Washington ? This year’s flu vaccine shortage could cost the nation up to $20 billion in lost productivity — almost twice as much as in a typical year — depending on the severity of the outbreak, according to one estimate.

The average worker misses about one to one and a half days a year because of the flu, said David Cutler, a health economist at Harvard. That absenteeism rate could double because of scant flu vaccine supplies this year.

“There’s an enormous margin of error: how bad the flu will be,” Cutler said of his estimate. “That’s where the real uncertainty is.”

One vaccine expert said early indications — and milder flu outbreaks in Australia and New Zealand — suggested this flu season in America could be mild.

The most recent statistics from the CDC indicate low flu activity in seven states.

“It’s not ‘We’re getting ready to close the school down, there are so many cases,'” Dr. Greg Poland said of the sporadic reports. Poland is director of the Mayo Clinic’s vaccine research group and one of the government’s vaccine experts.

The dominant virus strain, so far, is contained in this year’s vaccine and sickened up to 30 percent of Americans last year, providing some carryover immunity. Poland cautioned that the nation is just beginning the flu season, which most often peaks in January or later.

Federal authorities learned in early October that Chiron Corp. would be unable to supply 46 million to 48 million flu vaccine doses, due to British regulatory action. The FDA, which sent inspectors in early October, agreed Chiron’s flu vaccine was not safe to use.

The Centers for Disease Control and Prevention is working with Aventis Pasteur to ensure the next wave of flu vaccine — 24 million shots — reaches those at the highest risk.

People lined up to receive flu vaccinations fill out forms outside the Peak Senior Center in Lansdale, Pa. The first 250 in line, some of whom had been waiting since before dawn Thursday, were given numbers to receive shots from the Montgomery County Health Department; the rest were turned away.

The number of high-risk individuals seeking shots exceeds the 58 million flu doses Aventis expects to provide this year. It’s unclear how many healthy Americans will receive flu shots.

Employers typically purchase 10 million to 20 million flu shots to sponsor flu clinics at work. Sixty percent of companies responding to a Society for Human Resource Management survey in June said they would offer flu clinics this year. Many of those workplace flu clinics were shelved to funnel vaccine to high-risk individuals, including the very young, the very old and those with chronic medical conditions.

This year’s flu vaccine shortage could cause deaths to spike by 25 percent, said Dr. John Treanor, an infectious disease expert at the University of Rochester Medical Center. In a typical year, 36,000 Americans die from the flu. That mortality figure rises to 51,000 when flu-related complications, like heart attacks and strokes, are included.

Health and Human Services Secretary Tommy G. Thompson on Tuesday again urged seniors to get vaccinated to protect against pneumonia, another flu complication.

Merck & Co., the sole supplier of that vaccine, on Thursday said it would make an additional 11 million doses of Pneumovax 23. That raises this year’s production to 17 million doses. In a typical year, the company sells 6 million to 7 million doses of the pneumococcal vaccine.

Vaccinating healthy Americans against the flu, some studies indicate, is cost-effective, saving at least $13.66 per person.