Still need a flu shot? Matthew Stefanak has so many left over he is giving them away by the carload.

"I sent out a blast fax to 700 physicians in the Youngstown area offering to give it away if they just come pick it up," said Stefanak, the health commissioner of Mahoning County, Ohio, which includes Youngstown. So far, he said, there have been few takers.

Two years ago the nation was plagued by a severe shortage of flu shots, with huge lines at clinics and many people going without. This year it looks like there may be a glut.

Yet, somewhat perversely, because of distribution delays earlier in the season, this year's abundant supply has not meant that everyone who wanted a flu shot has received one.

The situation underscores the fragile nature of the nation's supply system for flu vaccine, a risky and volatile business with thin profits, in which the federal government has a limited role.

Because makers, distributors, doctors and health departments lose money from vaccine they cannot sell to patients, they may be discouraged from making or ordering as much in coming years — potentially leading to future shortages.

And there is little opportunity to stockpile for lean years, because flu vaccines must be changed each year to match the strains of influenza virus in circulation.

Some manufacturers dropped out of the flu vaccine business in the past, in part because they could not sell all they made. That set the stage for the shortage in 2004, when there were only two major suppliers and one of them, Chiron, had to suspend production because of sanitary problems.

This time, experts say millions of doses might go unused, in part because production is at an all-time high. The four vaccine suppliers are expected to make as many as 110 million to 115 million doses, with more than 100 million having already been delivered. The most vaccine ever distributed in any previous year was 83 million doses.

If drumming up demand was a challenge, the task was made harder by manufacturing and distribution snags that caused shortages in some places in September, October and early November — the months when most people are accustomed to getting vaccinated.

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When millions of doses did finally get delivered in late November or earlier this month, it was too much, too late, many health officials say. Holiday distractions make it difficult to get people to come for immunization in December. Also, the flu season has been relatively mild so far this year, making a flu shot seem less urgent.

"There's so many people out there that need it, and we know didn't get it yet," said Dr. Henry H. Bernstein, a pediatrician at Dartmouth Medical School and member of the infectious diseases committee of the American Academy of Pediatrics.