WASHINGTON (Reuters) – It is too soon to determine whether tens of thousands of doses of H1N1 swine flu vaccine may have to be thrown out if they are not used before their expiration date, a US health official said at the weekend.

The United States ordered enough antigen to make 229 million doses of vaccine as the swine flu pandemic began to ramp up a year ago. About 162 million doses have been shipped and between 81 and 91 million doses have been administered, according to the US Centers for Disease Control and Prevention.

The Washington Post reported that an estimated 71.5 million doses of H1N1 vaccine may have to be discarded if not used before expiry, costing millions in taxpayer dollars.

Dr. Anne Schuchat, head of the CDC'S National Center for Immunization and Respiratory, said it was difficult to say how many H1N1 vaccine doses may be be discarded.

She said most of the vaccine has yet to expire, some by the end of June and more next year. Schuchat encouraged providers, pharmacies and health departments to keep offering the vaccine as long as they could unless it had expired. "We really made a conscious decision to be prepared and to assure that we would have more than enough vaccine instead of less than enough vaccine," Schuchat said.

It was not uncommon to discard seasonal flu vaccine every year, she said, and the only difference with the H1N1 vaccine was that the federal government had bought the supplies.

When the swine flu outbreak was first detected last April, officials got flu manufacturers working on a vaccine within weeks, and the CDC said more than half the US population should be vaccinated quickly.

As of the end of January 2010, only about one fifth of US adults had been vaccinated and more than one third of American children, according to CDC estimates.