Americans who were worried about the safety of the swine flu vaccine are still worried and it may not be easy to convince them to get themselves or their children vaccinated, researchers said on Tuesday.

About 60 percent of parents polled say they plan to get their children vaccinated and 79 percent of adults will try to get the vaccines for themselves, but there is a hard core of resistance that has not been moved by entreaties by the U.S. government, pollsters said.

Thirty-five percent of parents say they are not going to get it and 60 percent say the major reason is safety, Robert Blendon of the Harvard School of Public Health said in a telephone interview. Our view is there just has to be more work on understanding how people think about the vaccine.

Several studies have shown the H1N1 swine flu vaccine does not cause unusual side effects and Blendon said it is not clear why so many parents are fearful.

The poll of 1,637 people aged 18 and older was taken last week, as 100 million vaccine doses had become available.

The U.S. Centers for Disease Control and Prevention estimates that nearly 50 million Americans have been infected with swine flu and 10,000 have been killed by it.

While this is not as high as the usual 36,000 deaths from seasonal influenza, officials note that swine flu infects and kills young adults and children more than seasonal flu does.

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The U.S. Health and Human Services Department has contracts with five companies to make 251 million doses of H1N1 vaccine.

The Harvard poll, which had an error margin of 3 percentage points, found that only 14 percent of adults had gotten the vaccine for themselves, but 79 percent would try to -- down from 92 percent who said they would in November.

HHS and CDC are working to get the vaccine distributed in so-called big box stores such as Wal-Mart.

John Roehm of Mollen Immunization Clinics, which operates the Wal-Mart clinics, said the company would have swine flu clinics in 48 states by the end of next week.

We will see how demand is, Roehm said in a telephone interview. He said demand rose as state health departments encouraged the wider population to be vaccinated.

Retail drug stores have also begun administering vaccines, with Walgreen Co offering it at more than 1,500 locations in 27 U.S. states and CVS Caremark Corp in 20 states and Washington. Rite Aid had clinics in 12 states.

Wal-Mart Medicine