At least 60 million people in the United States have been vaccinated against swine flu, and children are twice as likely as adults to have been innoculated, a top health official said Tuesday.

"We think right now at least 60 million people have been vaccinated" against pandemic H1N1 flu, said Anne Schuchat from the US Centers for Disease Control and Prevention (CDC).

Her findings were based on extrapolating data from separate CDC and Harvard University surveys conducted earlier this month on swine flu vaccine uptake.

The CDC survey showed that about 46 million people had been vaccinated by the week of December 6-12, while the Harvard survey, conducted 10 days later, showed around 56 million people had been vaccinated, said Schuchat.

Both studies showed that children were more likely to have been innoculated against swine flu than adults, she said.

"That's good news because usually with seasonal flu, there's a lot more coverage of adults than kids. With the H1N1 vaccine we were targeting children because they have been so hard-hit by the virus," Schuchat said.

US health authorities had acknowledged in October that there was a swine flu vaccine shortage, and that manufacturers were not likely to catch up until December.

But President Barack Obama on Tuesday said there was now plenty of the vaccine to go around, adding that he and his wife, Michelle, have been vaccinated and urging other Americans to do the same.

Some 100 million doses of A(H1N1) vaccine are now available to the public in the United States.

Seasonal flu usually hits the elderly hard, but swine flu has impacted children and young adults the most.

Schuchat also insisted that children under 10 need a second, follow-up swine flu shot, in spite of a report on the website of the Journal of the American Medical Association (JAMA) which said one dose was enough in infants and young children.

"The study in JAMA was one single study with one particular vaccination in one population at one point in time," Schuchat said.

The study reported in JAMA - which showed that most infants and very young children were immunized against swine flu after just one shot - was conducted in Australia on 346 children using a vaccine manufactured in Australia.

"There are other studies done of the H1N1 vaccine under the coordination of the coordination of the NIH (US National Institutes of Health), and it's through those NIH-organized studies that we strongly believe that two doses of vaccine are needed in children under the age of 10," Schuchat said.