A mercury-based preservative once used in many vaccines does not raise the risk of neurological problems in children, concludes a large federal study that researchers say should reassure parents about the safety of shots their kids received a decade or more ago.
However, the study did not examine autism — the developmental disorder that some critics blame on vaccines. A separate study due out in a year will look at that issue, said scientists at the federal Centers for Disease Control and Prevention, who led the latest analysis and published results in Thursday's New England Journal of Medicine.
They found no clear link between early exposure to the preservative thimerosal and problems with brain function and behavior in children age 7 to 10. The results are in line with past research that found no connection between vaccines and neurological problems or autism.
Thimerosal (pronounced thih-MEHR'-uh-sawl) has not been used in childhood vaccines since 2001, although it is still in some flu shots. The new findings apply to children immunized before then, or exposed to the preservative through shots their mothers received while pregnant. Thimerosal was put in vaccines to prevent contamination from bacteria.
Some doctors say the CDC study should reassure parents worried about the safety of vaccines.
"It's good news for families," said Dr. Michael Goldstein, vice president of the American Academy of Neurology who works in private practice in Salt Lake City. "There's no evidence that these vaccines have caused injury."
The study involved 1,047 children who were exposed to varying levels of thimerosal while in the womb or after birth in the 1990s. The children belonged to four health maintenance organizations that are part of a federal project to study the side effects of vaccines. Their mercury exposure was determined through medical and immunization records and interviews with parents.
Each child was tested for speech and language skills, motor coordination and intelligence. Parents, teachers and trained specialists also rated stuttering, attention span and tic disorders such as head shaking, eye blinking and neck jerking. A total of 42 neurological problems were analyzed.
On balance, researchers did not find a consistent pattern between increasing thimerosal exposure and the risk of these problems. However, they said one finding merited further study: Boys exposed to higher mercury levels seemed to have more tic problems — a link seen in previous research.
"The doses of mercury that children were exposed to because of immunization doesn't cause neuropsychological damage," said Dr. Bruce H. Cohen, a Cleveland Clinic pediatric neurology specialist who had no role in the study.
The CDC study was reviewed by an independent panel of scientists and statisticians who oversaw its design, reviewed results and contributed to writing the report.
The panel included one vaccine opponent — Sallie Bernard, executive director of the consumer group SafeMinds. Although she had a role in planning the study, she asked to be listed as a "dissenting member" because she disagreed with the study's conclusions.
The research was led by William Thompson, a CDC epidemiologist who once worked for vaccine maker Merck & Co. Four other researchers have received fees from drug companies and one has served as a consultant to a CDC committee on immunization.
The study was not designed to tease out the effects of mercury exposure on autism. Thompson is completing a separate study examining whether thimerosal exposure before or after birth causes autism. The study recruited 1,000 children including 250 with autism. Results are expected next year.
Autism is a major public health concern, with one in 150 American children diagnosed with the disorder characterized by repetitive behaviors and impaired social interaction.
Although past scientific studies have found no link between autism and thimerosal-containing vaccines, the highly charged issue went on trial this summer.
A court in Washington, D.C., heard from an Arizona mother who blamed vaccines on her 12-year-old daughter's severe autism. The case is being followed by about 5,000 families who filed similar claims to receive compensation from a federal vaccine injury fund. The fund so far has not paid out an autism claim.