(Adds CDC quotes paras 7 and 14, background para 10)

By Matthew Bigg

ATLANTA, April 19 (Reuters) - The United States is expected to have at least 127 million flu-vaccine doses on hand for this year's influenza season -- the most ever, companies told the U.S. Centers for Disease Control and Prevention on Thursday.

Sanofi Pasteur, the vaccine unit of Sanofi-Aventis <SASY.PA><SNY.N>, said it would have 50 million doses. Novartis AG <NOVN.VX><NVS.N> said it would have 40 million doses, GlaxoSmithKline Plc <GSK.L><GSK.N> said it would have 30 million to 35 million doses and MedImmune <MEDI.O> said it would provide 7 million doses.

An Australian vaccine maker, CSL Ltd. <CSL.AX>, said it could provide some vaccine as well, if it got approval from the U.S. Food and Drug Administration.

The CDC and other health officials are eager to increase the share of Americans who get vaccines each year against seasonal influenza, which kills about 36,000 people in an average U.S. flu season and puts 200,000 into the hospital.

Officials are also wary of the potential for an influenza pandemic, such as bird flu. They are trying to encourage companies to boost their capacity to make influenza vaccines for the U.S. market.

But making flu vaccines uses old and risky technology involving chicken eggs and months of incubation, and few companies want to make them. Batches must be mixed up fresh every year to match the various flu viruses that are circulating because they mutate every year.

"We are trying to make a vaccine to be prepared for an epidemic of influenza ... but we don't know which strains are in the epidemic, we don't know how virulent those strains will be and we don't know when the epidemic will peak," said Dr. Jeanne Santoli of CDC's immunization program.

The supply can also be wiped out easily -- as in 2004, when the United States lost half its flu vaccine supply when a British-based maker suffered contamination and was shut down for a year.

But usually, hundreds of thousands of doses of vaccine go unused every year and are thrown away.

For the 2006-7 season 120.9 million doses were produced, of which 102.5 million doses were distributed according to demand. One million went onto a government stockpile and 17 million were unsold and thrown away, the CDC said.

DISTRIBUTION

The system for distributing vaccines -- based mostly on private distributors -- is unpredictable and delivers vaccines at various times throughout the season.

The result is that people often cannot get vaccines when they want them. CDC says it is working on this problem, although the government agency has little authority over the private distributors.

The CDC says that most Americans should be vaccinated against the flu every year, but fewer than half of those who should get the immunization actually do.

"Our goal is to get to where we are vaccinating those recommended 218 million every year," Santoli said.

Because there is such a limited market for the vaccines, companies do not make them in large numbers. The total global supply is estimated at about 400 million doses a year, far short of what would be needed in a pandemic.

The H5N1 avian influenza virus circulating among birds in Asia, Europe, the Middle East and Africa is considered the No. 1 candidate for causing a pandemic. It does not easily infect people now but has killed 172 people out of 291 known to have been infected worldwide.

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