Study Unravels Reasons For Failure Of Experimental HIV Vaccine

Trials of a once-promising experimental HIV vaccine were cut short in 2007 because the drug may have increased the likelihood of HIV infection rather than preventing it, according to a new study published in the Journal of Experimental Medicine.

The HIV-1 vaccine, created by American drug maker Merck, was undergoing second stage trials when the problem was discovered in September 2007, according to researchers at the Montpellier Institute of Molecular Genetics in France.

The vaccine relied on a modified form of a common cold virus to carry elements of HIV (Human immunodeficiency virus) into the body, triggering the human immune system to start fighting off later infection with the virus.

But three years after the first trial, researchers discovered that more of the vaccine recipients who had prior immunity to the Ad5 virus had been infected with HIV than those not exposed to the vaccine.

The presence of long-lasting antibodies specifically catering to the Ad5 virus, generated during natural infections with the common cold, could have altered the response to the HIV vaccine, the study said.

HIV infection spread through cell cultures three times faster in the presence of antibodies from individuals immune to the Ad5 virus, because the HIV virus came in contact with more of its preferred "T" cells -- prompted to grow by the vaccine -- to infect.

The study said the vaccine reached the second phase of its trials because primates, used in the first phase, do not naturally come into contact with the human common cold, so the problem went unrecognized.

The vaccine prototype was tested on 700 HIV-negative persons in five hospitals in South Africa between February and September 2007, in the first clinical HIV trial of its magnitude ever conducted in Africa. Courtesy AFP.