In 1928, in a small town in the Australian outback called Bundaberg, a local doctor received a shipment of TAT ("toxin-antitoxin") vaccine from the State Serum Institute in Perth, Western Australia, at the time perhaps the finest manufacturer of vaccines in the world. The TAT vaccine being shipped was made by taking the toxin produced by the diphtheria bacterium and combining it with neutralizing antibodies obtained from horses. These antibodies rendered the toxin harmless without diminishing the vaccine's ability to stimulate its recipients to produce their own antibodies. At the time, the TAT vaccine represented a stunning medical breakthrough.
The TAT shipped to Bundaberg was injected in the local children and then given a second time as a booster shot two weeks later. All the children became violently ill after receiving the second shot, and 12 died. When news of the disaster spread, the first explanation proposed was that the toxin-antitoxin mixture had somehow separated, rendering the vaccine toxic again. An investigation by an Australian Royal Commission, headed by Macfarlane Burnett—who later received a Nobel Prize for his contribution to immunology—established that the real culprit was the common skin bacterium Staphylococcus aureus. The germ was accidentally introduced into the bottles of vaccine during the first round of immunization. In the absence of refrigeration and without an additional preservative, the bacterium multiplied and produced its own toxin (which we now know is the same toxin that causes toxic shock syndrome). To prevent future events like the Bundaberg disaster, the Royal Commission recommended that all vaccines stocked in multiple-dose containers—bottles that might be opened more than once—include a preservative that would prevent any introduced bacteria from multiplying. This recommendation immediately became the worldwide standard for vaccine production.
The preservative thimerosal was chosen because it seemed to be nontoxic and because it did not diminish the protective quality of the vaccines to which it was added. (Thimerosal is more commonly known by the name merthiolate, familiar to my generation as the non-stinging, red skin disinfectant that replaced tincture of iodine.)