The figures are not a prediction but a possibility, said epidemiologist Marc Lipsitch of the Harvard School of Public Health, who helped prepare the 86-page report that is available on the White House website.

Researchers expect the high incidence of infections because the new flu is dramatically different from strains that have been circulating in recent years, so that the bulk of the population has no residual immunity.

"This isn't the flu that we are used to," said Kathleen Sebelius, secretary of the Department of Health and Human Services, at a news conference at the CDC headquarters in Atlanta. "We won't know until we are in the middle of the flu season how serious the threat will be."