Several people speaking at a meeting of the National Vaccine Advisory Committee said the laws would be an impediment to efforts to speed delivery of vaccines should the H5N1 avian flu, or any other virus, mutate into a pandemic strain.

"The states that passed these laws have just introduced a huge barrier to influenza programs," said Mary Beth Koslap Petraco, coordinator of child health for the Suffolk County Department of Health Services in New York.

She noted recent shortages of influenza vaccine, and hitches this year with distribution of the vaccine.

"If we think we have a problem this year with getting angry phone calls, I can't imagine what's going to happen next year," Koslap Petraco said.

Most doctors say thimerosal is safe and studies have shown there is no association between vaccines of any type and neurological diseases such as autism. Experts say the type of mercury used in thimerosal does not affect the body in the same way as mercury found in pollutants.

But several groups dispute this and some high-profile books have claimed that mercury is causing an epidemic of neurological disease. They claim that US health officials have covered up evidence of this.

DISPELLING DOUBTS

Manufacturers are removing thimerosal from vaccines, saying that while it is not unsafe, it is important to dispel any doubts about the safety of immunization.

The "vast majority" of influenza vaccines still contain thimerosal, said Dr. Melinda Wharton of the National Vaccine Advisory Committee.

Some states exempt influenza vaccines, Wharton said. California's bill prohibits use of thimerosal in vaccines except for the influenza vaccine in pregnant women or children under the age of 3.

But Delaware, for instance, has passed a law prohibiting the use of any vaccines containing any mercury, even trace amounts, as of 2008.

Making vaccine mercury-free means packaging individual doses -- something that is not necessarily easy for manufacturers to do. Experts said this could be even more difficult in the case of a pandemic, when large batches of vaccine would have to be rushed out.

Wharton said legislators may not understand the dangers, and she said the bills appear to have been driven by activists.

"Some of the individuals who have been actively involved in this issue clearly been on a state-to-state road show," she said. "State health departments and state health officials clearly have not been involved in trying to mold legislation."

Spokespeople for SafeMinds, a group that lobbies against thimerosal, could not be immediately reached for comment.

Dr. Jerome Klein of the Boston University School of Medicine and a member of the committee, said groups should propose alternative legislation that would limit the use of thimerosal while leaving wiggle room for emergencies.

"All of the intellect in the world is not going to stop this in some states. It is only going to get worse," Klein told the meeting.

But Claire Hannan, of the Association of Immunization Managers, which groups state health departments, said her group was worried that this would convey the impression that thimerosal is indeed harmful.