In the span of less than a decade, the public went from being blissfully unaware of thimerosal, the mercury-based preservative that was used in a number of vaccines, to a place where death threats against vaccine advocates are now issued with frightening regularity. How exactly did this happen? PLoS Biology has produced an article that attempts to provide a historical perspective on these developments and fit them into the wider issue of how scientific information is communicated with the public. The story turns out to be one of historic accidents and missed opportunities, but one that could provide some valuable lessons to both public health authorities and the scientific community at large.

The article is well written, should be accessible to nonscientists, and is open access, so I'll only recap its outlines, while focusing on aspects of the story that the author didn't develop.

In the US, the vaccine scare can apparently be traced back to an apparently unrelated event in 1997, when a funding bill included a rider that called for the FDA to conduct an inventory of all possible sources of mercury exposure from the products it regulated. The inventory came at a time when the vaccination programs were benefitting from increased productivity—boosters were scheduled based on a better understanding of epidemiology, vaccines were becoming available for more illnesses, etc.—that had increased the number of injections children were receiving. The result was that the FDA found that children were exposed to unexpected levels of thimerosal, a preservative used in some vaccines that contains ethyl mercury.

The obvious question was whether this was significant, and scientists tended to respond with the default answer when information isn't available: we don't know yet. Although methyl mercury accumulates in nerve cells, leading to toxic effects, ethyl mercury wasn't well studied at the time. Public health authorities reacted with a message that was meant to reassure the public. In the absence of clear information, they'd apply the precautionary principle, and work with vaccine makers to eliminate thimerosal, which was done by 2001. What they failed to grasp, however, is that in the hands of interested parties, a message that was intended to reassure was presented as an admission of problems.

In any case, subsequent studies have shown that ethyl mercury doesn't accumulate in the same way as its chemical relative, and that the elimination of thimerosal hasn't put a dent in the increased rates of autism diagnoses. But, in the meantime, damage has been done. Pockets of unvaccinated children now exist in many US states, and a variety of childhood illnesses are making reappearances there.

Accident number two was a result of the fact that, at roughly the same time, a completely separate vaccine scare started in the UK, where Andrew Wakefield published a study claiming to find a link between the MMR vaccine (which contains no thimerosal) and autism. Wakefield has since been charged with research improprieties and conflicts of interest, and the original results could not be confirmed. But, because of the focus on MMR, only measles seems to be enjoying a resurgence there, where cases rose to over 1,000 last year.

Certainty in the face of contrary evidence

Despite the completely unrelated scares, the numbers are disturbingly similar. In both countries, a quarter of adults think that vaccines can contribute to autism, no matter that the evidence against the connection has become ever more decisive in recent years; the article has an entire section entitled "Evidence-Resistant Theories" describing this phenomenon.

There are two things I'd add to the author's analysis here. For one, despite the fact that the panics in the US and the UK were based on essentially unrelated mechanisms, it's safe to assume that the majority of the public doesn't recognize this. Instead, the fact that the fears extend across national boundaries is probably taken as evidence that there is something concrete behind them.

The second is that it doesn't follow the incredible morphing of fears in the face of contrary evidence. As the evidence for vaccine-driven mercury toxicity became shakier, a variety of other vaccine related factors took the blame. Vaccines were said to contain harmful chemicals aside from thimerosal, or the mercury there was said to interact with various other environmental factors, or the vaccines were scheduled too closely together. In short, advocates of a link between vaccines and autism are demanding that the scientific community follow them down an infinite regression of potential fears. In cases where the medical community has responded with a detailed evaluation of specific ideas, such as a CDC study released in 2003, the vaccine skeptics responded with death threats against agency officials.

These same skeptics also learned to use the tools of the scientific trade against science. The founders of one autism advocacy group (one of whom is identified as a marketing consultant) published an article proposing a mercury-autism link in a publication called Medical Hypotheses that is described as "an unconventional journal that welcomes 'even probably untrue papers.'" One physician published a case report—essentially a formalized description of an anecdote—without disclosing the case involved his own daughter, and that he was pursuing her case in the US vaccine court at the time. A number of vaccine skeptics also push unsupportable autism ideas at the same time they profit from ostensible "cures" based on these ideas.

Lessons to be learned

Oddly, despite these obvious conflicts of interest, it's the medical community that's frequently the target of accusations that it's too cozy with the pharmaceutical industry. Vaccines are manufactured products, and public health officials often assist in their development and/or advocate their use, so it's difficult to avoid these perceptions to a degree. What the article doesn't note, but should be said, is that the medical community as a whole has hurt its credibility through various practices that allowed doctors to extract cash from pharmaceutical companies. Even though these issues are not related to vaccines, they contribute to a general sense of corruption and undermine public confidence, which spills over to vaccination programs.

The second is that the public health community was unprepared to discuss this issue via the popular press. Reporters often attempt to present scientific issues with the same "balance" they use for political ones, even though it's wildly inappropriate. Vaccine advocates needed to be prepared to handle this. In several cases, they also chose to make no response to stories that appeared in the places like Rolling Stone which ran a misinformation-filled article penned by Robert F. Kennedy, Jr.

The final issue is that, despite the fact that they're a public health issue, many people view vaccines as an area of personal responsibility. If parents were to simply follow expert advice, they'd be left with the sense that they're not personally involved in decisions regarding their children. The net result is that they view public health experts as voicing just another opinion, and often adopt alternate vaccine schedules that leave their children at risk, simply because it provides them the opportunity to feel as if they have taken action to safeguard their children. The public health community needs to convey that their statements are based on the best available information, and vaccine schedules are formulated based on that.

Although these lessons are developed in response to the problems that the public health community has faced in convincing the public to accept the benefits of vaccines, it's easy to see how they can apply to other scientific fields.

PLoS Biology, 2009. DOI: 10.1371/journal.pbio.1000114