The National Institute of Allergy and Infectious Diseases, part of the U.S. government's National Institutes of Health, said on Thursday it was canceling the HIV vaccine study known as PAVE 100.

"However, NIAID believes the vaccine developed by its Vaccine Research Center (VRC) is scientifically intriguing and sufficiently different from previously tested HIV vaccines to consider testing it in a smaller, more focused clinical study," the institute said in a statement.

It is the first demonstration of a new approach being taken by federal vaccine developers after a trial called STEP showed last year that Merck's vaccine may have raised the risk of infection among certain men -- those who had pre-existing immune responses to the virus used in the vaccine and who were not circumcised.

The vaccine used a common virus known as an adenovirus to carry bits of the AIDS virus. "It didn't work, it wasn't beneficial and there were the issues of risk for people who were uncircumcised," NIAID director Dr. Anthony Fauci said in a telephone interview.

"I am entertaining a smaller, leaner, less-expensive trial," Fauci added.

The new vaccine being tested by the Partnership for AIDS Vaccine Evaluation, or PAVE, was designed to be given as three injections of HIV genetic material, followed with a final boost of adenovirus.

"I do believe that it is worthwhile pursuing this concept of a DNA vaccine, but I don't believe that we have enough information to warrant making that a large trial," Fauci said.

Scientists had argued whether it was justified to go ahead with a large trial of a vaccine that could not be expected to protect people against infection in the hope of finding something in their biological responses that could help design a better vaccine down the road.

AIDS has killed 25 million people since it was first identified in the 1980s and it infects 33 million people globally. There is no cure although a cocktail of drugs can keep the infection suppressed.

"An HIV vaccine continues to be our best hope for ending the HIV pandemic," the NIAID said.

'DIFFICULT PERIOD'

Fauci said he decided it would be better to do smaller, more focused tests and if and when one of those gets a result, to follow that lead.

"We are in an interesting and, some would say, difficult period," Fauci said. "The obvious empirical approaches have not worked."

But while about 30 vaccines are being tested, none has come even close to preventing infection in people.

"Because the PAVE vaccine candidate is different from the STEP candidate, we can still learn something from testing the PAVE candidate in humans. It's just not necessary to do so in a trial involving thousands, as called for in the PAVE 100 design," International AIDS Vaccine Initiative president Seth Berkley said in a statement.

"The decision by NIAID does not reflect paralysis in the AIDS vaccine field, or a lack of direction forward. In fact, it reflects the opposite. It reflects the dynamic learning that is the scientific process, that is pharmaceutical product development," Berkley added.