Washington, Nov 13 - At least 22 million Americans have come down with swine flu since the virus first surfaced in April and approximately 3,900 people have died, including an estimated 540 children, according to an US health official.

"What we are seeing in 2009 is unprecedented. As people know, we haven't had a (flu) pandemic since 1957," Anne Schuchat, director of the US Centres for Disease Control and Prevention's National Centre for Immunisation and Respiratory Diseases, said at a news conference Thursday.

"To have high rates of influenza in September and October is extremely unusual," she said. "We have already seen a larger number of paediatric deaths than we have seen for several years," she added.

"I do believe that the paediatric death toll from this pandemic will be extensive, and much greater than what we see with seasonal flu." In a normal year, there are 60 to 80 paediatric deaths from seasonal flu, Schuchat noted.

The statistics released Thursday only run through mid-October, Schuchat said. Swine flu infections have picked up significantly since then.

According to Schuchat:

* An estimated eight million children under age 18 have been infected by the swine flu.

* An estimated 98,000 Americans have been hospitalised by the swine flu, including 36,000 children.

* Among adults, there have been 12 million cases on infection, 53,000 hospitalisations and 2,900 deaths.

* Among those 65 and older, there have been two million infections, 9,000 hospitalisations and 440 deaths.

Schuchat said vaccination was the best protection against H1N1 swine flu.

As of Thursday, there were 41.6 million doses of the vaccine available. Original estimates had put the vaccine supply at 40 million doses by the end of October and 190 million doses by the end of the year.

So far, 94 million doses of seasonal flu vaccine have been distributed. Manufacturers plan to have an additional 18 million doses available early next year, Schuchat said. (IANS)