At a closely guarded mountainside factory, a woman plucks a vial from an assembly line and unceremoniously dumps its shimmering contents into what looks like a metal spittoon.

The precious liquid is freshly made flu vaccine. Although there is nothing wrong with the fluid, the worker must periodically discard some to make sure a machine is squirting the right amount into each vial.

Ironically, she will get none of it to protect herself. Aventis Pasteur has ordered employees to do as most Americans are being asked: Skip the flu shot this year unless you are at high risk of getting seriously ill from flu.

"Company policy," she said with a shrug.

The woman is one of 1,500 workers at this 50-building, 70-acre compound that is a two-hour drive north of Philadelphia. Iron gates protect it, and security guards record the license plates of visitors, who must be escorted at all times.

Inside, the mood is tense. Many workers start early, end late, and spend their days dealing with the simultaneous blessing and curse of having the only flu shots in the nation.

Perched on computers in the warren of cubicles used by the company's sales force are Beanie Baby-like stuffed animals that were given to reward workers who had twisted customers' arms to get orders for Aventis Pasteur.

Now workers are calling customers back with a very different request in an effort to steer vaccine supplies to places that have none.

"We're going to them and asking them voluntarily to accept fewer doses, and we're getting good cooperation," company spokesman Len Lavenda said.

Aventis Pasteur is in charge of figuring out which Peters it can rob; the federal government tells it which Pauls to pay.

No one at Aventis Pasteur expected to be in this position. Earlier this month, the company had finished making its planned 52 million doses, plus pilot lots of an experimental bird flu vaccine for the government to test. Production areas were closed for fumigation.

Only the filling and packaging of the rest of this year's vaccine remained to be done when word came that rival Chiron Corp. could not supply any shots this year because of contamination at its plant in Britain.

Now workers are hustling to fill 2 million to 3 million doses a week while gearing up for an unexpected new run of 2.6 million more that will not be ready until January.

Wearing yellow goggles, thick rubber gloves and cloth uniforms that cover them from top to toe, some work in a glass-enclosed room where clear plastic tubing carries hundreds of thousands of doses from a stainless steel tank to a dispensing machine.

Its eight needles squirt a little more than 10 doses into each sterile vial, which gets an aluminum safety seal and a turquoise-colored cap and moves on a belt past large microfiche-like viewing screens. Workers peer at the magnified images, searching for tiny particles in the swirling liquid that would make it unfit for use.

Once an hour, a worker snatches a vial, weighs it, tosses its contents, uses a yellow coiled air hose suspended from the ceiling to dry it, then weighs it again to make sure the right amount of vaccine had been inside.

Nobody winces, but Raymond "R.J." Fitch, a filling and processing team leader, acknowledges the difficulty of discarding a product that millions desperately want.

"Sure it's hard, but it's part of ensuring you have the right level of quality," he said.

Other workers scrutinize each label for smudges over the expiration date or identifying information. Still others sit with calculators and log sheets, poring over columns of numbers to ensure lots can be tracked through testing and distribution.

Sam Lee, 40, a chemical engineer who is Aventis Pasteur's operations team leader, has overseen much of this in more than nine years with the company. He had planned to go to a clinic to get a flu shot with his wife and three children, as they do every year.

"We make it. I know the benefits of it," he said of the vaccine.

Lee feels confident he will be able to get a shot next year, but Aventis Pasteur employees have learned not to try to predict the future.

"We have a saying around here: Once you've seen one flu season, you've seen one flu season," Lavenda said. "No two are the same."