UPDATE 1-Swine flu vaccine poses problem for makers-US FDA

WASHINGTON, July 23 (Reuters) - H1N1 vaccine makers are getting only 30 percent as much vaccine from swine flu as from seasonal flu, a U.S. Food and Drug Administration official said on Thursday.

"Development work has indicated the existing reference strains have an expected yield of around 30 percent of H1N1 seasonal vaccine strains," Dr. Jerry Weir, the FDA's director of the division of viral products, told an advisory meeting. This means makers may end up with fewer doses than expected of vaccine against the pandemic strain.

Earlier this month, the World Health Organization reported that vaccine makers were not getting the expected "yield" from swine flu virus samples they were sent, which they have been growing in eggs to then purify and make vaccines from.

AstraZeneca's (AZN.L) MedImmune unit, Australia's CSL Ltd (CSL.AX), Baxter International (BAX.N), GlaxoSmithKline Plc (GSK.L), Novartis AG (NOVN.VX) and Sanofi-Aventis SA (SASY.PA) all are making H1N1 vaccines and will tell the FDA committee about what they have learned as they work with the virus.

The U.S. National Institutes of Health reported on Wednesday that it had assigned human trials of vaccines from CSL and Sanofi to start in August at various academic centers and clinics. But the FDA must approve these before they can start.

The FDA's Vaccines and Related Biological Products Advisory Committee is meeting to consider those clinical trials. Health officials have said they hope to be able to start vaccinating people, if approved, in October.

The FDA's Dr. Wellington Sun told the meeting the agency would approve the new H1N1 vaccine as a strain change -- akin to the annual new formulations of seasonal flu vaccine -- as opposed to an entirely new vaccine. This should speed approval. (Reporting by Maggie Fox; Editing by Lisa Von Ahn)