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 up to 1.5 days a year because of the flu, said David Cutler, a health economist at Harvard. That absenteeism rate could double because of scant vaccine supplies.

“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 says early indications – and milder than usual flu outbreaks in Australia and New Zealand – suggest this flu season in America could be mild.

The most recent statistics from the Centers for Disease Control and Prevention indicate low flu activity in seven states.

“It’s not ‘We’re getting ready to close the school down there are so many cases,’” said Dr. Greg Poland 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 contained in this year’s vaccine sickened up to 30 percent of Americans last year, and that provided some carry-over immunity. Poland cautioned that it is just beginning the flu season, which most often peaks in January or later.

Emergency physicians, however, are bracing for the worst.

“If we have a mild flu season, I will be the first one to be thrilled,” said Dr. Brian Hancock, past president of the American College of Emergency Physicians. “There is a very good chance we will have an influx of patients with flu to emergency departments that are already full.”

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 to Britain in early October, agreed that Chiron’s flu vaccine was not safe to use.

The CDC is working with the other major flu vaccine producer, Aventis Pasteur, to ensure that the next wave of vaccine – 24 million shots – reaches those at highest risk of the flu.

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 sponsor flu clinics this year. Many of those workplace clinics were canceled to funnel vaccine to high-risk individuals, including the very young, the very old and those with chronic medical conditions.