WASHINGTON, Oct 01
(Reuters Health) - Parents and doctors should avoid childhood vaccines containing thimerosol, even though there is no hard evidence connecting the mercury-containing preservative to autism and other neurodevelopmental disorders, a panel of government experts concluded Monday.
The authors of an Institute of Medicine (IOM) report announced that they had not found enough evidence to either accept or reject the idea that thimerosol-containing vaccines could lead to brain damage in children. But the link is "biologically plausible" given evidence that high doses of the chemical are known to be neurotoxic, the report states.
From its introduction in the 1930s until a near-complete government ban in 1999, thimerosol was used in several vaccines recommended for infants in the United States, including vaccines against hepatitis B, haemophilus influenza, and a combined vaccine against diphtheria, tetanus, and pertussis known as the DTaP vaccine.
The government acted because of concerns that vaccine doses recommended by the Centers for Disease Control and Prevention (CDC) could be linked to autism, attention-deficit/hyperactivity disorder (ADHD) or speech delay in children. Companies began making vaccine in single-dose packages so that thimerosol was not needed.
But the transition to thimerosol-free vaccine was not immediate, and pediatricians have long worried that safety fears would spur parents to avoid vaccinating their children, possibly exposing them to dangerous infectious diseases.
"It is far better to be vaccinated with a thimerosol-containing vaccine than to not be vaccinated," said Dr. Marie McCormick, a professor of maternal and child health at Harvard University and chair of the IOM committee.
A federal registry of vaccine-related adverse events posted nine cases of autism and nine cases of speech or language problems on children who had receive thimerosol-containing vaccines between November 1990 and May 2001. Preliminary evidence from one study suggested the compound could be linked to speech problems or ADHD.
Thimerosol contains ethyl mercury as a guard against bacterial contamination. But because direct studies of thimerosol are few and far between, reviewers were forced to try to draw conclusions from studies of methyl mercury, a related and highly toxic compound.
"If thimerosol can be removed from these products without risking bacterial contamination, it makes sense to do so," McCormick said.
Doctors may still stock some DTaP vaccine containing thimerosol since the chemical was only fully removed in March of this year, according to Len Lavenda, a spokesman for Aventis Pasteur, the maker of the Tripedia brand of the vaccine.
The company does not know how much thimerosol-containing DTaP is still on the market, "but our supposition is that most of it is gone," he said.
Vaccine safety groups praised the IOM's conclusions, but warned that some vaccines still contain thimerosol. Parents should ask to see the package inserts of vaccines before allowing their children to receive shots, said Barbara Loe Fisher, president of the National Vaccine Information Center.
Fisher also expressed concerns about adult flu vaccines, many of which still contain thimerosol. "Doctors should inform pregnant mothers that the flu vaccine contains mercury," she said.
Childhood vaccine safety has also garnered the attention of Congress, which has held several hearings on possible links between vaccines and neurodevelopmental problems. "Hearings on the childhood vaccine issue will continue," said Mark Corallo, a spokesman for the House Government Reform and Oversight Committee.