UPDATE 2-New U.S. flu plant to boost vaccine availability

* Plant increases Sanofi flu vaccine capacity

* Facility could be used for H1N1 vaccines if needed

* FDA: more vaccine production capacity needed (Adds details from FDA, company; adds byline)

By Susan Heavey and Lisa Richwine

WASHINGTON, May 6 (Reuters) - U.S. regulators approved a new flu vaccine manufacturing plant to boost not only production of seasonal flu shots, but also possible vaccines to protect against the H1N1 swine flu.

The facility, owned by Sanofi-Aventis SA (SASY.PA), was cleared to make the company's seasonal Fluzone influenza vaccine in Swiftwater, Pennsylvania, and will eventually more than double the company's yearly flu vaccine production, the Food and Drug Administration said.

It could also be used to produce immunizations against the new H1N1 flu strain if needed, the agency said.

"This is very, very good news for a major important supplier ... it immediately (and) substantially increases their production capacity," Dr. Jesse Goodman, the FDA's acting chief scientist and deputy commissioner for scientific and medical programs, told Reuters.

The company's Sanofi Pasteur unit -- the major supplier for the U.S. market -- already operates another plant in Swiftwater that produces 50 million doses of seasonal flu vaccines a year. The new plant could produce another 50 million doses this year and up to 100 million doses later on, the company has said.