On Wednesday the British medical journal Lancet issued a formal retraction of a widely discredited 1998 study by Andrew Wakefield that claimed to show the MMR vaccine caused autism. The retraction came after the recent revelation that Wakefield was accepting personal payments from lawyers for parents who claimed their children were harmed by the shots.
The study hurt public confidence in the vaccine for measles, mumps and rubella. Vaccination rates fell dramatically in the United Kingdom and several other European nations and have yet to recover. According to Britain's Sunday Times newspaper, in just 10 years the number of confirmed cases of measles skyrocketed to 1,348 compared with 56 in 1998.
In addition to the ethical problems inherent in Wakefield's research, it has several methodology problems.
First, the study relied on an extremely small sample size. The research included only 12 children. Studies with very few participants are unlikely to apply to the broader population because they have a high risk of not representing the average person.
Second, the study relied on retrospective evidence. Rather than starting out with a sample of children who neither had autism nor had yet been vaccinated, the study used parents who reported autism symptoms after the child was vaccinated. In other words, the researchers relied on parents of autistic children's memories of events - parents who were understandably upset and far from objective observers.
Third, the study used a biased sample that was not randomly selected. Almost half of the kids' parents were recruited by a lawyer who planned to sue the vaccine manufacturers.
Finally, the conclusions overstated the data. The published article stopped short of concluding that there was a causal link between the MMR vaccine and autism. However, when Wakefield spoke to the press he made no such disclaimer. And to this day, outspoken celebrities and the ill-informed continue to jump on the bandwagon.
One reason some parents think vaccines caused their child's autism is the unfortunate and purely coincidental timing of diagnosis. The first signs of autism don't appear until around 18 months to 2 years of age, the same time period in which kids are vaccinated for MMR. It is this unfortunate timing that makes parents think the vaccines caused the condition.
If vaccines really were causing autism, you'd at least expect to see the incidence of autism rise and fall with the rise and fall of vaccination rates - but it doesn't.
The Associated Press contributed to this report.