Hilleman is a former senior vice president of Merck who developed numerous vaccines for the company. A 1999 profile in the Philadelphia Inquirer said that "it is no exaggeration to assert, as many scientists do, that Maurice Hilleman has saved more lives than any other living scientist."
Hilleman, 85, currently director of the Merck Institute for Vaccinology, had officially retired and was a consultant to Merck when he wrote the '91 memo. He declined to be interviewed.
The memo was sent to Dr. Gordon Douglas, then head of Merck's vaccine division and now a consultant for the Vaccine Research Center at the National Institutes of Health. Douglas also declined to comment.
The memo stated that regulators in several countries had raised concerns about thimerosal, including in Sweden, where the chemical was being removed from vaccines.
"The public awareness has been raised by the sequential wave of experiences in Sweden including mercury exposure from additives, fish, contaminated air, bird deaths from eating mercury-treated seed grains, dental amalgam leakage, mercury allergy, etc.," the memo said.
It noted that Sweden had set a daily maximum allowance of mercury from fish of 30 micrograms for a 160-pound adult, roughly the same guideline used by the FDA. Adjusting for the body weight of infants, Hilleman calculated that babies who received their shots on schedule could get 87 times the mercury allowance.
The Swedish and FDA guidelines work out to about four-tenths of a microgram of mercury per kilogram of body weight. A stricter standard of one-tenth of a microgram per kilogram has been adopted by the Environmental Protection Agency and endorsed by the National Research Council.
These standards are based on methyl mercury, the type found in fish and airborne emissions from power plants. Though toxic, the ethyl mercury in thimerosal may be less hazardous than methyl mercury, some scientists say, because it is more quickly purged from the body.
"It appears essentially impossible, based on current information, to ascertain whether thimerosal in vaccines constitutes or does not constitute a significant addition to the normal daily input of mercury from diverse sources," the memo said.
"It is reasonable to conclude" that it should be eliminated where possible, he said, "especially where use in infants and young children is anticipated."
