Studies of the new H1N1 (swine) flu vaccine show children 10 and older will need just one shot for protection against swine flu - but younger kids almost certainly will need two.
Protection kicks in for older children within eight to 10 days, 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 shot, 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."
But how reliable are the age guidelines -- 10 and older, one shot, younger than 10, two shots?
Fauci told "Early Show" co-anchor Harry Smith on the broadcast Tuesday, the guidelines aren't official yet, but said he feels "pretty confident about it."
"We tested the vaccine in children of various ages from 6 months up to 17 years. And what we saw with regard to the response, first the safety looked good. But the response that they gave, which means a measurement of how you would predict would they be protected with this vaccine, and that looked good. (It was) similar to the seasonal flu vaccine, which we give every year to children."
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.
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.
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 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.
Responding to critics, Fauci said the government's response to the H1N1 flu has been prompt.
"This virus was just recognized for to the first time in April in Mexico and in California and Texas," he said, "And to get to the point where we now have a vaccine that's ready to be queued up to be given out in early to mid-October. ... We have orders that will keep them coming in, in a flow right through the fall, and the winter."
He added, "And we're learning. ... Just about a week and a half ago, we did not know if this (vaccine) would be able to induce a response that would be protective. We know now that in adults and elderly we can, and we just found out yesterday that in children that's also the case, that the kinds of things that we would expect from the seasonal flu vaccine, we're seeing exactly the same with the H1N1 vaccine."
Who needs which flu vaccine?
Who needs vaccine against regular winter flu, and who should be first in line for the swine flu shot? There's lots of overlap.
Regular flu vaccine is available now, and people who need it most include:
-Adults 50 and older.
-All children age 6 months to 18 years.
-People of any age with chronic health problems like asthma, heart disease or a weakened immune system.
-Caregivers to the high-risk, including babies younger than 6 months.
Don't like shots? There's a nasal spray version of the vaccine, called FluMist, available for people ages 2 to 49.
Once the swine flu vaccine starts arriving next month, the government says first in line will be:
-The young, from age 6 months up through age 24.
-Younger and middle-aged adults with those chronic health conditions.