This Oct. 5, 2007 file photo shows a nurse filling a syringe with a flu vaccination at a CVS pharmacy in the Brooklyn Borough of New York. U.S. health officials say swine flu could strike up to 40 percent of Americans over the next two years and as many as several hundred thousand could die if a vaccine campaign and other measures aren't successful.[Agencies]

The estimates by the Centers for Disease Control and Prevention are roughly twice the number of those who catch flu in a normal season and add greater weight to hurried efforts to get a new vaccine ready for the fall flu season.

Swine flu has already hit the United States harder than any other nation, but it has struck something of a glancing blow that's more surprising than devastating. The virus has killed about 300 Americans and experts believe it has sickened more than 1 million, comparable to a seasonal flu with the weird ability to keep spreading in the summer.

Health officials say flu cases may explode in the fall, when schools open and become germ factories, and the new estimates dramatize the need to have vaccines and other measures in place.

A world health official said the first vaccines are expected in September and October. The United States expects to begin testing on some volunteers in August, with 160 million doses ready in October.

The CDC came up with the new projections for the virus' spread last month, but it was first disclosed in an interview this week with The Associated Press.

The estimates are based on a flu pandemic from 1957, which killed nearly 70,000 in the United States but was not as severe as the infamous Spanish flu pandemic of 1918-19. The number of deaths and illnesses from the new swine flu virus would drop if the pandemic peters out or if efforts to slow its spread are successful, said CDC spokesman Tom Skinner.

"Hopefully, mitigation efforts will have a big impact on future cases," he said. Besides pushing flu shots, health officials might urge measures such as avoiding crowded places, handwashing, cough covering and timely use of medicines like Tamiflu.

Because so many more people are expected to catch the new flu, the number of deaths over two years could range from 90,000 to several hundred thousand, the CDC calculated. Again, that is if a new vaccine and other efforts fail.

In a normal flu season, about 36,000 people die from flu and its complications, according to the American Medical Association. That too is an estimate, because death certificates don't typically list flu as a cause of death. Instead, they attribute a fatality to pneumonia or other complications.

Influenza is notoriously hard to predict, and some experts have shied away from a forecast. At a CDC swine flu briefing Friday, one official declined to answer repeated questions about her agency's own estimate.

"I don't think that influenza and its behavior in the population lends itself very well to these kinds of models," said the official, Dr. Anne Schuchat, who oversees the CDC's flu vaccination programs.