For example, the NIH vaccine, being overseen by Nabel and up next for clinical trial, includes a cold virus vector like Merck's, but it is in a booster dose given after three shots that do not include the cold virus. Nabel said that minimizes the chance of a repeat of what appears to have happened with the Merck vaccine.
Nabel said the majority of vaccine candidates in development are, like Merck's, based on the cellular model, because the antibody approach has thus far proved "much tougher to solve." Still, he said, "we desperately need the antibodies."
Meanwhile, condoms, circumcision and AIDS-prevention education are not working fast enough to halt the spread of AIDS. According to the NIH's National Institute of Allergy and Infectious Diseases, approximately 40 million people live with HIV, primarily in the developing world. This year, there have been an estimated 4.3 million new HIV infections.
In Baltimore, public health officials have struggled to contain the epidemic, especially in the hardest-hit neighborhoods.
"There is this sense that if we had a vaccine we could end this epidemic," Warren said. "When you have a setback, people might get so discouraged and say, we might never get an AIDS vaccine so let's just give up.
"It's not at all the end of the line."
Dr. John G. Bartlett, chief of infectious diseases at the Johns Hopkins School of Medicine, said that fighting AIDS thus far has been a losing battle and that hearing of another vaccine failure is "disheartening; it's really depressing."
"We're all going to be better off if we get this vaccine," he said. But, he added, "The AIDS vaccine program has been a disappointment from the beginning. Some people will say we're right where we were in 1985 and, in some ways, we are."
Nabel insisted that the knowledge about the AIDS virus and the immune system gained since 1985 - the year after HIV was discovered - has grown exponentially.
"At the end of the day, the search for a vaccine is a marathon, not a sprint," said Dr. Seth F. Berkley, president and CEO of the nonprofit International AIDS Vaccine Initiative. "The great challenge is to keep the world focused on the long-term prize."