ATLANTA - For some parents, fears are compounded by worries about thimerosal, a mercury-based preservative that will be in roughly 60 percent of the 225 million swine flu doses ordered for Americans.

The preservative is not in the FluMist nasal spray, which can be given to healthy kids age 2 and older. But it's in many injectable doses, which are packaged in multi-dose vials that require thimerosal to prevent bacterial contamination.

Fears that the preservative or something in vaccines themselves can lead to autism remain entrenched in some quarters — despite no evidence from the most rigorous scientific studies.

Some autism advocacy groups echo parents' concerns about swine flu vaccine, and also argue it's a bad idea to spend so much time and money on the new flu.

"We're flipping out over swine flu, but it's only affected a few thousand people. Why isn't somebody freaking out about the autism epidemic?" said Wendy Fournier, president of the National Autism Association.

Vaccine makers are sensitive to demand for preservative-free shots. Parents can ask their doctors to order preservative-free, single-dose vaccine for their kids, said Dr. Tom Frieden, head of the Centers of Disease Control and Prevention.

As for his own two school-age children, Frieden said in a recent interview: "I would have no hesitation about getting my kids vaccinated by thimerosal-containing vaccines."

Health officials and many parents are strong believers in the vaccine, and warn about the potential dangers of a virus that has caused at least 9,000 U.S. hospitalizations and at least 600 deaths, including 60 children.

Jennifer Barnes enrolled herself and her two children in one of the government studies of the new vaccine, seizing an opportunity to get them all immunized before the illness became widespread.

"I thought, 'This is an opportunity to get the kids vaccinated, and I better jump on it,'" said Barnes, 32, a speech language pathologist who lives in Decatur, Ga.

Barnes said she gets her kids vaccinated against flu each year not only for their own health but to protect others. "My kids hang around kids who might have lowered immune systems. I would hate for them to get something and pass it on," she said.

Shea said she appreciates those arguments, but she's hesitated to talk about swine flu vaccine with other parents, who seem polarized on the topic. "There's the crunchy granola group" against flu vaccinations, she said, "and the very staunch, follow everything group" who extol them.

She also worries that swine flu could become more widespread and dangerous than it is now. If that happens, she said, she would probably try to get her son vaccinated, though she's aware there are risks in waiting, too.

"It's one of those things where you're almost damned if you do, damned if you don't," she said.

The AP-GfK poll was based on a nationally representative sample of 1,003 adults age 18 or older, contacted by telephone on land lines and cell phones. The margin of sampling error is plus or minus 3.1 percentage points for all adults, 5.2 percentage points for parents.