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In all of human history, the invention of vaccination should be classed as one of our greatest medical triumphs. This innovation has saved millions of lives and prevented untold suffering and misery; it has brought many once-epidemic diseases under control, and eradicated others altogether. Tragically, our era has seen a dangerous new strain of pseudoscience emerge, one that threatens all of these gains.

The theory behind vaccines is conceptually simple. Human beings possess an exquisitely evolved immune system with a remarkable ability to learn from experience. For many diseases, once we’ve had them, we develop antibodies that “recognize” the particular pathogen that causes it, giving us lifelong protection against ever catching the same disease again. The innovation was to realize that we could administer killed or weakened germs – not enough to make the recipient sick, but sufficient to trigger the immune system and stimulate it to develop antibodies, so we can gain the immunity without ever having had the disease. In modern times, this technique has been refined by introducing not whole germs, but characteristic molecules that appear in a bacterium’s cell membrane or a virus’ protein coat. Done properly, this is sufficient to trigger the creation of antibodies.

For decades, the benefit of vaccines was unquestioned. But in the last few years, a vehement anti-vaccination movement has erupted. Hysterical, paranoid rhetoric about how doctors and pharmaceutical companies are “poisoning children” for the sake of profit are the stock in trade of this movement, which makes up in shrillness what it lacks in scientific support.

The anti-vaccine movement got its start in 1998, when a British researcher named Andrew Wakefield published a paper in the Lancet. This paper suggested that there was a link between childhood vaccination and autism, claiming that twelve (!) children who had received the measles-mumps-rubella (MMR) vaccine showed developmental regression soon thereafter. (Ten of Wakefield’s twelve co-authors have subsequently retracted this interpretation, and it’s been reported that Wakefield himself was being paid by trial lawyers seeking to file a lawsuit against vaccine manufacturers. This conflict of interest had not been previously disclosed, and the Lancet‘s editor has said he would not have accepted the paper if he had known about it.)

Despite the collapse of its scientific claims, Wakefield’s paper caused a firestorm. Vaccination rates in the UK showed a significant drop soon afterward. Soon, the antivaccinationists had even identified a supposed causative agent: thimerosal, a preservative that was used in MMR and some other vaccines. The molecular structure of thimerosal contains an atom of mercury, and it was this that antivaccinationists labeled the culprit. Some went so far as to label autism a kind of mercury poisoning – an obvious falsehood for anyone who knows the symptoms of the two conditions.

In response to public fear, scientific bodies such as the CDC asked vaccine manufacturers to remove thimerosal from their products. Although this was an understandable effort to reassure people who were worried, it only added fuel to the fire. Leading antivaccinationists boldly predicted that, as thimerosal was phased out, we would see a dramatic drop in autism rates.

This did not happen. Even after thimerosal was completely removed from vaccines, autism rates continued to rise, confounding the antivaccinationists’ predictions. (This is probably due to better screening and a widening of the diagnostic criteria, rather than a real increase.) In addition, numerous large, well-run studies have failed to find any causal connection between vaccination and autism.

An evidence-based movement would have dwindled away by now, but the antivaccination movement is not based on evidence. It is a pseudoscientific movement based on irrational fear, on obstinate mistrust of the medical establishment, and on the cultish sense of us-vs.-them which the movement’s leaders have taken care to cultivate. The evidence weighing against thimerosal as a cause of autism has grown so overwhelming that even some antivaccinationists can no longer ignore it, but rather than change their position, many of them have simply shifted the cause of concern to conveniently undefined “toxins” (a common, meaningless buzzword of quack-medicine communities), and continue to rail against the medical establishment with equal fervor.

In some respects, vaccines are a victim of their own success: the terrible diseases they were invented to combat have been so effectively defeated that people have forgotten just how bad they were, and so they no longer fear the consequences of not vaccinating. But few of those diseases have been completely wiped out. Instead, they linger on the margins… and when a significant number of people in a community refuse vaccination, they can come back with a vengeance. In one community in Colorado, whooping cough has reemerged, with sometimes fatal results:

In 2000 it killed seventeen people in the United States, including two Colorado babies, both of whom were taken to the hospital too late. “It was very sad,” Tina Albertson, a pediatric resident who cared for one of the infants, told me. “She was a six-week-old girl with a sister and a brother, four and six. The family had chosen not to immunize, and the week she was born, her siblings both had whooping cough. When they’re real little, the babies don’t whoop—they just stop breathing. This little girl was septic by the time they got her here.”

And see also:

“It is a frightening illness to see the paroxysms of coughing, especially in very young children,” Clark said. “They can cough uncontrollably and turn blue and not be able to get a breath. And it’s all so concerning because it is so exquisitely transmissible.”

Worst of all, parents who choose not to vaccinate are putting not just their own families but other people at risk as well. Few vaccines are 100% effective; instead, vaccination as a public health strategy relies on “herd immunity”, the idea that an epidemic can never catch hold in a population of resistant individuals. But even a small number of unvaccinated people can serve as reservoirs of disease, providing a repeated source of exposure and increasing the chances that even people who are vaccinated will get sick. This form of pseudoscience is a particularly vivid illustration of the dangers of credulity.

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DAYLIGHT ATHEISM Adam Lee is an atheist author and speaker from New York City. His previously published books include "Daylight Atheism," "Meta: On God, the Big Questions, and the Just City," and most...