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 U.S. 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 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 U.S. population should be vaccinated quickly.

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

Schuchat said it was still a good idea for people to be vaccinated to protect against swine flu. We may see situations like what we're seeing in Georgia where ongoing vaccinations could be very beneficial, she said.

Health officials reported a spike in H1N1 cases in the southeastern United States this week, with Georgia being the hardest hit.

Early in the pandemic, there were long lines and chaos with people clamoring for H1N1 vaccines but there was not enough to go round. By the time vaccines were available in ample supplies, most of the public had lost interest.

The United States has contracts with five influenza vaccine makers -- Novartis, AstraZeneca unit MedImmune, Sanofi Aventis, GlaxoSmithKline and Australian vaccine maker CSL.

Schuchat said U.S. officials are reviewing their response to the pandemic to see what they could have done better.

The CDC estimates H1N1 has killed about 12,000 Americans and put 265,000 into the hospital.

Schuchat said health official were seeing few signs of seasonal flu, which kills about 36,000 people in the United States each year and puts 200,000 in the hospital.