UPDATE 2-H1N1 has killed 3,900 Americans, U.S. CDC says

* 22 million Americans infected

* 540 children killed

* Numbers are better estimates, not worsening of pandemic (Adds details throughout)

By Maggie Fox, Health and Science Editor

WASHINGTON, Nov 12 (Reuters) - H1N1 swine flu killed an estimated 3,900 Americans from April to October, including more than 500 children, U.S. health officials said on Thursday.

More complete data and more comprehensive calculations than previously released now show that, in the first six months of the pandemic, H1N1 infected an estimated 22 million Americans and put 98,000 in the hospital, the U.S. Centers for Disease Control and Prevention said.

Of these totals, children account for 8 million of the infected, 36,000 of those in hospital and 540 deaths.

"I have already seen a larger number of deaths than we have had for several years," the CDC's Dr. Anne Schuchat told reporters. "I do believe the pediatric death toll from this pandemic will be extensive and much greater than what we see with seasonal flu."

In an average flu season, about 82 children die in the United States, the CDC says. But those are lab-confirmed cases, which Schuchat points out are far lower than the kinds of estimates released on Thursday.

For the first six months of this pandemic, 129 U.S. children who died had lab-confirmed H1N1 flu.

The CDC said swine flu is causing the worst flu season in the United States since 1997, when current measurements started.

"What we are seeing in 2009 is unprecedented," Schuchat said. "Influenza is really serious. The vaccines we have are the best way to protect patients."

But the vaccines are being produced and distributed slowly. Schuchat said 41.6 million doses were available or had been distributed but this is far below what had been expected.

"Last week we did mention that we had been led to expect about 8 million doses this week," Schuchat said. "Based on what we have today, we aren't expecting to meet that estimate that the manufacturers gave us."

Many things can go wrong with flu vaccine production, Schuchat noted.

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