The virus vector approach is a common one in vaccine research generally, and HIV/AIDS experts had high hopes for the Merck vaccine, which was meant to be tested in more than 3,000 volunteers uninfected with HIV.
Unfortunately, the vaccine failed to deliver. In September, a preliminary analysis of the data showed no statistical difference between those who got the shot and those who got a placebo, in terms of new infections. The trial was halted at that time.
Reporting last Wednesday at a scientific meeting in Seattle, the research team said an updated review of the numbers had since revealed a widening gap in infections that actually favored the placebo.
So far, the researchers said, 49 of 914 men vaccinated have tested positive for HIV, compared to 33 of 922 men who got the placebo shot.
And in a puzzling twist, individuals who had higher levels of preexisting immunity to the adenovirus before vaccination were actually much more prone to developing HIV infection, compared to participants with low levels of immunity, the researchers said.
Among 778 male volunteers with a high level of preexisting adenovirus immunity, 21 of those vaccinated are now HIV-positive, versus nine in the placebo arm of the trial.
In terms of vaccine's effectiveness at slowing progression from HIV infection to AIDS, the trial was also a bust. Among participants infected with HIV, researchers have so far seen no difference in "viral load" -- HIV levels in the blood -- between those who received a shot and those who did not.
The vaccine's failure comes as a disappointment to AIDS researchers, the experts said. However, the notion that the vaccine actually heightened users' risk for infection is still far from certain, they added.
First of all, the numbers of cases of new infection recorded in the trial simply didn't reach statistical significance, Fauci said. However, the trend "is noticeable enough that
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