As lawmakers in about 20 states press for bans on mercury in children's vaccines, they are meeting stiff resistance from influential health and medical organizations, including groups that get substantial funding from drug makers and the Centers for Disease Control and Prevention.
Seven states have adopted the anti-mercury bills -- California being one of the first.
California's law, passed in 2004 and to take effect July 1, will prohibit shots with more than a trace of thimerosal for pregnant women and children younger than 3. In recent weeks, similar bills have been defeated in at least five states.
The push for legislation comes long after the uproar over continued use of thimerosal, a mercury-based antibacterial agent, appeared to subside in 1999, when manufacturers began phasing it out of routine pediatric vaccines.
But the controversy flared anew when flu shots containing thimerosal were added to the childhood immunization schedule in 2004 and the CDC refused to recommend thimerosal-free shots for infants and pregnant women.
Angered by the CDC's refusal -- and fearing a backslide into more thimerosal use -- state lawmakers and anti-mercury advocates began pushing for outright thimerosal bans.
The legislation faces opposition from groups such as the American Academy of Pediatrics and the Immunization Action Coalition -- a stance that anti-mercury advocates say defies logic.
"We're trying to get [mercury] out of the environment," said Marilyn Rasmussen, a Washington state senator and sponsor of a thimerosal bill that was signed into law last month.
"Why would we be injecting it into babies? We've got to be smarter than that."
Mercury can damage the nervous system, and infants and toddlers are thought to be particularly at risk because of their low body weight and rapidly developing brains. That concern is behind wide-ranging initiatives to cut mercury pollution from industrial plants and warn pregnant women to limit intake of some types of fish.
The American Academy of Pediatrics and its allies, including some state health departments, say there is no proof that the small amount of mercury in vaccines is harmful. They argue that legal restrictions could undermine confidence in vaccines -- causing people to skip their shots -- and lead to shortages.
The academy, an organization of 60,000 pediatricians, has generally taken a zero tolerance stance on mercury, even joining a federal lawsuit seeking stricter controls on power plant emissions. Its official policy, published in July 2001 in its journal Pediatrics, states in part:
"Mercury in all of its forms is toxic to the fetus and children, and efforts should be made to reduce exposure to the extent possible to pregnant women and children as well as the general population."
Dr. Louis Z. Cooper, the academy's former president and chairman of its Center for Child Health Research, acknowledged the group's stand on thimerosal "does appear to be a paradox."
But Cooper said he did not believe "the science justifies codifying in state law that we ban all mercury-containing vaccines."
He also voiced concern about the effect on immunization programs in the developing world. The World Health Organization relies heavily on thimerosal to immunize millions of children in poor nations, and could face cost and logistical problems if forced to abandon it.
"If we banned mercury-containing vaccines by statute in the United States," Cooper said, "it would make it a lot harder to explain in other parts of the world" why they should accept them.
Vaccine producers generally oppose the bills but have kept a low profile, leaving health groups to lead the charge. Last year, pharmaceutical companies contributed about $1.54 million to the academy out of a budget of $68.2 million. Among the donors were vaccine giants Merck & Co., GlaxoSmithKline and Sanofi Pasteur Inc., according to tax filings and academy officials.
The group also got about $1.55 million from the CDC for several health programs.
Although CDC officials are not permitted to lobby states, they have warned that thimerosal bans could create confusion about which vaccines are acceptable and lead some parents to delay or forgo immunization of their children.
Cooper said financial ties to drug makers and the CDC had not influenced the academy. "The bottom line has always been what would be best for the child," he said.
Another opponent of bans, the Immunization Action Coalition, based in St. Paul, Minn., runs websites and newsletters promoting immunization. The group has been dispensing strategy advice and materials to critics of the anti-mercury bills.
"We're scaring people away needlessly from very safe vaccines," said Diane Peterson, the group's associate director.