An estimated 57 million Americans have contracted pandemic H1N1 influenza since the outbreak began last April, about 257,000 have been hospitalized with complications from it, and nearly 12,000 have died, according to estimates released Friday by the Centers for Disease Control and Prevention.

The total number of infected represents an increase of about 7 million cases since the last estimate was released in December, a modest gain that correlates with other data suggesting the swine flu pandemic has been waning.

Most cases in all categories have involved children and adults younger than 65, a sharp change from normal flu seasons, in which the elderly suffer disproportionately.

Although the death total is much lower than the estimated 35,000 U.S. deaths in a normal flu season, the numbers among people younger than 65 are much higher than normal.

Virtually all cases of influenza that were tested have been caused by the H1N1 virus rather than by seasonal flu viruses, a finding that leads some experts to predict -- rather hopefully -- that the country will not see a regular flu season this year.

Nonetheless, the CDC report says, “flu activity, caused by either 2009 H1N1 or seasonal flu viruses, may rise and fall, but it is expected to continue for several more months.”

thomas.maugh@ latimes.com