Study Suggests Mercury in Vaccine Was Not Harmful

By DONALD G. McNEIL Jr.

small but groundbreaking study of infants who received vaccines containing a mercury-based preservative has found that the levels of mercury in their blood were well within federal safety limits.

The study, reported on Saturday in the British medical journal The Lancet, also found that infants excrete the mercury much faster than expected, suggesting that it does not build up from one vaccination to the next.

The preservative, thimerosal, is no longer used in American vaccines for infants under 6 months old, but the issue is important to parents of children who did receive thimerosal-containing vaccines as infants and are now autistic. Thousands of those parents have filed damage claims or lawsuits against thimerosal's maker, Eli Lilly & Company, although a clause protecting Lilly from such suits was mysteriously slipped into the domestic security bill signed into law by President Bush on Nov. 25.

Dr. Neal Halsey, director of the Institute for Vaccine Safety at Johns Hopkins University, who was not involved with the study, praised it as "much needed and done quite well," although he said more work was needed, especially on underweight babies and long-term follow-up.

But Sallie Bernard, director of Safe Minds, a parents' group suing the vaccine industry, vehemently attacked the report, calling its optimistic conclusions "very much off base."

Mercury is unquestionably poisonous. At extreme doses, it causes tremors and madness. Children who accidentally get high doses tend to speak and walk later and to have tics and lower intelligence. But no medical evidence has ever shown that it causes autism.

Small amounts of mercury are common in soil and plants, in power-plant fumes and in dental fillings. Fish are the largest source for humans: a tuna sandwich may contain more mercury than a vaccine shot.

Though no scientific study has proved that thimerosal causes any ill effects, vaccine makers began eliminating it in mid-1999 at the urging of federal health officials. The newly reported study commenced with that recommendation.

Thimerosal, which kills fungi and bacteria, is still used to preserve vaccines sent to the third world, and the World Health Organization defends it. The diseases that vaccines prevent are far more common there, so vaccines save more lives than can be harmed by rare side effects. Further, vaccines there must survive dirtier storage conditions and are shipped in multiple-dose bottles to save money, factors that make thimerosal's abilities as a germ-killer all the more appealing.

The study in The Lancet was led by Dr. Michael E. Pichichero of the University of Rochester. The researchers tested the blood, urine and stools of 33 infants from 2 to 6 months old, all of whom were seen by Rochester pediatricians injecting thimerosal-containing vaccines. These infants were compared with 15 infants seen at a clinic in Bethesda, Md., that used mercury-free vaccines.

The Environmental Protection Agency sets the safe level of mercury in children's blood at no more than 5.9 parts per billion. That limit, Dr. Pichichero explained, is based on a study of children in the Faeroe Islands, southeast of Iceland, whose mothers ate whale blubber polluted with mercury and PCB's. Mothers who had 59 or more parts per billion of mercury in their blood while pregnant gave birth to children who scored lower on intelligence tests several years later. The E.P.A. took one tenth of that — 5.9 parts per billion — as the safe-level limit.

In the Rochester study, all but one of the infants in the group exposed to thimerosal had mercury levels of 1 to 3 parts per billion in their blood; the one exception went to 4.1. In the thimerosal-free control group, only one baby had even a measurable level of mercury.

The babies exposed to thimerosal had high concentrations of mercury in their stools, indicating that it was being rapidly excreted. Its half-life in the blood appeared to be seven days, a finding that surprised the investigators, Dr. Pichichero said, since methyl mercury, the kind found in fish, has a half-life of 45 days.

Quick excretion suggests that ethyl mercury, the kind in thimerosal, would not build up in the brain from a child's two-month shots to the four-month and six-month shots, causing cumulative damage, said Dr. Halsey, of the vaccine safety institute. That conclusion is reassuring, he said.

"I give the investigators lots of credit for acting quickly and doing a study that a lot of people didn't want to do," he added. "Who wants to sit down and say to parents, `We're giving your babies vaccines containing mercury'?"

But Mrs. Bernard, of Safe Minds, called the study "small and mediocre" and said it was "alarming" that The Lancet had given it prominence.

She complained among other things that Dr. Pichichero had done studies for vaccine makers, that 33 children were too few from which to draw conclusive findings and that the infants' blood had not been drawn until several days after immunizations, so peak levels might have been missed.

Dr. Pichichero acknowledged that he had done antibiotics studies for Lilly and vaccine studies for other companies, but noted that the National Institutes of Health had paid for this one.

He also acknowledged that 33 children were relatively few, and said a follow-up study of 200 children was under way in Argentina, where thimerosal is still used.

But he argued that his conclusions were valid. Mercury levels in the body take days to stabilize, he said, damage to fetuses is greater than to infants, and neurological damage is thought to result from long-term high mercury levels, not brief peaks.

Suggesting that a fetus's exposure to more than 59 parts per billion of methyl mercury for weeks is comparable to a 2-month-old child's exposure to 5.9 parts of ethyl mercury for a few hours or days "is mixing apples and oranges," he said.

One immunization expert not connected to the study, Dr. Charles G. Prober, a professor of pediatrics at Stanford University, also defended the work, saying it "addressed an issue we had no data about." He found the procedures "quite valid" and the sample size acceptable.

"I'm not sure that peak exposures are relevant," Dr. Prober said. "Most problems are associated with chronic exposure."