WASHINGTON (Reuters) - H1N1 swine flu has killed as many as 17,000 Americans, including 1,800 children, the U.S. Centers for Disease Control and Prevention reported on Friday.

The H1N1 flu virus (red) is pictured in this handout photograph taken on July 9, 2009 and released on July 13, 2009. REUTERS/Image courtesy of Yoshihiro Kawaoka/University of Wisconsin-Madiso/Handout

The swine flu pandemic put as many people into the hospital as during the normal influenza season -- but most were younger adults and children instead of the elderly, and it was during the months when usually very little or no flu is circulating, the CDC said.

In a separate study that helped shed light on why so many children were seriously affected, researchers reported on four children who developed a serious infection of the heart from swine flu, including one who died.

“CDC estimates that between 41 million and 84 million cases of 2009 H1N1 occurred between April 2009 and January 16, 2010,” the agency said in a statement. Usually the CDC goes with a middle number, which it puts at about 57 million people infected.

Between 8,330 and 17,160 people died during that time from H1N1, with a middle range of about 12,000, the CDC said. But between 880 and 1,800 children died, up to 13,000 adults under the age of 65 and only 1,000 to 2,000 elderly.

In a normal flu season, the CDC estimates that 36,000 Americans die of flu, but 90 percent are over the age of 65. The CDC estimates that 200,000 go into the hospital, again mostly frail elderly people with other health conditions.

The swine flu pandemic has affected much younger people.

The CDC estimate shows that between 183,000 and 378,000 people were hospitalized with H1N1 swine flu from April to January.

In an average flu season, about 82 children die in the United States, the CDC says. But those are lab-confirmed cases.

ACCURATE ESTIMATES

The CDC and the World Health Organization stopped trying to count all the actual cases months ago, once it became clear that H1N1 was a pandemic that would infect millions.

WHO’s count of lab-confirmed cases showed that at least 15,292 people had died in 212 countries and territories.

But WHO and the CDC note there are nowhere near enough diagnostic tests to give to everyone with flu-like symptoms to see if they really have swine flu.

The CDC therefore does its estimates based on models, calculated by looking intensively at small groups of people, gathering data on overall reports of sickness and death, and reconciling the two.

That is also how the CDC comes up with its annual estimates for seasonal flu, and experts agree these estimates are far more accurate than counting confirmed cases.

H1N1 is causing some unusual symptoms. In a report published in the Journal of the American College of Cardiology, Dr. Andras Bratincsak of Rady Children’s Hospital and the University of California San Diego reported on four cases of a heart inflammation called myocarditis among children there in just one month.

“We present the first known report of acute myocarditis in pediatric population associated with the present pandemic H1N1 influenza A virus infection,” they wrote.

Viral myocarditis is extremely rare and one child died and two required extracorporeal membrane oxygenation support or ECMO -- an extreme type of life support similar to heart-lung bypass machines used during open-heart surgery.

“Our observations warrant a high index of suspicion for myocarditis in children with H1N1 influenza A infection. Early detection and aggressive management are paramount,” Bratincsak’s team wrote.