Shortages of flu vaccine are nothing new in America, but this year's is a whopper. Until last week, it appeared that 100 million Americans would have access to flu shots this fall. Then British authorities, concerned about quality-control problems at a production plant in Liverpool, barred all further shipments by the Chiron Corp. Overnight, the U. S. vaccine supply dwindled by nearly half and federal health officials found themselves making an unusual plea. Instead of beseeching us all to get vaccinated, they' re now urging most healthy people between the ages of 2 and 64 not to. 'This reemphasizes the fragility of our vaccine supply, ' says Dr. Martin Myers of the National Network for Immunization Information, ' and the lack of redundancy in our system. ' Why is such a basic health service so easily knocked out? Mainly because private companies have had little incentive to pursue it. To create a single dose of flu vaccine, a manufacturer has to grow live virus in a 2-week-old fertilized chicken egg, then crack the egg, harvest the virus and extract the proteins used to provoke an immune re-sponse. Profit margins are narrow, demand is fickle and, because each year' s flu virus is different, any leftover vaccine goes to waste. As a result, the United States now has only two major suppliers(Chiron and Aventis Pasteur)—and when one of them runs into trouble, there isn't much the other can do about it. 'A vaccine maker can't just call up and order 40 million more fertilized eggs, ' says Manon Cox, of Connecticut-based Protein Sciences Corp. ' There' s a whole industry that' s scheduled to produce a certain number of eggs at a certain time. ' Sleeker technologies are now in the works, and experts are hoping that this year' s fiasco will speed the pace of innovation. The main challenge is to shift production from eggs into cell cultures—a medium already used to make most other vaccines. Flu vaccines are harder than most to produce this way, but several biotech companies are now pursuing this strategy, and one culture-based product(Solvay Pharmaceuticals' Invivac)has been cleared for marketing in Europe. For Americans, the immediate challenge is to make the most of a limited supply. The government estimates that 95 million people still qualify for shots under the voluntary restrictions announced last week. That' s nearly twice the number of doses that clinics will have on hand, but only 60 million Americans seek out shots in a normal year. In fact, many experts are hoping the shortage will serve as an awareness campaign—encouraging the people who really need a flu shot to get one. Shortages of flue vaccine show that______.