Filed at 3:26 p.m. ET

WASHINGTON (AP) -- Important unanswered questions remain in the development of a broadly effective AIDS vaccine, which could be a decade or more away, a top AIDS researcher said Friday.

``Do I think in five years we are going to have a vaccine that is going to prevent AIDS? Probably not,'' Dr. Anthony Fauci told the President's Advisory Council on HIV/AIDS.

He said clinical trials are underway on a vaccine that seems to have at least partial effect, and ``some global health good could come of that.''

In countries with high rates of AIDS infection a vaccine of even modest effectiveness can have an impact, he explained.

Last month, researchers told an AIDS conference that a new vaccine is showing promise in early human testing, but they cautioned they are still years away from proving it works.

The approach, called prime-boost, is highly effective in monkeys and early results indicate the same would be true in people.

``There are still a lot of important unanswered questions,'' Fauci said.

Experts believe a vaccine is the only way to stop the worldwide AIDS epidemic, which has already killed 20 million people and infected 40 million more.

Fauci, director of the National Institute of Allergy and Infections Diseases at the National Institute of Health, outlined his agency's AIDS research for the panel.

Asked if he thought an eventual AIDS vaccine would be a one-time injection or would need annual repeats like the flu vaccine, he indicated that periodic boosters will probably be needed.

Fauci also warned against becoming complacent, commenting that ``we might not yet be at the peak of the HIV pandemic worldwide.''

Earlier in the session Hank McKinnell, head of the drug company Pfizer, Inc., announced that his company is freezing the price of Viracept, a widely used AIDS treatment.

``We want all patients to have access to needed medicines,'' he said, promising that Pfizer will not raise the price of Viracept for two years. Currently Pfizer's wholesale price for the drug is $2.02 per tablet and dosage is one tablet twice a day.