In the U.S., however, thimerosal continued to be added throughout the '90s to a number of widely used pediatric vaccines for hepatitis B, bacterial meningitis, diphtheria, whooping cough and tetanus.
It was added to multi-dose vials of vaccine to prevent contamination from repeated insertion of needles to extract the medicine. It was not needed in single-dose vials, but most doctors and clinics preferred to order vaccine in multi-dose containers because of the lower cost and easier storage.
The Hilleman memo said that unlike regulators in Sweden and some other countries, "the U.S. Food and Drug Administration ... does not have this concern for thimerosal."
A turning point came in 1997 when Congress passed a bill ordering an FDA review of mercury ingredients in food and drugs.
Completed in 1999, the review revealed the high level of mercury exposure from pediatric vaccines and raised a furor. In e-mails later released at a congressional hearing, an FDA official said health authorities could be criticized for "being 'asleep at the switch' for decades by allowing a potentially hazardous compound to remain in many childhood vaccines, and not forcing manufacturers to exclude it from new products."
It would not have taken "rocket science" to add up the amount of exposure as the prescribed number of shots was increasing, one of the e-mails said.
While asserting that there was no proof of harm, the U.S. Public Health Service in July 1999 called on manufacturers to go mercury-free by switching to single-dose vials. Soon after, Merck introduced a mercury-free version of its hepatitis B vaccine, replacing the only thimerosal-containing vaccine it was still marketing at the time, a company spokesman said.
By 2002, thimerosal had been eliminated or reduced to trace levels in nearly all childhood vaccines. One exception is the pediatric flu vaccine made by Aventis and still sold mainly in multi-dose vials.
