Six-month-old children who received shots could get a mercury dose up to 87 times higher than guidelines for the maximum daily consumption of mercury from fish, according to the March 1991 memorandum obtained by the Times.
"When viewed in this way, the mercury load appears rather large," Dr. Maurice R. Hilleman, an internationally renowned vaccinologist, wrote to the president of Merck's vaccine division.
The memo came at a time when health authorities were recommending shots for children that contained an anti-bacterial compound called thimerosal, the Times reported. Thimerosal contains mercury and was once used in the measles-mumps-rubella vaccine.
In 1999, federal health officials concluded that routine vaccinations were exposing many infants to quantities of mercury above health guidelines. The U.S. Public Health Service said there was no evidence of harm but urged manufacturers to avoid mercury in vaccines.
Merck later introduced a hepatitis B vaccine that was mercury-free to replace the only thimerosal-containing vaccine, a company spokesman said.
Merck officials declined to discuss details of the memo with the Times because of pending litigation.
Mercury-laced vaccines have led to more than 4,200 claims in a special federal tribunal by parents who claim their children were harmed as a result. Alleged injuries include autism and other neurodevelopment disorders.
The newspaper obtained the memo from a lawyer who works with parent groups on vaccine safety issues and who said he acquired it from an unidentified whistle-blower.
Thimerosal has been largely removed from pediatric vaccines in recent years.
Separately,over Vioxx, the popular painkiller it introduced in 1999 and .
The company, based in Whitehouse Station, N.J., has been accused in the suits of marketing a drug that caused heart problems and concealing the risks. Merck has denied the allegations.