The U.S. Public Health Service and the American Academy of Pediatrics (AAP) have asked the manufacturers of many childhood vaccines to reduce or eliminate small amounts of an organic form of mercury that has been used for decades in immunizations as an antibacterial preservative.

Called thimerosal, this mercury can be found found in more than 30 vaccines, including those against hepatitis B, whooping cough, diphtheria, tetanus and bacterial meningitis. None is found in live virus vaccines such as immunizations against measles and mumps.

Health officials stressed that today's vaccines with thimerosal are safe, and said that the current action is being taken to avoid a hazard that theoretically could occur if an infant received many shots of vaccines with thimerosal.

Nonetheless, they have acted quickly to reduce this potential hazard. It was only two weeks ago that the Food and Drug Administration informed them that mercury levels in small babies who receive many thimerosal-containing vaccines during one doctor's visit could exceed Environmental Protection Agency's precautionary limits. At very high levels, mercury can cause neurological damage in children.

"The news came as a surprise to everybody," said Louis Z. Cooper, a Columbia University professor of pediatrics and a member of the board of directors of the AAP.

"We pediatricians have been using vaccines with thimerosal for 50 years and haven't given it a second thought," he said. "It's important to take these steps now," he continued, adding: "If there was a problem here regarding harm to infants, we would have picked it up sooner."

Health officials said thimerosal has been used in vaccines since the 1940s and that there have been no reports of actual cases of mercury poisoning from vaccines. But infants today receive more immunizations than was true a generation ago. Some of the current concern is fueled by that increased number of recommended vaccinations--generally six in their early months and 11 by their second birthdays.

Some companies already make vaccines without thimerosal, and health officials said they were confident the manufacturers would be able to reduce or eliminate the mercury soon. In a statement issued last week, the Pharmaceutical Research and Manufacturers of America, which represents five vaccine producers, said that while there is no clinical evidence of harm from thimerosal, "there is a general consensus that it would be preferable to eliminate thimerosal from vaccines whenever possible. The vaccine industry is working closely with FDA and other government agencies to meet this objective."

Meanwhile, health officials strongly recommended that parents continue inoculating their children during the changeover to thimerosal-free vaccines. They said the dangers from childhood diseases were far greater than those from very small doses of mercury.

"We weighed the very real and known risks of withholding vaccine with the mercury and the very small risk of using it," said Jon Abramson, a Wake Forest University doctor and chairman of the AAP infectious disease committee. "It wasn't close. The truth is that every medicine and every vaccine carries a risk, and our job is to determine when the risk is worth taking. And it certainly is an appropriate risk here."

At the same time, officials did propose one modification in infants' vaccine schedule. They recommended that most infants be given their first hepatitis B shot when they are between 2 and 6 months old, rather than at birth. That delay would decrease the mercury intake when infants are most vulnerable, they said, and it would also pose little danger of disease to the babies.

That change in particular was welcomed by Barbara Loe Fisher, president of the National Vaccine Information Center, which has been critical of many national vaccine policies. During congressional hearings in May chaired by Rep. John Mica (R-Fla.), numerous parents testified that their children had been harmed by adverse reactions to the hepatitis B vaccine, including the development of arthritis-like symptoms.

She applauded the action taken by the Health Service and the AAP but said, however, that "there is a real question as to whether current stocks of childhood vaccines containing mercury should be used and whether vaccination of babies under 6 months of age with multiple vaccines containing mercury should be delayed."

The mercury problem comes at a time when public health officials are often on the defensive when they promote vaccinations. With many childhood diseases now largely controlled, parents have become less concerned about missing vaccinations for their children.

With this dynamic in mind, Surgeon General David Satcher issued the following statement last week: "Terrible childhood diseases like whooping cough, bacterial meningitis, polio and diphtheria are waiting for us to let our guard down. The risk of devastating childhood diseases from failure to vaccinate far outweighs the minimal, if any, risk of exposure to cumulative levels of mercury in vaccines."