U.S. funding for AIDS vaccine research is tightening, the government’s top HIV expert warned Monday, even as he said scientists still must overcome a big hurdle in the hunt: how to harness the body’s first defenders to repel infection.

Today’s leading vaccine candidates can’t do that, noted Dr. Anthony Fauci of the National Institutes of Health. They work on a different pathway, one of cell-based immunity.

Indeed, scientists are approaching a critical crossroads where they must determine if that cellular approach alone will prove protective enough, said Seth Berkley, president of the International AIDS Vaccine Initiative.

“If cellular immunity turns out to be quite positive, we’ve got a series of candidate vaccines,” Berkley said at a meeting of the American Association for the Advancement of Science. “If it turns out that cellular immunity is not very useful ... it’s a much longer time period” to a useful vaccine.

That question looms just as researchers face tough new battles for U.S. funding.

After years of record budget increases in the 1990s, the NIH next year expects its total budget to increase just half a percent.

Fauci, head of NIH’s National Institute of Allergy and Infectious Diseases, didn’t say how much of that allocation is expected for research into an AIDS vaccine, although he said it remained a top priority.