New Delhi: Scientists told that virus used in experimental HIV vaccines given to volunteers in India, Europe and South Africa weakens the immune system defenses against the AIDS virus.

Researchers at The Wistar Institute in Philadelphia, who led the study, said their findings show possibility of vaccines might do more harm than good.

Hildegund Ertl, director of the Wistar Institute Vaccine Center in Philadelphia, said, “Mice given a vaccine containing Adeno-associated virus had slow-growing immune cells that produced low amounts of protective chemicals. The same virus, which doesn’t cause symptoms, is used in an HIV shot being developed by Targeted Genetics Corp, working on HIV vaccines since 2000.

The latest study may pressure researchers to rethink methods to develop a vaccine against AIDS virus. Scientists are analyzing the failure of Merck & Co.’s vaccine that may have linked to cold virus “called a vector,” used to deliver proteins that trigger immune response.

Wistar researchers found immune system cells, called CD8 T (help in destroying HIV-infected cells) are impaired by AAV vaccines.

Ertl said, “The AAV vaccines against HIV may do more harm than good by robbing people of their natural immune response to HIV. If, after a vaccination, you have impaired CD8 T cells that cannot proliferate, you could be worse off than if you did not have the vaccine.”

Meanwhile, other researchers said that relevance of the study’s findings to humans is unclear as they may be the effect of high dose of AAV given to mice by the Wistar scientists.

Patricia Fast, medical affairs director of the New York-based International AIDS Vaccine Initiative, an agency leading a global effort to find a safe and effective HIV vaccine, “The dose given to these mice was equivalent to 3,000 to 4,000 times the highest dose given to humans in the study in India.”

Earlier, the Indian and European trials of AAV-based HIV vaccines were concluded. Officials of AIDS vaccine initiative and scientists at Pune’s National AIDS Research Institute, who conducted Indian trials, said that vaccine has had no serious effects on any of volunteers.

Satyajit Rath, a scientist at Delhi’s National Institute of Immunology, “These findings are a reminder of just how little we know about the human immune system.”

“We gave the vaccine only to healthy volunteers who are not at risk of picking up HIV,” said, institute director Ramesh Paranjpe.

The vaccine tested in India, Belgium and Germany generated an immune response in only 25 per cent of volunteers at the highest dose tested.

The study is published in the Journal of Clinical Investigation.