Some parents have voiced concern that mercury-containing thimerosal, formerly used extensively as a preservative for vaccines, could cause autism. But health authorities say that except for some flu vaccines, immunizations for children 6 and younger have not contained thimerosal as a preservative since 2001.

Only harmless trace amounts of the substance, used for cleaning equipment, may still be in some vaccines, the Centers for Disease Control and Prevention says. Manufacturers are working to produce thimerosal-free flu vaccines, the agency says.

Dr. Julie Gerberding, CDC director, also said recently that "the preponderance of evidence consistently does not reveal an association between thimerosal and autism." The independent Institute of Medicine, which analyzes scientific issues in medicine, reviewed studies relating to autism and could not find a link to thimerosal. The National Institutes of Health continues to study the issue.

Immune-system overload?

A study in the Aug. 10 edition of the Journal of the American Medical Association also addressed some parents' concerns that multiple vaccines could "overload" the immune system, making children more vulnerable to other diseases. The study by Danish scientists analyzed 805,206 children who had received routine vaccines for diphtheria, tetanus, pertussis, measles, mumps, rubella, polio and Haemophilus influenzae type b (Hib).

The researchers looked for any link between the immunizations and hospitalizations for acute upper-respiratory infections, viral or bacterial pneumonia, septicemia (a blood infection), viral central-nervous system infection, bacterial meningitis or diarrhea. They found a slight increase in the risk of acute upper-respiratory infection hospitalization after Hib vaccination, but said the effect could have been due to chance and was unlikely to be a real result of the vaccine. They found no other risks of hospitalization from the immunizations.

A 2002 review in the journal Pediatrics by U.S. researchers had a similar conclusion that multiple vaccines do not weaken the body's defenses:

"On the contrary, young infants have an enormous capacity to respond to multiple vaccines, as well as to many other challenges present in the environment. By providing protection against a number of bacterial and viral pathogens, vaccines prevent the 'weakening' of the immune system and consequent secondary bacterial infections caused by the natural infection."

Warren King: 206-464-2247 or wking@seattletimes.com