Further analysis has found that Merck's failed AIDS vaccine, MRKAd5, did not actually enhance volunteers' risk of HIV infection as was reported when the trial was halted in 2007, researchers have announced. Initial findings suggested that some vaccinated participants in the Step Study, such as uncircumcised men, may have been at increased risk of subsequent HIV infection.
"With ongoing follow-up, the trend in the wrong direction is diminishing," Susan Buchbinder of San Francisco's public-health department told the AIDS Vaccine 2009 conference. "Either they were at risk -- and that has gone away -- or they were never at increased risk."
The Step Study enrolled 3,000 people from South America, the United States, Canada, and Australia, and researchers continue to monitor its participants, Buchbinder says.
Merck's vaccine used weakened adenovirus-5 as a vector and was designed to encourage cell-mediated immunity, using T cells to stop or slow HIV infection. The Step Study continues to be important in analyzing the effect of such vectors on the immune system, according to Buchbinder.
"We never understood the complexity of the immune system against the vector, and we think that is a very important thing to understand," she says. Animal studies can't address that because adenovirus-5 does not normally infect nonhuman primates."
"These efficacy trials are really moving science forward," Buchbinder says. "With each step we are learning more information that we couldn't get any other way. We don't know what it is going to take to make a safe and effective vaccine. Each of these studies, particularly larger trials in humans, helps shine a light on issues that we didn't know or understand before."
