the small size of the non-Hispanic subgroup, the results cannot be definitive, said Dr. Tom Coates, director of the AIDS Research Institute at the University of California, San Francisco.
Out of the more than 5,000 study participants, 498 were black or from a non-Hispanic ethnic group, VaxGen reported. Although infection rates were 67 percent less among the vaccine recipients than in the control group, the percentage translates into only 16 in the control group who became HIV infected over the course of the trial vs. 12 in the vaccinated group who became infected.
While disappointing, the trial results raise interesting questions, experts said.
"Is there a way to build on this?" Coates asked. "What is it that conferred protection to some people? Should future vaccine trials include more African-Americans? African-Americans are bearing the biggest burden for HIV."
Of the approximately 40,000 new HIV infections in the United States each year, about half of the men are black, according to figures from the National Institute of Allergy and Infectious Diseases in Bethesda, Md. About 64 percent of newly infected women are black, NIAID reported.
Coates said it would be difficult to reach a biological explanation for the different responses to the vaccine. But he added that studying susceptibility among groups could be intriguing.
No conclusive evidence shows blacks in America are more susceptible to HIV infection, but previous studies have found cells in some Africans have a second receptor to which the virus attaches and other groups, such as northern Europeans, lack that receptor, Coates said.
One positive outcome of the study, Coates remarked, was it tended to discourage behavior that increases a person's risk of infection. Prior to the trial, he said, "There was a concern people in a HIV vaccine trial would increase risk behavior." However, the opposite proved true, probably due to the counseling participants received during the trial, he added.
Although risky practices decreased, Dr. Brooks Jackson, director of pathology at Johns Hopkins University in Baltimore, said: "Vaccines are really the only way to cure the epidemic. Human behavior is very difficult to change."
Jackson predicts a vaccine will not be available for at least five to 10 years. In the meantime, he is researching as a prophylaxis the use of nevirapine, a drug used to treat HIV and reduce chances an HIV-positive mother will pass the virus to her baby during childbirth.
A preliminary study has yielded promising results. At low doses -- and taken as infrequently as once per week -- nevirapine prevented infection over a 12-week period, Jackson said. The regimen, which could cost as little as 40 cents per week, is not a long-term solution, he added, but if approved, it could be helpful in preventing infection over short but high-risk periods of time.
The danger of the drug is the buildup of resistance, Jackson explained. If someone is already infected with HIV and takes nevirapine at a low dose, the virus would likely build up immunity. He emphasized the need for HIV testing before taking the drug.
"But until there is a vaccine, this might be a very effective way to protect yourself," Jackson said.
The company's mixed results sent shares of VaxGen down 47.31 percent, or $6.16, to close Monday at $6.86 on heavy volume of 14.7 million shares traded on the Nasdaq stock market. On an average day, 659,636 shares of VaxGen are traded.
(Reported by Christine Suh, UPI Science News, in Washington)