AstraZeneca P.L.C.'s MedImmune Inc. is churning out nearly three million doses of its vaccine a week at its facility on Red Lion Road near the Northeast Philadelphia Airport.

Production problems at other makers of H1N1 vaccine have proved to be an unintended boon for MedImmune.

Other manufacturers, including Novartis AG, Sanofi Pasteur Inc., GlaxoSmithKline P.L.C., and CSL Behring, initially could not get the H1N1 virus to grow as quickly as they had predicted, creating unexpected shortages.

Those companies say they have largely fixed the problems, but in the meantime, MedImmune's product has helped fill the void. In the third quarter of this year, AstraZeneca said sales of its H1N1 vaccine in the United States brought in $152 million in revenue.

U.S. government officials have boosted their orders from MedImmune, which so far has produced 40 million doses valued at $453 million.

One reason for MedImmune's success is the way it produces its vaccine.

Other pharmaceutical companies get the seed strain of the virus that they use to make their flu vaccine from the Centers for Disease Control and Prevention. Those companies, however, use a strain of the H1N1 virus that is "dead," or inactivated.

MedImmune uses a live version of the virus that it weakens, or attenuates. Live virus is easier to grow than inactivated virus. The CDC gives MedImmune virus components, but the company makes its own seed.

Ben Machielse, MedImmune's executive vice president of operations, said the company's process had helped it identify the seed candidate with the best chances for growth.

"Because we have to do it every year, we have obtained a lot of experience," he said.

Pharmaceutical companies grow material for flu vaccines in eggs, which serve as a factory for the virus.

MedImmune gets about 90 vaccine doses from each egg, compared with less than one for makers of inactivated vaccine when they first started H1N1 production.

The company's Red Lion Road site is operating around the clock as it has ramped up production from 1.7 million doses weekly over the summer to nearly three million now, Machielse said.

In earlier flu seasons, the Gaithersburg, Md., company struggled to compete.

That was a disappointment to AstraZeneca, a London company with U.S. headquarters that paid $15.6 billion in 2007 for MedImmune, partly on the hope that a needle-free vaccine would prove popular with consumers.

In recent years, MedImmune lowered some barriers to boost sales of its seasonal flu vaccine, sold as FluMist. The company more than halved the price, bringing FluMist's cost closer to the cost of injectable versions of the vaccine. It won U.S. Food and Drug Administration approval to give FluMist to children as young as 2 years old.

Earlier versions of FluMist needed to be frozen, but MedImmune developed a new version that requires only refrigeration, just as injected vaccines do.

Although MedImmune uses the FluMist name only for its seasonal flu product and does not have a name for its H1N1 product, doctors and others often call both products FluMist.

Just as the company finished revamping FluMist, governments around the world started ordering large amounts of vaccine to cope with the threats posed by swine flu, which health officials feared could spread quickly and lethally.

The company has added 110 seasonal workers at the Red Lion Road site, where it fills and packages the vaccine, bringing total employment there to 263.

But there are natural limits to FluMist's growth. And it is not clear that FluMist will remain in such high demand in more typical flu seasons.

Because the FluMist vaccine is live, it is recommended only for people between the ages of 2 and 49 who are not pregnant. People with illnesses such as asthma or diabetes, which increase their risk for flu complications, also should not take FluMist.

Some of those groups, including children under age 2 and people with certain illnesses, are considered most vulnerable to H1N1.

Some studies have questioned whether FluMist works as well in adults as other vaccines; the company says the data are mixed.

A recent study on seasonal flu in the New England Journal of Medicine found that FluMist provided less protection against flu in adults than injected vaccines. Scientists, however, do not know whether the results apply to H1N1.

"The attenuated vaccine works very nicely in children. It's not clear that it works quite as nicely in adults," said Dr. Hildegund C. Ertl, director of the Wistar Institute Vaccine Center.