Overall, 20 percent to 40 percent of the population could develop symptoms of the strain commonly known as swine flu, and 30,000 to 90,000 could die, according to the report.
During a normal flu season, the virus kills about 35,000 Americans.
The numbers confirm those previously released by the Centers for Disease Control and Prevention, said CDC director Dr. Thomas R. Frieden, but he emphasized the great unpredictability of flu outbreaks and cautioned that this winter's could be much milder.
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."
The report also made several recommendations, including:
- A senior member of the White House staff should be responsible for coordination of all decision-making about the pandemic. The most likely candidate would be the president's homeland security advisor.
- The CDC's surveillance systems should be expanded to improve the chances of detecting new variants of the virus.
- The government should accelerate production of vaccines to have them become available in the middle of September rather than the middle of October, as currently planned. That would allow an initial immunization of 40 million of the most vulnerable people, including pregnant women, health workers and children.
To achieve that goal, vaccine manufacturers should be allowed to fill vaccine vials and prepare them for shipping while they await results from the clinical trials, a process known as "fill and finish." The process normally takes a month, and waiting for results from clinical trials before beginning it would significantly delay distribution.
Sebelius said that manufacturers have been given such permission.
She also noted that no final decision had been made about using the vaccine. But, she said, "We are anticipating moving ahead with vaccination" if the clinical trials show that it works and do not raise any questions about safety.