At a Seattle meeting, researchers reported Wednesday that participants who received an experimental Merck & Co. HIV vaccine might be at increased risk of HIV infection compared to those who received a placebo. The divergence was principally noted among those who had pre-existing immunity to adenovirus type 5, a common cold virus that was modified and used as the vector for three synthetic HIV genes. Participants at the HIV Vaccine Trials Network meeting emphasized that the vaccine itself could not have caused infection and that the findings on infection risk could be a statistical fluke. Nonbiological factors such as circumcision rates and sexual practices of participants could explain the number of infections seen among vaccine recipients. One theory suggests that the adenovirus vector may have activated the immune system and made recipients more susceptible to HIV infection upon subsequent exposure. In late September, the trial involving 3,000 volunteers in nine countries was stopped when preliminary analyses found the vaccine neither blocked HIV infection nor curtailed HIV levels in the blood of those who contracted the virus. Among trial participants, there were 49 infections among those vaccinated and 33 infections among those who received a placebo. However, the divergence was greater among 778 males with pre-existing adenovirus immunity: 21 vaccine recipients later became infected, compared with nine infections among the placebo group. Researchers said it is difficult to define statistical significance for that difference, since the trial has been stopped. For those with low pre-existing adenovirus immunity, the difference in infections between vaccinated subjects and the placebo group were not statistically significant. Women accounted for 35-40 percent of volunteers, but just one woman later acquired HIV. The reasons for the gender difference are unknown, but the case was removed from statistical analysis. It will take months to years to complete tests to find out the underlying biological mechanisms for the vaccine's failure, said Dr. Keith Gottesdiener, a Merck vice president. The meeting continues today, when researchers will mull such issues as whether to tell volunteers which trial group (vaccine vs. placebo) they were in and if they had pre-existing immunity to the vector. A recommendation will be made in about 10 days, Gottesdiener said.
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