Washington  - The death toll from swine flu is much higher than previously thought, with the virus having claimed an estimated 3,900 lives in a six-month period, the nation's disease control agency said Thursday.

That number included an estimated 540 children under age 18, more than four times the 129 pediatric deaths reported as late as last week, the Centres for Disease Control in Atlanta, Georgia, said.

A similar comparison on adult deaths could not be made because the agency did not release cumulative tallies for those age groups in the past.

Dr Anne Schuchat, a top CDC official who has been in charge of H1N1 prevention and control, attributed the huge jump in the death toll to a new method of counting that includes people who died of secondary infections such as pneumonia or who did not have laboratory confirmation that they had swine flu.

"It's not a switch or a change from how we've been counting cases. It just gives us a bigger picture," she told reporters. "We don't think things have changed from last week to this week. We believe the 540 number is a better picture of what is out there."

The new numbers are even more sobering than first thought. In a normal flu season, a range of 40 to 88 children die of seasonal flu, she said.

A similar comparison on adult deaths could not be made because the agency did not release cumulative tallies for those age groups in the past.

Schuchat described the fatality level as "pretty severe."

Under the new, broader counting methods, CDC estimated that 22 million people have contracted the H1N1 virus in the US from mid- April to October 17. That includes 8 million children, 12 million adults ages 18-64 and 2 million people over 65.

The flu pandemic, a newly emerging virus that surfaced in April in Mexico, has reversed the pattern of normal seasonal flu, hitting children and young adults hardest.

In normal seasonal flu, those 65 and older are the most vulnerable.

"We know that a number of deaths we are seeing are occurring outside hospital, where testing is not possible," Schuchat said. "Not every person with influenza gets a flu diagnosis."

Bacterial pneumonia has been a particular problem connected to the H1N1 virus. The new numbers, she said, are "probably a more accurate picture of the full scope of the pandemic."

Schuchat cautioned that the number of 98,000 hospitalized, for example, was the middle of a range that could go as high as 153,000. Likewise, the extimated death toll of 3,900 could be as low as 2,500 and as high as 6,100.

The new counting method drew information from two databases: one that collects information from counties in 10 states and a second that gives an aggregate of US states and territories.

The CDC is in charge of distributing scarce H1N1 vaccine to the states, with set priorities for pregnant women, children, caretakers of children and people with chronic disease such as asthma and diabetes.

To date, more than 40 million doses have been made available nationwide, less than one-third of the ultimate goal of 130 million doses.

The scarcity of vaccine has provoked criticism nationwide as people form long lines to get shots.

"We are making progress," Schuchat said. "It's a marathon, not a sprint."

Experts said even vaccine given months from now in the northern hemisphere's spring, at the end of the seasonal flu season, would help protect against the H1N1 virus. (dpa)