The US government has decided not to go ahead with a proposed AIDS vaccine trial called "PAVE 100." The decision comes a few weeks before the 17th International AIDS Conference in Mexico City ? and after a controversy earlier this year about the safety of another AIDS vaccine trial.

To find out what it means for future AIDS vaccine research, VOA English to Africa Service reporter Joe De Capua spoke to Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition. From New York, he spoke about the proposed "PAVE 100" vaccine trials.

"Pave 100 had been an AIDS vaccine efficacy trial that had been in design for quite a number of months. It was actually supposed to start back in September last year. And it's testing a vaccine that's been developed by the NIH's (National Institutes of Health) own vaccine research center. And the vaccine is actually a combination of two products. It has what's called a DNA Prime and an ADNO-5 (cold virus) boost. So two different prime boosts together making one vaccine candidate," he says.

However, just before trials were to begin, news about problems in a Merck vaccine trial came to light. At first, researchers learned the Merck vaccine candidate wasn't effective. Later, it was discovered it actually made some people more susceptible to HIV infection. It did not cause an infection, however.

"Now that vaccine also uses ADNO-5 as its?vector. So, basically a vector is the vehicle that we (use to) try to deliver the elements of HIV (AIDS virus) to teach the immune system," he says. Warren says that this vaccine candidate also could not infect a person with HIV.

"So, all of this caused a lot of extra analysis about the PAVE 100 trial," he says. Last Thursday, Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, decided to cancel it.

Warren considers this "a very big point" in AIDS vaccine history but is not sure whether it will be a big story at next month's International AIDS Conference. "This is an important point on what is an increasingly a long road to an AIDS vaccine and it's an important milestone in many respects. This is the way products get developed. Lots of things don't work and we're on the road to finding something that does," he says.

With the Merck and PAVE 100 trials canceled, what vaccine candidates remain? He says, "It's important to recognize that the pipeline for AIDS vaccine development still includes several dozen concepts that are in phase one and phase two trials (early stages)?. And should they prove successful in those trials, they will then graduate up into these larger efficacy trials."

He says that while the trials may not yield a vaccine, they usually do yield valuable scientific information that can help other candidates. "They have taught us so much more about the body's response to HIV," he says.

The head of the AIDS Vaccine Advocacy Coalition says history shows us it takes a long time to develop a vaccine for any disease.

"It is a setback for sure?. Absolutely. But I think what it does is teach us is an invaluable lesson that we sometimes forget. In our optimism, in the enthusiasm to end the AIDS epidemic, we have to remember what science teaches us. And that is developing any new technology, certainly vaccines, is a long and difficult road. For almost every vaccine we use today?these took decades and decades to develop. We are still on the very same path to an AIDS vaccine. The path is longer than we thought, but these setbacks don't knock us off the road at all," he says.