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.