Roughly 50 million doses of vaccine are expected to be available by mid-October. But for those who get initial doses right away, that will only mark the beginning of a vaccination process that will take five or more weeks.

Here's why: Health officials believe most people will need two shots, spaced three weeks apart, and it will take a week or two after the second dose before immunity kicks in. That's five or six weeks in all.

That means large numbers of Americans won't be fully immunized until Thanksgiving, said U.S. Health and Human Services Secretary Kathleen Sebelius, speaking to reporters in Atlanta.

Health officials don't really know if that's good or bad news. Since it was first reported in April, swine flu has turned out to be not much more dangerous than seasonal flu, overall. Government experts say it may soon become just another variety of the flu, and perhaps will conform to the seasonal flu calendar - vaccinated against in the fall, suffered through every winter.

But swine flu cases have persisted through this summer, especially in camps and other places where kids congregate. It's possible they will explode not long after kids return to schools and colleges this fall.

"Hopefully we will have a mild presentation this fall" with limited cases of serious illness and few deaths, Sebelius said during a visit to the Centers for Disease Control and Prevention.

Clinical trials started just this month to evaluate the new vaccine's safety and effectiveness, and the first substantial set of results is not expected until next month. Health officials presume there will be a fall campaign to encourage people to get swine flu vaccinations that will be called off only if something unexpected and alarming emerges in the clinical trials, CDC spokespeople said.

Also Monday, the White House released a report from the President's Council of Advisors on Science and Technology that assessed the nation's swine flu preparations. The panel predicted 20 to 40 percent of the U.S. population will suffer swine flu symptoms this fall, and about half will get sick enough to go to the doctor.

The panel also estimated the virus will cause between 30,000 and 90,000 deaths, concentrated among children and young adults. Seasonal flu, in contrast, kills an estimated 36,000 people every year.

The panel also recommended that vaccine manufacturers step up production so at least 40 million doses are available by mid-September to start immunizing children and others at higher risk of serious swine flu complications.

However, vaccine makers have said they've had trouble producing large quantities of doses as quickly as was originally hoped.

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