New York ? If the United States wants to avoid future shortages of flu vaccine it must take steps to draw drug companies back into the business of making the inoculations, flu experts say.

In a bad year, the stakes could be higher than just saving people from fever, headaches and a runny nose.

“What if you had 20 or 30 percent of your population not able to go to work or to school? It would affect the economy. It would affect, in a sense, our security,” said Dr. Gregory Poland of the Mayo Clinic, who sits on a federal vaccine advisory board.

Health officials warned of shortages last Tuesday after one of the two companies that supply most of the nation’s flu inoculations said it couldn’t provide any. British authorities had suspended the license of Chiron Corp. because of problems at its vaccine plant in Liverpool. That chopped the nation’s supply of flu vaccine in half.

The basic problem is that “we’ve lost most of our domestic manufacturers” of flu vaccine, said Richard Webby at St. Jude Children’s Research Hospital in Memphis. “When you’re relying on two manufacturers … and one goes down, you’re up the creek.”

Drug companies have pulled out of flu vaccine production because it’s not very profitable and it’s financially risky, health experts said.

One big problem is that demand for flu shots fluctuates from year to year as public interest waxes and wanes. Last season brought huge demand for a flu shot; the year before saw little interest, Poland said. But flu shots have to be made far in advance, so the manufacturers must rely on estimates, and then they’re “out there naked in the marketplace,” said Dr. William Schaffner of Vanderbilt University.

If a flu shot isn’t used during the season, it must be discarded. So companies generally throw away millions of doses a year, Poland said.

What’s more, making vaccines requires massive capital investment and involves the costs of complying with federal regulations, and the market is relatively small, he said.

People stand in line for flu shots Saturday at Memorial Medical Center in Las Cruces, N.M. Health officials warned of shortages last Tuesday after one of the two companies that supply most of the nation's flu inoculations said it couldn't provide any. Drug companies have pulled out of flu vaccine production because it's not very profitable and it's financially risky, health experts said.

So what can be done to draw more companies into making flu vaccines?

l One strategy would be to make demand higher and more reliable by getting more healthy adults to get flu shots regularly, Webby said. Between the publicity over last year’s early flu season and the current concern that bird flu could spawn a lethal human pandemic, “flu certainly has had a relatively high profile the last couple years,” Webby said. “All of this I think is certainly raising public awareness.”

l Other experts suggest having the government commit to buying a certain number of doses at a given price each year, buffering the uncertainty. Currently, most flu vaccine in the United States is bought by the private sector.

l Not only could government buy more flu vaccine and promote flu shots more aggressively, it also could require health insurance companies to cover flu shots as well as other vaccines for adults, Schaffner said.

“It would create a huge new market,” he said.