WASHINGTON, Nov 3, 2008 (AFP) - 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 study published Monday.
The HIV-1 vaccine, which raised hopes in the fight against AIDS as it was being developed by US pharmaceutical giant Merck and Co., was undergoing second stage trials when the problem was discovered in September 2007, said researchers at the Montpellier Institute of Molecular Genetics in France.
The vaccine relied on a modified form of a common cold virus -- Adenovirus 5 (Ad5) -- to carry elements of HIV (Human immunodeficiency virus) into the body.
The smaller HIV parts, the Merck trials contended, would trigger the human immune system to start fighting off later infection with the virus.
One of main worries about the approach was that widespread immunity to the vaccine might cause the drug to be rejected by the body before an effective anti-HIV response could develop.
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, according to the study, published online in the Journal of Experimental Medicine.
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.
Meanwhile, tests had been conducted since 2004 in the United States, Australia, Peru, Brazil and Puerto Rico.
HIV can lead to acquired immunodeficiency syndrome (AIDS).
According to the World Health Organization, 33 million people around the world are infected with the AIDS virus, mostly in the sub-Sahara Africa.
Some two million people died worldwide of AIDS in 2007.
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