They say it also protected two-thirds of monkeys against an HIV-like virus.
Though results of animal studies are not always the same in humans, researchers are encouraged by this early-stage study, which included nearly 400 healthy people. For their next step, they are launching a new vaccine trial that will include 2,600 women in southern Africa who are at risk of HIV infection.
The experimental HIV-1 vaccine is one of five that have progressed to tests of effectiveness in humans.
While previous experimental HIV-1 vaccines have usually been limited to specific regions of the world, this vaccine combines different HIV viruses. The aim is to trigger immune responses against a wide variety of HIV strains, according to authors of the study published Friday in The Lancet medical journal.
"These results should be interpreted cautiously," study leader Dr. Dan Barouch said in a journal news release.
"The challenges in the development of an HIV vaccine are unprecedented, and the ability to induce HIV-specific immune responses does not necessarily indicate that a vaccine will protect humans from HIV infection," he added.
Barouch is director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston. He is also a professor at Harvard Medical School.
Dr. George Pavlakis and Dr. Barbara Felber of the U.S. Cancer Institute wrote an editorial that accompanied the study.
"Despite unprecedented advances in HIV treatment and prophylaxis, the number of people living with HIV infection continues to increase worldwide," they wrote.
"Implementation of even a moderately effective HIV vaccine together with the existing HIV prevention and treatment strategies is expected to contribute greatly to the evolving HIV/AIDS response," the editorial continued. "It is therefore essential that a commitment to pursue multiple vaccine development strategies continues at all stages."
About 37 million people worldwide have HIV/AIDS, and there are 1.8 million new cases a year.