he recent backroom political maneuver that gave Eli Lilly protection against lawsuits for damage allegedly caused by a mercury-containing preservative in vaccines was not only an abuse of Congressional process. Its more pernicious effect was to fan fears about the safety of vaccines and the ingredients used to protect them from dangerous contamination.

The preservative in question, known as thimerosal, was used in many vaccines to prevent microbial contamination until concerns were raised in 1999 that cumulative doses of mercury might cause subtle harm to the developing brain. Since then, thimerosal has been dropped from the vaccines routinely administered to infants in America, but the issue remains important because thousands of parents whose children had previously received mercury-containing vaccines have filed damage claims or lawsuits alleging harm.

Although mercury is known to be toxic at high doses, there is very little data on whether very low doses of ethyl mercury, the form found in thimerosal, can be harmful. Last year the Institute of Medicine, an arm of the National Academy of Sciences, concluded that the scientific evidence neither proved nor disproved a link between thimerosal and neurodevelopmental disorders in children. But this year the World Health Organization endorsed the preservative for global use, concluding that there is no evidence of toxicity in infants, children or adults exposed to thimerosal in vaccines. That judgment was buttressed by an encouraging study published last Saturday in The Lancet, a British medical journal. It found that vaccines containing thimerosal did not raise the amount of mercury in infants above federal safety limits and that the mercury was excreted quickly, suggesting that it would not accumulate with repeated vaccine injections and cause damage.

Whatever risks might be posed by thimerosal are remote compared with the risk from not getting vaccinated. That is why the American Academy of Pediatrics recommended this week that infants as young as six months be given influenza vaccine, which still contains small amounts of thimerosal. The academy said healthy infants were at relatively high risk of hospitalization for the flu; thus the benefits of vaccination "outweigh the theoretical risk of adverse effects, if any, from the small volume of thimerosal" in the vaccine.