There have been widespread concerns that mercury-based preservatives used in vaccines might impair the neurological development of children, but the opposite seems to be true.

Immunizing infants with vaccines containing the preservative thimerosal may actually be associated with improved behavior and mental performance, according to two British studies published in the medical journal Pediatrics.

Dr. Jon Heron of the University of Bristol, and colleagues followed 12,956 children, born in 1991 and 1992, until they were about 7-1/2 years old. Information was collected on doses of thimerosal-containing diphtheria-tetanus-pertussis vaccines given at ages 3, 4, and 6 months, as well as on measures of behavior, fine motor skills, speech, tics and special education needs.

Instead of finding that outcomes were worse with increasing exposure to thimerosal, the authors saw less hyperactivity and conduct problems at 47 months, better motor development at 6 months and at 30 months, and reduced difficulty with sounds and need for speech therapy.

In the second report, Dr. Nick Andrews of the Communicable Disease Surveillance Centre, London, and colleagues conducted a look-back study involving 107,152 children born between 1988 and 1997.

There appeared to be protective effects from thimerosal-containing vaccine exposure for general developmental disorders, attention-deficit disorder, and unspecified developmental delay.

The only condition associated with increased risk with increasing thimerosal exposure was tics. However, “the vast majority of tics were minor transient events,” the authors note.

Hence, they conclude that “there is no reason to change current immunization practices with thimerosal-containing vaccines on grounds of safety.”

SOURCE: Pediatrics, September 2004.