The three-decade-old quest for an AIDS vaccine received a shot of hope Monday when developers announced that a prototype triggered the immune system in an early phase of human trials.

Tested in 393 people in the United States, Rwanda, Uganda, South Africa and Thailand, the drug “raised antibody responses in 100 percent of vaccine recipients,” Dan Barouch, a member of the research team, said in Paris.

“These promising… data, together with advances from many other investigators in the field, support a new sense of optimism that development of an HIV vaccine might in fact be possible,” he told journalists at an HIV science conference organised by the International AIDS Society (IAS).

Developing a vaccine to stop HIV is thought to be among the most daunting challenges in medicine for one big reason: The virus is extraordinarily genetically diverse, even more so than the flu. So it’s difficult to think about how a single shot might work against all the different HIV subtypes circulating around the world.

But scientists may be inching toward a vaccine that could tackle HIV’s genetic diversity and prevent the virus from taking hold in people.

Researchers from the National Institutes of Health and Johnson & Johnson at the International AIDS Society conference in Paris Monday morning presented data on a clinical trial of what’s called the “Ad26-env mosaic vaccine.”

The mosaic vaccine was developed using a computer algorithm to analyze HIV data from around the world and select a range of HIV sequences to include in a shot. It’s called a “mosaic” because it involves taking pieces of different viruses and sticking them together to generate immune responses that can cover a broad range of HIV subtypes.

“One of the great challenges for development of HIV vaccine is viral diversity,” said Dan Barouch, a lead researcher on the vaccine and director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston. “The mosaic strategy is one way to attempt to deal with the global virus diversity.”

A word of caution, though: We’re only in the early phase of human testing, and the vaccine could ultimately fail to prevent the virus from spreading among people. But so far, the HIV community is carefully watching the Ad26 vaccine research because the approach seems quite promising.