Cautioning that the final answer may be years away, scientists in Baltimore say they have made significant but "incremental" progress toward a vaccine that could protect people against the virus that causes AIDS.
Researchers with the Johns Hopkins School of Hygiene and Public Health said yesterday that an experimental vaccine given to 20 human volunteers caused the majority to produce "neutralizing antibodies" -- proteins that keep the free-floating virus from infecting healthy cells.
The results, published in this week's issue of The Lancet, a British medical journal, were noticed when blood samples taken from vaccinated people were mixed in test tubes with samples of the AIDS virus. The volunteers were all healthy and believed to be at low risk for contracting the virus.
Dr. David Schwartz, the chief investigator, cautioned that the vaccine was based on a viral strain that has been maintained for research purposes in laboratories throughout the world. Eight of 10 people who took the highest dose of the vaccine produced antibodies that neutralized this lab strain.
But, encouragingly, six of the people taking the high-dose vaccine also produced antibodies against a different strain that is much more common among patients.
"Obviously, any vaccine that is really put forward as a potential preventive vaccine would have to be effective not only against laboratory strains but also strains in circulation in the population," Dr. Schwartz said.
The vaccine, produced by Genentech, a California biotechnology company, was the same one tried successfully three years ago on chimpanzees at a research facility in San Antonio, Texas, when scientists vaccinated the chimpanzees and injected them with the AIDS virus.
Most of the animals remained free of infection, an indication that the vaccine armed their immune system to fight off infection. But the vaccine remained to be tried in human volunteers -- a more difficult proposition, because scientists cannot ethically expose people to the virus to see what happens.
Ultimately, for a vaccine to be proved effective, it will have to be given to thousands of healthy individuals who are thought to be at risk for infection because of unsafe sexual or drug-taking habits. The volunteers would have to be tracked for several years to see if they remain healthy.
Before ending its work a month ago, the National Commission on AIDS said a vaccine for general use in humans is at least five to 10 years away.
If an effective vaccine ever does emerge, Dr. Schwartz said, it will probably be a cocktail of several vaccines, each one targeted against a different strain or group of strains.
The Genentech vaccine contains a protein that appears on the surface of the AIDS virus. Once it enters the bloodstream, the immune system mistakenly recognizes it as the virus itself and generates antibodies to ward off infection.