TUESDAY, Sept. 30, 2003 (HealthDayNews) -- A child-by-child study finds no relationship between thimerosal, a mercury preservative that until recently was commonly added to vaccines, and childhood developmental problems such as autism.
But that report will not stop a flood of lawsuits claiming just such a relationship, a lawyer says.
The Danish study, reported in the Oct. 1 issue of the Journal of the American Medical Association, supports the findings of other medical analyses, most notable one issued last year by a panel appointed by the U.S. Institute of Medicine, which found no increased incidence of problems in children who got thimerosal-containing vaccines.
But that panel recommended that thimerosal be removed from vaccines, saying it is "biologically plausible" that cumulative exposure to the chemical might make children vulnerable to mercury-related disorders.
Vaccine makers began adding thimerosal to their products in the 1930s to kill potentially dangerous germs. It contains ethylmercury, a cousin of methylmercury, the compound that has diners worried about eating too much fish. High doses of both compounds are known to damage brain cells, but whether low doses are hazardous is unclear.
Manufacturers already had started to remove thimerosal from vaccines in 1999, because of a recommendation made by the U. S. Public Health Service and other medical groups. All childhood vaccines in this country, and many in other countries, now are available in thimerosal-free formulations.
So Danish researchers at the Statens Serum Institute in Copenhagen were able to compare the incidence of childhood problems including autism, attention deficit/hyperactivity disorder and developmental delays in children who were vaccinated with a thimerosal-containing pertussis (whooping cough) vaccine and those who got an additive-free vaccine.
"This is a study that overcomes the problem previous studies have had," says study author Dr. Mads Melbye, a professor of epidemiology at the institute. "In Denmark, we have an identifiable registry on all childhood vaccinations going back to 1990."
Thimerosal was removed from the Danish pertussis vaccine in 1992. "Since then we have been able to follow up all these children with respect to the eventual development of autism," Melbye says. "We compute that there is no difference in the risk of autism in those children who got the vaccine with or without thimerosal."
The exhaustive study includes records on all the 467,450 children born in Denmark between Jan. 1, 1990, and Dec. 31, 1996. It finds no significant difference between the incidence of autism and other such problems in children who got vaccine with or without thimerosal, and no indications of a dose-response relationship between autism and the amount of ethylmercury received through thimerosal.
"Previous studies have not looked at personally identifiable data," Melbye says. "We were able to look at the incidence curve for autism and find no sign that it correlates with thimerosal exposure."
But the researchers did find "a dramatic increase in the number of diagnosed cases of autism-spectrum disorders during the study period, similar to what has been observed in other countries."
"No one knows why this increase is taking place," Melbye says, and the study was not designed to answer that question.
But from a lawyer's point of view, "the Danish study is not very good," says James Farrell, a member of the Houston law firm of R. G. Taylor Associates, which is actively involved in thimerosal litigation. "From the scientific standpoint, it really doesn't answer the question."
Farrell estimates that 3,800 lawsuits claiming damage done by thimerosal-containing vaccines now are before courts all across the country. Most courts have ruled that class-action suits are not appropriate, he says. "They think that the damage varies too much from child to child, as well as the types of exposure," Farrell says.
"Eventually, all these cases will go before juries around the country," he says. "All of them are early on in the discovery process."
An explanation of concerns about vaccines that contain thimerosal and how they are being met can be found at the U.S. Centers for Disease Control and Prevention. Learn about vaccines in general from the American Academy of Family Physicians.