By Jon Hurdle

SWIFTWATER, Pa., July 19 (Reuters) - Influenza vaccine maker Sanofi Pasteur <SASY.PA> unveiled a factory on Wednesday that will eventually triple its production of the vaccine as U.S. officials seek to boost output amid fears of a pandemic.

The vaccine arm of Sanofi-Aventis is the only U.S.-based producer of injectable flu vaccine. The plant will produce 150 million doses a year after the new 140,000-square-foot (13,000- square-metre) facility comes on line by 2010.

The $150 million facility will also be capable of producing vaccine against pandemic strains of flu. Experts expect a pandemic of influenza of some sort and the No. 1 suspect is the H5N1 avian flu virus, which has so far infected 318 people and killed 192 of them.

In the event of a pandemic, the plant will switch its entire production to that strain, officials told reporters.

"A pandemic would pose a unique challenge," Health and Human Services Secretary Mike Leavitt said. "You can't start planning vaccines (after) a virus appears."

About 70 million of the 120 million flu vaccine doses available on the U.S. market last year were made overseas. U.S. officials fear that in the event of a pandemic, plants might be nationalized by governments eager to protect their own populations.

In 2004, British officials unexpectedly closed a flu vaccine plant because of contamination, cutting in half that season's supply of vaccines for the U.S. market.

Sanofi's increased production is being aided by a $77 million grant for the retooled facility from the federal government. The federal government is spending $1 billion to expand vaccine production over the next five years.

Sanofi produced 50 million doses last year. The rest were made by Novartis <NOVN.VX> in Britain and GlaxoSmithKline <GSK.L> <GSK.N> in Canada. Maryland-based MedImmune, a unit of AstraZeneca <AZN.L>, makes an inhaled flu vaccine as well.

INCREASING DEMAND

Sanofi anticipates increased demand for seasonal flu vaccine as a result of a stepped-up campaign by federal health authorities to encourage people to get vaccinated. Seasonal flu kills 36,000 people in an average year in the United States.

While the Centers for Disease Control and Prevention said 218 million Americans should get seasonal flu shots, only about 90 million got one last year.

At the new facility in northeast Pennsylvania, vaccine is generated from millions of chicken eggs. In super-clean, climate-controlled rooms, the eggs are inoculated with the virus strain that is identified by international health authorities in February each year.

The virus is then incubated for two to three days, and the eggs then go through a process called candling in which each one is inspected to check that is intact, fertile, and contains clear fluids.

Technicians wearing gowns, masks, gloves and goggles harvest the virus by using machines to decap the eggs and emptying their contents. The resulting fluids are separated and concentrated to produce a clear liquid that is finally made into the vaccine.

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