A vaccine designed to provide immunity against HIV actually infects animal test subjects, researchers at the Harvard Medical School (HMS) recently discovered, dealing a serious blow to hopes that a vaccine can be developed in the near future.

HMS researchers, led by Assistant Professor of Medicine Ruth Ruprecht, tested the vaccine on eight newborn monkeys and 16 adults, according to data published in the February issue of Nature Medicine.

After three to five years, one of the adults died of AIDS and levels of the virus rose for four others. Six of the infants developed AIDS, while the other two showed damages to their immune systems.

Researchers infected the monkeys with a weakened strain of the virus that has a limited ability to replicate itself.

The less potent virus mimics HIV but is missing a single gene, known as the "nef" gene. Ronald Desrosiers, professor of microbiology and molecular genetics at HMS, developed the strain used for these trials.

Scientists had hoped the virus would infect immune system cells, but would prove unable to replicate itself to the point that it would seriously damage the host's immune system.

A population of infected but controlled cells would allow the host to develop the immune response necessary to defeat the real virus. Disabled viruses have offered the solution to two of the modern era's most problematic viruses, smallpox and polio.

But the HMS researchers' theory fell victim to the resourcefulness of HIV. The weakened virus used in the experiment mutated, allowing it to debilitate the host.

These results demonstrate the adaptability of the virus as well as the danger of using vaccines derived from HIV.

"With these results," Ruprecht said, "the whole approach is under question."

The idea that a less virulent strain of HIV might offer a path to immunity is only one of several options being considered, but it is unique because of the dangers involved.

For example, other vaccines attempt to introduce the immune system to the surface proteins of HIV. But when these methods fail, the results are less dire because proteins are not genetically capable of infecting a cell.

Previous experiments led researchers to believe that the most promising vaccine to fight HIV would use a weakened virus, but this idea now has to be revised, Ruprecht said.

Currently, the dominant treatment for HIV involves a cocktail of drugs that attack the virus once it has infected. The drugs can significantly reduce the presence of the virus but there is little evidence to suggest that it can actually cure the virus.