Studies of the new H1N1 vaccine show children 10 and older will need just one shot for protection against swine flu — but younger kids will need two.

The National Institutes of Health says that protection kicks in for older children within eight to 10 days, just like it does for adults.

But the preliminary data shows that younger children may need two shots 21 days apart — not a surprise, since the very young often need two doses of vaccine against regular winter flu the very first time they're immunized against that influenza version.

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Dr. Anthony Fauci, director of the U.S. Institute of Allergy and Infectious Diseases, said young children will likely need to have their doses 21 days apart, but he said they could receive seasonal flu shots and H1N1 shots on the same day — something that could ease the logistics of vaccinating children multiple times.

"Immunologically this is acting like seasonal flu and we are very pleased with that," Fauci told reporters in a telephone briefing. "The response in younger children is less robust but that is not unexpected."

The U.S. Centers for Disease Control and Prevention said 47 U.S. children have died from swine flu.

The children's study found that 76 percent of 10- to 17-year-olds showed good protection with one dose of H1N1 vaccine. That's similar to protection from the regular winter flu vaccine — and doesn't mean the other quarter didn't respond at all, just not quite as robustly.

But just over a third of 3- to 9-year-olds had a good response to the swine flu shot, and only a quarter of babies and toddlers ages 6 months to 3 years, Fauci said.

That response was measured eight to 10 days after the shot, and flu protection usually builds over several weeks so the numbers could improve somewhat, he said — but he wasn't optimistic that the under-10 crowd would be able to skip the booster dose.

Younger children simply "don't have as mature an immune system," Fauci explained. So a first dose of vaccine against a flu strain they've never experienced acts as an introduction to it, while a booster shortly thereafter revs up their immune response.

The study, government officials said Monday, didn't examine FluMist, the nasal-spray flu vaccine. The first 3.4 million doses of swine flu vaccine to be shipped early next month will be the FluMist version, which can be used by healthy people ages 2 to 49. But health officials said they expected FluMist to act similarly in all age groups.

The vast majority of swine flu vaccine will be the shot version, and about 45 million doses of the shots are expected to be available around Oct. 15, with more shipping each week. The U.S. has ordered a total of 195 million doses, enough to meet anticipated demand.

The Associated Press and Reuters contributed to this report.