There is an interesting discussion in the public health arena about whether the removal of thimerosal from vaccines was actually harmful to the nation’s public health. Remember that when the AAP and USPHS recommended removing thimerosal from vaccines in 1999 and deferring certain vaccines, there was no concrete proof that there was any harm from ethylmercury exposure. However, it was a plausible concern given the known toxicity of a related chemical, methylmercury. Public health officials at the time felt it was a prudent decision, pending additional research.
An unintended consequence of the decision, though, was to diminish public trust in the vaccine supply. Both anecdotally in our own practice and statistically across the nation, many more parents became hesitant to vaccinate, even after the thimerosal was gone. They regularly asked us about other ingredients in vaccines and worried about other unknown risks from formaldehyde or animal tissue protein.
Over the past few decades, a lack of public trust in vaccinations has occasionally led to lower immunization rates in various countries. When these decreased rates occurred in Japan and Great Britain, small epidemics occurred of diseases that had been under control in the past. Statistically, if enough cases occur, then some bad outcomes will also occur. Indeed, with a mounting number of cases, some developed countries started reporting deaths from pertussis and measles.
This doesn’t seem to have occurred in the United States in the last eight years, but it is a concern. In order to recommend the prudent course of action (avoiding mercury in vaccines), public health officials may have contributed to lower vaccine rates and increased the risk of contracting the disease. In short, they may have traded an unknown risk that appears to have been proven harmless for a known risk of increased disease.
Of course all this is in hindsight, and we now have reasonable evidence that the thimerosal in vaccines was not harmful. If ethylmercury had been shown to be harmful, the decision would have been applauded as brilliant and farsighted. But it does raise the question of what level of proof is necessary to act. Do you have to actually show that something is harmful? Or do you simply need a plausible theory?
When the CDC recommended against using the rotavirus vaccine in the late 1990s, it based its decision on hard evidence. There was clear proof of the increase in intussusceptions in the months following the administration of the vaccine. Should the U.S. Public Health Service have waited for similar clear proof in the case of thimerosal? But what about all the children who would be receiving the potential “toxin” while the research was being done? Can you justify harming some children while waiting for an answer? we have no clear answers to these questions.