Two new studies cast further doubt on the theory that a mercury-based preservative in vaccines causes autism. Called thimerosal, the preservative has already been phased out in many industrialized countries but is still used in the developing world. The new findings "provide additional, extremely reassuring data," says William Schaffner of Vanderbilt University School of Medicine in Nashville, Tennessee.
In 1999, parent advocate groups such as Safe Minds proposed that thimerosal might explain the rising incidence of autism, which often appears at about the same time that 2-year-olds get a round of booster shots. Many scientists were skeptical, but in 2001 an Institute of Medicine panel concluded that there wasn't enough evidence to rule out or accept the link.
Now the first big epidemiological studies weigh in. One comes from Denmark, which eliminated thimerosal from childhood vaccines in 1992. A team led by Kreesten Madsen of the Danish Epidemiology Science Centre in Aarhus reasoned that if thimerosal were a major cause of autism, incidence should drop once it was removed. But it continued to skyrocket after the 1992 phaseout, according to a report in the September issue of the journal Pediatrics. A similar pattern emerges from health statistics from Sweden, where total mercury in childhood vaccines began to decline in the late 1980s, as reported in the August issue of the American Journal of Preventive Medicine.
But Mark Blaxill of Safe Minds argues that the Danish study is "distorted and misleading." He notes that in 1995, the Danish health registry began tracking a new category of patient, called autism outpatients. This and other factors, he says, are artifacts that confound the interpretation. Madsen responds that an unpublished analysis without outpatients showed the same increasing trend.
Epidemiologist Craig Newschaffer of Johns Hopkins University in Baltimore says that these ecological studies have inherent limitations, because they look at populations rather than studying individual exposure. Although the current studies are unlikely to end the controversy in the United States, where many lawsuits have been filed, the new findings are reassuring to the World Health Organization, which continues to recommend the use of small amounts of thimerosal to keep down the costs of essential vaccines.