Younger children will need two doses of the vaccine against the new pandemic of H1N1 influenza, according to US officials.
They said tests of Sanofi-Pasteur's swine flu vaccine showed children respond to it just as they do to seasonal flu vaccines, with children over 10 needing only a single dose but children under nine needing two.
Separately, Sanofi said it had won a US government order for 27.3 million more doses of its vaccine and AstraZeneca's MedImmune unit said the US government has ordered 29 million more doses of its needle-free vaccine.
Anthony Fauci, director of the National 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.
"As we had hoped, in children the 2009 H1N1 vaccine is acting just like the seasonal flu vaccine," Dr Fauci told reporters in a telephone briefing.
Children aged 10 to 17 mounted an immune response that should protect them from H1N1 within 8 to 10 days, Dr Fauci said.
The US Centres for Disease Control and Prevention said 46 US children have died from swine flu, which appears to have first emerged in Mexico in March and which spread around the world to cause a pandemic in only six weeks. As soon as the new virus was identified, the US government and companies started making a vaccine against it and testing began in August to ensure it was safe and to determine what dose would be needed.
About 25 companies globally are now making H1N1 vaccine.
The US had ordered 195 million doses of H1N1 vaccine from five makers - GlaxoSmithKline, Sanofi, Australia's CSL, MedImmune and Novartis. With the new orders from Sanofi and MedImmune, that would make more than 250 million doses.
Most countries ordered H1N1 vaccine with the expectation that two doses would be needed, so many now have more than anticipated.
The CDC has designated about 160 million people to be vaccinated first, including pregnant women, people with heart disease, asthma or diabetes and school-aged children.
With seasonal flu vaccine, children under nine who are getting a flu vaccine for the first time need two doses, so Dr Fauci said it is likely that young children who have never had a flu vaccine before will need four doses this year - two seasonal flu doses and two swine flu doses.
But that may not mean four visits to a clinic or doctor's office, the CDC's Anne Schuchat said.
"We do expect, based on what we know about vaccines, that it should be fine for the shot to be given on the same day," she said.
It gets a bit more complicated with MedImmune's nasal spray vaccine, she said. Children getting that needle-free vaccine will likely need to get FluMist, the seasonal version, separately from the swine flu formulated nose spray.
H1N1 is now the dominant strain of influenza circulating globally and Dr Fauci said it is possible it may replace the seasonal form of H1N1 also circulating.
"If you look at the history of where new viruses come in, frequently what they do is come back the next year and displace one or even more than one of the circulating strains," he said.
"It is a distinct possibility that this might ultimately be incorporated into a seasonal flu vaccine."