The U.S. government may end up throwing away unused doses of swine flu vaccine if the 250 million doses ordered by the federal government aren't used by the end of the season, the director of the U.S. Centers for Disease Control and Prevention said on Tuesday.

Some government officials worry that by the time all of the H1N1 shots are received people will already have had the H1N1 virus or will no longer want the vaccine. In that case, leftovers would be thrown away.

Next year, the H1N1 virus will be included in the regular seasonal flu shot, rather than a separate one, CDC director Dr. Thomas Frieden said Tuesday.

Members of Congress questioned whether federal officials were too rosy in their estimates of how much vaccine would be available and when, and companies said they were still struggling to produce immunizations against H1N1.

LiveShots Blog: H1N1

Full Coverage H1N1 -- Click here

Frieden said 22.4 million doses were now available to states, which can get them a day after they order them.

"It's quite likely that too little vaccine is one of the things that's making people more interested in getting vaccinated, frankly," Frieden told reporters.

"We think it will get easier to find vaccine in the weeks that come."

President Barack Obama's daughters found it. "Malia and Sasha were both vaccinated for H1N1 last week, after the vaccine became available to Washington, D.C. schoolchildren," a White House blog reads.

"President and Mrs. Obama have not yet been vaccinated for H1N1, and they will wait until the needs of the priority groups identified by the CDC — including young people under the age of 24, pregnant women, and people with underlying conditions — have been met."

Many states and cities say they have received about one-tenth as much vaccine as they originally had expected by this time. Frieden said the delays may discourage people who are lining up for vaccine.

"It is likely also as we produce more vaccine and as both people are given the opportunity to get vaccinated, and as disease maybe wanes in the future, we will have significant amounts of vaccine that can't be used," Frieden said.

"One of the messages for states, localities and health providers is not to reserve vaccine that they have available, to give it out as soon as it comes in, because more is on the way."

In September, U.S. officials said 40 million vaccine doses would be available by the end of October and they estimated 20 million doses a week would be delivered, with a goal of 250 million doses by the end of flu season in March or April.

UNRELIABLE ESTIMATES

Maine Republican Senator Susan Collins asked why the estimates were so far off.

"It now appears that much of the vaccine could arrive only after many people have already been infected with H1N1," she said in a letter to Health and Human Services Secretary Kathleen Sebelius, released late on Monday.

"It seems that HHS gave its assurance of sufficient supply in August without adequate information to make such a commitment."

Connecticut independent Senator Joseph Lieberman weighed in on Tuesday.

"Unfortunately, these missteps in estimating available doses of H1N1 vaccine have effects beyond just growing public frustration; they have the potential to critically undermine our vaccine distribution efforts, which depend on accurate estimates of vaccine availability," he said.

But HHS spokeswoman Jenny Backus said the agency was simply passing on information as it became available.

"We have been very clear and open and told the American people what we know when we know it," she said in a telephone interview.

"We have passed on the manufacturing estimates, and as they have changed, we have conveyed the information to the American people, too."