As public health officials around the world scramble to protect their citizens from swine flu, some in the US are grappling with an additional issue: state laws that limit the use of a mercury preservative in vaccines.

Between 2004 and 2006, six states—California, Delaware, Illinois, Missouri, New York and Washington—enacted laws limiting the use of the preservative, thimerosal, in flu vaccines and other shots given to children and pregnant women. The move was made in response to fears that thimerosal might cause neurological conditions such as autism.

This fall, however, all six states temporarily lifted the restriction in response to the outbreak of pandemic swine flu and the shortage of H1N1 vaccine. Many of the available vaccines came in multidose vials, which are quicker to manufacture and contain thimerosal to prevent contamination with repeated inserts of a needle.

Although the laws allow for such temporary suspensions, many people question their utility in the first place, given their feeble scientific foundation. The laws are “absolutely not” supported by research, says Diane Peterson, an associate director at the Immunization Action Coalition, a vaccine advocacy group in St. Paul, Minnesota. Even in 2004, before the laws were enacted, a US Institute of Medicine panel found that the scientific evidence did not support a link between thimerosal-containing vaccines and autism, and several subsequent studies have backed up that conclusion (N. Engl. J. Med. 357, 1281–1292; 2007)

But vaccine experts say that antithimerosal advocates remain vocal. Mary Selecky, Washington's secretary of health, received a flurry of complaints after she suspended the thimerosal restrictions in September. “They were quite angry at me,” she says. “Clearly, folks were very pointed about feeling I was wrong.”

Peterson is concerned that the thimerosal bans have helped fuel a backlash against vaccination. “The laws have contributed to the doubts people have about the safety of vaccines,” she says. An ABC/Washington Post poll from October found that nearly half of parents did not intend to vaccinate their children against swine flu, in part because of safety concerns.

But as the scientific evidence mounts, laws limiting thimerosal have been harder to get through state legislatures. This year alone, advocacy groups and individuals in 12 states tried—and failed—to pass similar restrictions, according to Peterson. What's more, the laws are mostly redundant, as thimerosal was removed from nearly all vaccines by 2001. Even so, the six states' thimerosal bans remain on the books; Washington's is scheduled to go back into effect in March.