"It was a big decision," Dr. Jose Esparza, chief of vaccine development for WHO's Global Programme on AIDS, said in an interview Thursday. "There is an instinctive reaction of people to say no to vaccination with (the live AIDS virus). . . . There are concerns, and they should be addressed. But the reaction to that concern should not be to say no."

Anthony Fauci, a respected scientist who is also the top AIDS official at the U.S. National Institutes of Health, said the U.S. government is not opposed to financing research on live-virus vaccines. But, he said, top priority will continue to go to safer genetically engineered vaccines, which are formulated using only a protein from the virus's outer membrane, or "envelope," and thus carry no risk of infection.

"I think ethically speaking, if (a live-virus vaccine) is developed in the U.S., it perhaps should be tested in the U.S.," said David Ho, director of the Aaron Diamond AIDS Research Institute in New York. "Otherwise everyone involved is going to be accused of taking advantage of a developing country."