Many employees at the company's Poconos plant here volunteered to work 12-hour shifts seven days a week, some have canceled honeymoons, and one reported to work in the afternoon after his child was born in the morning, company officials said.

As thousands of people have lined up across the nation to get scarce H1N1 shots, vaccine-makers and the Obama administration have scrambled to explain why production has gone much more slowly than expected.

And Sanofi, as the only company of the five that makes seasonal and H1N1 flu vaccine in the United States, is at the heart of the story.

By the end of this week, Sanofi will have produced and delivered 20 million doses.

Chief executive officer Chris Viehbacher yesterday opened up the company's vaccine plant to about 20 reporters to try to help the public understand the challenges of producing vaccines.

"I think what you've been able to see today is that this is Mother Nature at work," Viehbacher told reporters after they had completed a tour of the company's 540-acre facility here. "It's the same as when you plant ears of corn and you try to predict how many you are going to have by the end of the year."

Along with Sanofi, MedImmune Inc., a division of AstraZeneca P.L.C.; CSL Ltd.; GlaxoSmithKline P.L.C.; and Novartis AG make flu vaccine.

When the Centers for Disease Control and Prevention earlier this year identified H1N1 as a flu strain that could strike hard and kill, Sanofi knew the virus grew slowly. Scientists at the company's vaccine division, Sanofi Pasteur, believed H1N1 would grow about half as fast as the seasonal flu vaccine. Initially, the virus grew only 30 percent as fast, slowing production. Sanofi employees have since figured out how to boost yields to about 80 percent of what it typically gets with the seasonal product, said Wayne Pisano, president and chief executive of Sanofi Pasteur.

Companies grow flu vaccine in eggs, and Sanofi uses more than a million a week as 2,000 of its 3,200 employees rush to make the H1N1 shot.

"The eggs are a perfect little factory for the vaccine," said Sam Lee, director of manufacturing technology for Sanofi Pasteur.

After injecting a form of the virus into the eggs, they are incubated for a few days while the virus grows. Sanofi then extracts the fluid from the eggs and purifies it in large stainless steel tanks that look like those used in breweries. Once the virus has been inactivated, the company conducts about 50 tests to make sure the product is sterile and potent. Employees wear helmets and inflated plastic suits and work inside plastic structures to keep contaminants at bay.

Once put into vials and syringes, digital cameras take photos and use software to detect defects. It takes about four months from the time an egg arrives at the Swiftwater plant until the vaccine is ready to ship.

Sanofi produces more than 1.6 billion vaccine doses against 20 diseases every year, enough to immunize more than 500 million people, according to the company Web site.

Also yesterday, Sanofi said its H1N1 vaccine produced a "robust" immune response on adult test subjects, even those aged over 65. Sanofi said the drug's safety and effectiveness held up during a six-week follow-up period, according to final results of a clinical trial. Two doses are needed for children 9 and under, with one dose for everyone else.

Some scientists and public-health officials have called for flu-vaccine manufacturers to find new ways to produce the product, but Pisano said Sanofi did not plan to switch away from eggs soon.

"Our experience is that eggs are very effective," he said. "We've been producing it that way for over 40 years."

The company is working on a universal flu vaccine, which would work against all type A flu strains, and is considering a cell-based production method that does not use eggs for that product. A universal vaccine, however, is not expected to be available for many years.

Viehbacher urged public patience.

"I don't think we need to have any kind of panic here," he said.

"Everybody will get a vaccination."