Experts are concerned about the US Centres for Disease Control & Prevention's (CDC) plan to use the Vaccine Safety Datalink (VSD) to probe an association between vaccines and autism, despite decades of data demonstrating the two are not linked.
Details of the proposed CDC study have not been formally announced, but The Washington Post said the agency plans to analyse VSD information.
The VSD is a collaborative project between the CDC and several healthcare organisations in the US that relies on electronic health record data to monitor vaccine safety.
VSD records have been used to identify real-time safety signals; they identified higher rates of myocarditis or pericarditis with the Covid-19 vaccines, for example. Whether they can identify longer-term, nuanced outcomes like an autism diagnosis is less clear.
“Could such a study conceivably be done through the VSD? Yes. The question is why you would want to do that,” said Gregory Poland, MD, President of the Atria Health and Research Institute in New York City and emeritus editor-in-chief of the journal Vaccine.
“The last time I counted, I think there were 28 studies about vaccines and autism that had been done across two or three decades, across multiple countries. The two hallmarks of science are generalisability – so, in different populations, different countries, different time frames – and repeatability," Poland told MedPage Today.
“Every single one of those studies has shown no association. Not all 28 studies have been of equally high value, but some have had superb study design.”
One of the largest was a population-based study of more than 537 000 children in Denmark published in the New England Journal of Medicine, which showed the risk of autism diagnosis was similar between those who received the measles, mumps and rubella (MMR) vaccine and those who did not.
Another study in the mix was a case-control analysis from CDC researchers using VSD data that showed that thimerosal, a mercury-containing preservative in vaccines and immunoglobulin preparations, did not raise the risk of autism.
Studies examining autism and vaccines are very hard to conduct, observed Paul Offit, MD, of Children’s Hospital of Philadelphia.
“The reason is that when people don’t get a vaccine, for example, the MMR vaccine, it’s usually because of an issue of access or an issue of choice; in other words, they chose not to do that,” he told MedPage Today.
Those people are different from people who get vaccines, Offit added.
“When you do these studies, you have to control for confounding variables like healthcare-seeking behaviour, medical background, or socio-economic background.
“You have to control for those three things, and then see whether or not there’s a difference. Because if someone didn't get a vaccine because they were scared of getting autism or because they didn’t trust the medical community, they may be less likely to go to the doctor and less likely to be diagnosed with autism.
“It could look as if the unvaccinated are less likely to develop autism when in fact, that’s not true.”
A rigorous study also needs to “develop a case definition for the outcome of interest – in this case, autism”, said William Schaffner, MD, of Vanderbilt University Medical Centre in Nashville.
“Autism is not a diagnosis associated with a specific laboratory test. It’s a syndrome,” he said. “How the diagnosis of autism is defined and corroborated in a study like this is absolutely critical.”
An autism diagnosis as it appears in the VSD database may not be sufficient, Poland said. “There are lots of different autism phenotypes or subtypes.
“How are you going to define those? It would be a massive amount of work because those aren’t always coded properly in medical records,” he continued. “You’re going to have to get permission to access those records. It’s harder, particularly in children, who are considered a vulnerable population from a research point of view, to access mental health psychiatry records.”
Patients in the VSD are enrolled in medical care systems and ought to have extremely high vaccination compliance, Schaffner added. “It will be very challenging to find a sufficiently sizable vaccine-negative population,” he said.
“And that population is likely to differ from the majority in a number of ways.”
Still, he sees possible value in yet another vaccine study. “I’m of two minds: the scientific half of my brain says this is a settled question. We don’t need to look at it again.
“However, the public health side of my brain acknowledges the reality that there are many people who still raise questions about the relationship of vaccines to autism. If a rigorous study were done under the aegis of the current HHS secretary and it came out with the same answer, perhaps that would put this issue largely to rest.”
This would have to be a very nuanced, very labour-intensive, very costly study, Poland pointed out. “That’s why I asked, why would you want to do it? I don’t think people who still believe that there’s an association will be convinced by a study like this.
“When you want to do a large, expensive, labour-intensive study, it’s because you have preliminary data that suggest this is a wise investment of material, time, and financial resources. Where are those preliminary data? Or is this an ideologic belief that we’re just going to keep doing studies until we get the answer we want? That’s not science. That’s ideology.”
Study details
A Population-Based Study of Measles, Mumps, and Rubella Vaccination and Autism
Kreesten Meldgaard Madsen, Anders Hviid, Mogens Vestergaard et al.
Published in The New England Journal of Medicine on 7 November 2002
Abstract
Background
It has been suggested that vaccination against measles, mumps, and rubella (MMR) is a cause of autism.
Methods
We conducted a retrospective cohort study of all children born in Denmark from January 1991 through December 1998. The cohort was selected on the basis of data from the Danish Civil Registration System, which assigns a unique identification number to every live-born infant and new resident in Denmark. MMR-vaccination status was obtained from the Danish National Board of Health. Information on the children's autism status was obtained from the Danish Psychiatric Central Register, which contains information on all diagnoses received by patients in psychiatric hospitals and outpatient clinics in Denmark. We obtained information on potential confounders from the Danish Medical Birth Registry, the National Hospital Registry, and Statistics Denmark.
Results
Of the 537,303 children in the cohort (representing 2,129,864 person-years), 440,655 (82.0 percent) had received the MMR vaccine. We identified 316 children with a diagnosis of autistic disorder and 422 with a diagnosis of other autistic-spectrum disorders. After adjustment for potential confounders, the relative risk of autistic disorder in the group of vaccinated children, as compared with the unvaccinated group, was 0.92 (95 percent confidence interval, 0.68 to 1.24), and the relative risk of another autistic-spectrum disorder was 0.83 (95 percent confidence interval, 0.65 to 1.07). There was no association between the age at the time of vaccination, the time since vaccination, or the date of vaccination and the development of autistic disorder.
Conclusions
This study provides strong evidence against the hypothesis that MMR vaccination causes autism.
Medpage Today article – Why CDC's Planned Vaccine-Autism Study Is Raising Eyebrows (Open access)
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