WASHINGTON - The manufacturer of the first pain-free flu vaccine sought Tuesday to ease federal scientists' safety concerns by withdrawing its attempt to market the product to children under 5 years old.

A Food and Drug Administration committee was considering whether to recommend approval of MedImmune Inc.'s FluMist, a nasal spray.

The big issue is just how safe MedImmune Inc.'s FluMist is, because the nasal vaccine is made with live flu virus instead of the killed virus in today's flu shots.

In July 2001, the Food and Drug Administration's advisers said FluMist hadn't proved safe enough to sell yet.

Tuesday, MedImmune argued its case again, this time withdrawing its initial attempt to sell the spray vaccine for toddlers - the age most likely to benefit from a pain-free option - to settle one of those safety questions. Instead, if FluMist is allowed to sell, MedImmune said it would be for children 5 and older, because in younger children the spray seems to increase the risk of an asthma attack.

Another complication: No one yet knows if FluMist is safe for anyone over 65 and people with asthma or other illnesses, those most at risk from the flu, because studies in those people haven't yet been done.

"In a way, that makes the safety bar go higher," noted Dr. Martin Myers of the University of Texas, Galveston, one of the FDA advisers debating FluMist's fate Tuesday.

And because it's made with a live virus, there is some concern that vaccinated children might spread flu to unvaccinated people, including other children in day care or their elderly grandparents.

Flu experts have longed for a painless vaccine as a way to persuade more people to seek the annual protection. About 70 million people get today's flu shots each year. Yet tens of millions of Americans get the flu each year, it kills 20,000 of them and hospitalizes about 100,000.

The nasal vaccine works by stimulating the immune system through the same nose tissue where the flu virus attacks. But no direct comparison of nasal versus injected vaccines has been done to prove if one type is any better than the other.

FluMist does offer protection, MedImmune scientists told the FDA advisers Tuesday.

In one study, 1,600 healthy children - ages 15 months to 6 years - got either FluMist or a dummy nasal spray squirted up each nostril. Children who later got sick were tested for influenza. FluMist proved 93 percent effective in preventing the flu.

Side effects primarily included runny nose, muscle aches and fever.

But children under age 5 occasionally suffered asthma attacks or asthma-like wheezing, a fairly rare side effect that FDA estimated could occur in up to 1.5 percent of the youngsters.

So MedImmune responded that FluMist would be for use only by children over 5, who didn't seem to have that asthma risk.

In adults, the vaccine wasn't quite as promising. Some 4,561 healthy, working adults ages 18 to 64 were given either FluMist or a dummy spray, and then reported any symptoms to researchers.

FluMist recipients were just as likely as the nonvaccinated to experience a flulike illness, although MedImmune said vaccination did cut severe illness by about 17 percent, as well as cutting lost days of work and doctor visits.

But the FDA argued that people age 50 to 64 may not experience that benefit.

The FDA's advisers will weigh those studies before voting late Tuesday on whether FluMist is safe and effective enough for sale to certain people. The FDA is not bound by its advisers' recommendations but typically follows them. It's unlikely a decision would come in time for the tail end of this winter's flu season.