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Studies of the new swine flu vaccine in the U.S. show that children 10 years old and older need just one shot for protection, but younger kids appear to need two.

The studies have shown that the vaccine provides good protection in children over the age of 10 within eight to 10 days, just as it does for adults, the National Institutes of Health announced Monday.

But kids younger than 10 appear to need two shots 21 days apart, said Dr. Anthony Fauci, head of the NIH's National Institute of Allergy and Infectious Disease.

That's not a surprise, since very young kids often need two doses of vaccine against regular winter flu, particularly the first time they're immunized against a new strain of influenza.

Younger children simply "don't have as mature an immune system," Fauci explained. So the first dose acts as "an introduction" to a flu strain they've never experienced, while the second shot revs up their immune response further.

"They need a little extra . . . a little nudge to the immune system to respond," Fauci added.

They say the finding that one standard sized shot will be enough for adolescents and teens is good news, because that means there will be more vaccine for others.

"This is acting strikingly similar to seasonal flu" vaccine, Fauci said. "Overall, this is very good news for the vaccination program."

The studies have found no significant side-effects in the 600 children who received the vaccine.

But parents of children under age 10 face a potentially complicated flu vaccine schedule this year. That's because officials still recommend kids also get two shots of seasonal flu vaccine. So that could mean that kids under 10 could be getting four shots altogether this fall -- two against regular flu and two against swine flu.

Dr. Anne Schuchat of the Centers for Disease Control and Prevention said it also appears safe to receive shots of each vaccine in each arm on the same visit.

The children's study found that 76 per cent 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

The other quarter did respond to the vaccine, just not as robustly.

But in younger kids aged three to nine years old, only a third had a good immune response to the swine flu shot. Only a quarter of babies and toddlers aged six months to three years responded well, Fauci said.

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

The study results apply only to the vaccine being tested in the U.S., which doesn't use an adjuvant.

There has been no data released yet on the vaccine being developed in Canada, which does use an adjuvant.

"We are dealing with a different vaccine . . . which may have a stronger immune response, even in children, but until the safety data is in we can't really say," infectious disease expert Dr. Neil Rau told Â鶹ӰÊÓ.