WASHINGTON – Spraying flu vaccine into the noses of babies and preschoolers offers significantly more protection than shots, says one of the largest comparisons of flu inoculations ever performed.
The study, being presented Monday at a child-health meeting, found the spray vaccine was 55 percent more effective than traditional flu shots when given to nearly 8,000 children under age 5.
The nasal spray FluMist, the only flu vaccine made of live but weakened influenza virus, now is sold only for children 5 and older. Manufacturer MedImmune Inc., which funded the new research, plans to seek government approval to sell FluMist for younger children as well.
But to flu experts, the findings have important public health implications. Doctors have long known that flu shots don't work quite as well in very young children as they do in older youngsters or adults.
And children are the prime spreaders of flu virus, fueling infections in older people who may not survive. Each winter, flu kills 36,000 Americans, most of them elderly.
"Our current thinking is that to control influenza, we really have to vaccinate all children," said Dr. Robert Belshe, a prominent vaccine specialist at St. Louis University who led the new study. "Anything that makes it easier and more effective (to vaccinate) children is going to contribute a lot to the protection against influenza."
The study did find a safety concern: A few of the very youngest patients, those ages 6 months to 2 years, had an episode of asthma-like wheezing in the weeks after the first FluMist dose.
The increased risk was slight — 1 percent more children wheezed after FluMist than after flu shots — and the reaction was temporary. But Belshe still is analyzing whether the risk would offset the increased flu protection, and regulators undoubtedly will ask whether it means FluMist should be used only after age 2.
Belshe and colleagues in 16 countries enrolled youngsters ages 6 months to 5 years during the 2004 flu season. Every child got both a nasal spray and a shot, but only one was the real vaccine instead of salt water, to allow unbiased comparison.
By winter's end, just 3.9 percent of nasal-spray recipients also got sick with influenza, compared with 8.6 percent of shot recipients.
More intriguing: That winter, a slightly different strain than was in the vaccine began circulating, and the nasal spray was more protective against that new strain, too.
"It's clear that FluMist is an influenza vaccine whose potential has not yet been either reached or appreciated," said Dr. William Schaffner of Vanderbilt University, a vaccine expert who wasn't involved in the study.
Easier administration aside, it "apparently provides broader protection than does the injected vaccine," Schaffner added. "That's really very, very exciting because we all know the influenza virus has a tendency to drift," or mutate.
Why, biologically, might FluMist work better in tots?
The live-virus nasal spray mimics real flu infection by stimulating immune defenses first in the nose and then body-wide. Flu shots, made with dead influenza virus, don't give the extra nose reaction.
So while flu shots are good at boosting previous immunity in healthy people who've caught flu before or been inoculated many times, more immune-naive babies and preschoolers may need that more flulike nasal response, Belshe said.
His study didn't use the exact version of FluMist sold today, but one slightly altered so that it requires refrigerator instead of freezer storage.
Apart from regular winter flu, the government is experimenting with brewing FluMist for bird flu, just in case the worrisome H5N1 strain ever begins spreading easily among people. The theory is that the extra nasal reaction might prove more protective there, too.
It makes sense, Belshe said. "When it comes to pandemic flu, we're all behaving like young children — we're naive."