Trials of Merck's experimental HIV vaccine were halted in September 2007 because the drug might have increased the likelihood of contracting the virus rather than preventing it, according to a study published Monday in the Journal of Experimental Medicine, AFP/Google.com reports.

The vaccine was based on the idea that a modified form of a common cold virus -- Adenovirus 5 -- would carry elements of HIV into the body, which would then trigger the immune system to start fighting a subsequent HIV infection. An initial concern was that widespread immunity to the vaccine might cause it to be rejected by the body before the body could develop an effective response against HIV. However, three years after the trial began, researchers at the Montpellier Institute of Molecular Genetics in France said that more of the recipients who had prior immunity to the Ad5 virus had contracted HIV than those who had not received the vaccine. The study found that the presence of long-lasting antibodies specific to the Ad5 virus, which were generated during natural infections with the common cold, could have altered the response to the vaccine. Furthermore, HIV spread through cell cultures three times faster in the presence of antibodies from individuals immune to the Ad5 virus because HIV came in contact with an increased amount of CD4+ T cells to infect. In addition, the study found that the problem was not documented during Phase II trials because nonhuman primates, which were used in Phase I trials, do not naturally come into contact with the human common cold (AFP/Google.com, 11/3).

The study is available online.