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WASHINGTON -- Studies of the new swine flu vaccine show children 10 and older will need just one shot for protection -- but younger kids almost certainly will need two.
Protection kicks in for older children within eight to 10 days of the shot, just like it does for adults, the National Institutes of Health announced Monday.
But younger children aren't having nearly as robust an immune reaction to the swine flu vaccine, and it appears they'll need two shots 21 days apart, said Dr. Anthony Fauci, head of NIH's National Institute of Allergy and Infectious Disease.
That's 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 version of influenza, too, Fauci stressed.
"This is acting strikingly similar to seasonal flu" vaccine, Fauci said. "Overall, this is very good news for the vaccination program."
It means that most people in the U.S. will have to line up for influenza vaccinations twice this year instead of three times -- once for the regular winter flu shot and a second time to be inoculated against swine flu, what doctors call the 2009 H1N1 strain.
But here's a twist: If a very young child happens to be getting their first-ever seasonal flu vaccination this year, that tot would need a total of four shots -- two against regular flu and two against swine flu.
Once swine flu shots start arriving next month, it will be OK for kids -- or people of any age -- to get one in each arm on the same visit, said Dr. Anne Schuchat of the Centers for Disease Control and Prevention. But doctors already have supplies of regular flu vaccine, and the CDC wants people to go ahead and get that first inoculation out of the way now.
"This is going to be a complicated flu season," Schuchat warned.
Also Monday, the U.S. ordered more swine flu vaccine, bringing its eventual total to 251 million doses, up from the long-planned 195 million doses. That's an ambitious undertaking for a country where fewer than 100 million people normally get a winter flu shot. The extra orders were from Sanofi Pasteur's Pennsylvania flu-shot factory and from Maryland-based MedImmune LLC, which makes a nasal-spray version.
The new swine flu seems no more deadly than regular winter flu, which every year kills 36,000 Americans and hospitalizes 200,000. But there's an important difference: This H1N1 strain sickens younger people more frequently than the over-65 population who are seasonal flu's main victims. So children are among the priority groups who are supposed to be first in line once swine flu vaccine starts arriving next month, and many schools around the country are expected to offer mass vaccinations.
To determine the right child dose, the NIH set up studies involving 600 children, from babies to teenagers.
About 76 percent of 10- to 17-year-olds showed strong protection after one H1N1 shot. That's similar to the protection seen with regular winter flu vaccine. It doesn't mean the rest didn't respond at all, just that they didn't have as strong a response, officials cautioned.
But just over a third of 3- to 9-year-olds showed strong protection from 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. Doubling the dose in the shot from a standard 15 micrograms of antigen to 30 micrograms didn't improve the response.
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 for their immune system, and a booster shortly thereafter revs up that immune response.
Side effects are no different from those with regular flu vaccine, such as redness and soreness at the injection site and occasional low fever or headache.
Monday's study 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 young children to need two doses of FluMist as well.