Breaking silence on vaccines, measles, autism
UNICEF recently warned that global cases of measles are surging to alarmingly high levels. Worldwide, 98 countries reported more cases of measles in 2018 than in 2017, eroding progress against that highly preventable but potentially deadly disease. In the U.S., the CDC has confirmed more than 200 cases of measles from 11 states in just the first two months of the year. There were 372 cases in the whole of last year. The trend is indeed alarming.
After reading online comments and engaging in a heated Twitter chat with people who do not believe in vaccines, I realized there are many who still see a link between vaccines and autism. That belief is largely based on discredited and retracted research by Andrew Wakefield, a British physician who was struck off the medical register in 2010 for deliberately falsifying his findings. But once fear-inducing information has spread, it is hard to take back.
As a parent, I can understand the visceral fear of possibly placing your child in harm’s way. But the reality is, you can potentially cause more harm by not vaccinating. Vaccines have saved lives and eliminated diseases such as polio in the U.S. and smallpox in my home country of Nigeria.
Despite the proven benefits of vaccines, misinformation still spreads, so public health experts have developed guidelines to help separate unfounded fears from serious risks. One set is Hill’s Criteria for Causation, named after British epidemiologist Sir Austin Bradford Hill, who clearly demonstrated the connection between cigarette smoking and the development of lung cancer. Hill’s criteria are widely accepted in the modern era as a logical structure for investigating and defining causality in epidemiology.
How do concerns that vaccines cause autism stack up against those criteria?
The prevalence of autism in the U.S. increased by an astounding 754 percent from 1994 to 2005. The numbers have risen from 1 in 2,500 children in 1985 to 1 in 68 children in 2014. Vaccine coverage also has increased over the past few decades. That has contributed to the fear that vaccines may cause autism.
But does that actually infer causation? That is where the role of evidence in understanding cause and effect is so important.
Hill’s criteria for causation are a powerful guide to separating factors that lead to problems from those that do not. Among the nine criteria, strength (of association), plausibility and analogy have been most frequently used by opponents of vaccines to assert a causal link between vaccination and autism. But how strong is the evidence?
It is true that there has been an increase in the prevalence of autism as vaccine coverage has increased. It is also true that organic mercury is a neurotoxin. Up until 2001 in the U.S., several vaccines contained small amounts of the mercury compound thimerasol as a preservative. It is tempting, therefore, to assume a connection between the two.
However, well-designed epidemiological studies have not revealed any statistically significant association between the MMR vaccine and autism. That is also true for case-control studies that have compared autistic children to those without autism with regards to exposure to vaccines.
And, importantly, not all forms of organic mercury are equal, when it comes to toxicity: thimerasol contains the compound ethylmercury, which is safe, as opposed to methylmercury, the compound people are regularly exposed to when we eat fish.
The bottom line is this: The fact that events A and B occur together does not mean that A caused B or vice-versa. When examined in the light of Hill’s other six criteria, the vaccines-cause-autism arguments do not hold up.
∫ Consistency: Autism also occurs in unvaccinated children.
∫ Specificity: Autism does not occur in every (MMR) vaccinated child.
∫ Temporality: The development of autism is not related to the time of vaccination.
∫ Biological gradient: The incidence of autism among vaccinated children does not increase with the number of vaccines received.
∫ Coherence: Follow up studies show no significant elevation in blood mercury levels following vaccination.
∫ Experiment: Most vaccinated children do not develop autism.
In light of those facts, the World Health Organization and the U.S. Centers for Disease Control and Prevention continually reassure the public about vaccine safety. Like drugs, vaccines are safe and save lives in the vast majority of cases when used as directed, and they do not cause autism.
I left our Twitter debate with a promise to write a convincing piece. It is my hope that this post will at least help some people consider other alternatives to the cause of autism. For me and most public health scientists, the jury is in: Vaccines are innocent in this matter.
Dr. Utibe Effiong is a certified internal medicine physician with MidMichigan Health. He is also a public health scientist and an international development fellow at the Aspen Institute.