Merck and Co's failed AIDS vaccine may not have worked, but it probably did not raise the risk of infection either, doctors said Tuesday.

Data analyzed after the large clinical trial was stopped in 2007 contradict earlier findings that suggested some groups, such as uncircumcised men, may have been more vulnerable to infection if they got the vaccine, Dr. Susan Buchbinder of the San Francisco Department of Public Health told an AIDS vaccine conference.

Merck's STEP trial involved 3,000 people in South America, the United States, Canada and Australia. Researchers were not only dismayed that it appeared to do no good, but may have harmed some of the volunteers.

With ongoing follow-up, the trend in the wrong direction is diminishing, Buchbinder said in an interview. Either they were at risk, and that has gone away, or they were never at increased risk. It was never significant.

Researchers working to produce AIDS vaccines are in Paris this week to pore over the results of the latest experiments.

Of particular interest is a trial in Thailand that has just shown it may be possible to make a vaccine to prevent AIDS; the first hint of success in the 25 years since the pandemic began.

Buchbinder said researchers who worked on the Merck vaccine continue to monitor participants from the trial and are drawing insights from it even now. Lessons learned will be crucial in the making of AIDS vaccines.


Twenty-nine people in the STEP trial who were infected showed a slight decrease in their viral load -- a measure of virus in the blood -- for a short time, she said.

The bigger picture issue is that we see some clues here and some clues there about ways where the vaccine may be providing some protection. Overall it didn't protect, but can we learn something about places where the vaccine may have provided even small amounts of protection so that we can build on those, she said.

The vaccine aimed to fight AIDS by encouraging so-called cell-mediated immunity, jump-starting T-cells to tackle the virus and stop or slow the progress of the human immunodeficiency virus. It used a weakened virus called adenovirus-5 as a vector to transport the vaccine into the body.

Buchbinder said the STEP study also showed the importance of studying the effects of such vectors on the immune system.

We never understood the complexity of the immune response against the vector and we think that is a very important thing to understand. Animal studies can't address that because adenovirus 5 does not normally infect non-human primates, she said.

These efficacy trials are really moving science forward, Buchbinder added.

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, help shine a light on issues that we didn't know or understand before.