The United States Centers for Disease Control and Prevention (CDC) has revised its long-standing guidance about vaccines and autism.

The guidance once stated clearly and correctly that the evidence shows no link between vaccines and the development of autism.

Now it claims “studies supporting a link [between vaccines and autism] have been ignored by health authorities”. It also says:

The claim “vaccines do not cause autism” is not an evidence-based claim because studies have not ruled out the possibility that infant vaccines cause autism.

Health Secretary Robert F. Kennedy Jr directed the CDC to make these changes, despite promising at his confirmation not to alter the CDC’s vaccine advice.

With this change in wording on the website the CDC has been dragged to a new low. The CDC once stood as a global benchmark of scientific integrity. Sadly, it now risks becoming a megaphone for misinformation and a tool for those whose goal is to undermine science.

Let’s look at the updated CDC statement about vaccines and autism, and how this is at odds with how science works.

Science can’t prove universal negatives

Saying “studies have not ruled out the possibility that infant vaccines cause autism” is in direct conflict with how science works.

Using science, we can demonstrate that two things are linked by showing consistent, reproducible associations that stand up across multiple study designs. We can also test a hypothesis repeatedly and from many angles.

Therefore, for example, when high-quality studies using different methods, populations and measurements, all fail to find a link between vaccines and autism, the rational conclusion is there is no causal connection.

But we cannot prove the universal absence of a link.

If we were to accept this notion, someone could always claim they aren’t convinced by the current evidence because maybe the next study will find something. Using this same logic, it’s impossible to rule out the Earth is flat or that fairies exist.

It’s wrong to reverse the burden of proof

Another dangerous premise in the CDC’s new framing on vaccines and autism is it reverses the burden of proof.

In science, the person making a claim, especially one that argues against the available consensus, must provide the evidence for it.

The rhetorical manoeuvre on the CDC website suggesting proof is required to show the absence of a link, however, flips this principle on its head. It suggests it’s reasonable to expect scientists to defend against an infinite list of hypothetical possibilities.

But as US astronomer Carl Sagan famously put it, “extraordinary claims require extraordinary evidence”. In science, if you want to assert something that contradicts the scientific consensus, the burden is on you to produce evidence strong enough to justify overturning what we already know.

The more implausible a claim is, the higher the bar in providing high quality, reproducible and methodologically sound research to support it.

By asking the CDC to alter its website guidance, RFK Jr wants you to accept the opposite: that he or anyone can make any claim and it’s the responsibility of everyone else to disprove these claims.

It’s also unclear what evidence would change RFK Jr’s mind on vaccines and autism. This leaves the door open for him to claim any amount of evidence that doesn’t support his preferred narrative is insufficient.

But what about the study that claimed to be proof?

Speculation about a link between the measles, mumps and rubella (MMR) vaccine and autism began with a fraudulent and now-retracted 1998 Lancet paper by the discredited doctor Andrew Wakefield.

Even if you accepted everything in Wakefield’s paper as true (it wasn’t) and assumed he was an honest researcher (he wasn’t), you would still be left with nothing more than a case series of 12 children. This study design is incapable of establishing a causal link between the MMR vaccine and autism.

Subsequent investigations also uncovered a long list of damning findings about Wakefield, including:

1) He hid major financial conflicts of interest

Wakefield was paid large sums by lawyers preparing a lawsuit against MMR manufacturers, money he failed to disclose. He was contracted to find evidence supporting a link between MMR and autism.

At the same time, he had filed patents for a single-dose measles vaccine and a diagnostic test that stood to profit if public fear about MMR increased.

2) He committed serious ethical violations

Wakefield falsely claimed the study had ethics approval. It did not. Children with developmental conditions were subjected to invasive procedures, including colonoscopies and lumbar punctures, without valid clinical justification or proper oversight.

3) He misrepresented how the children were recruited

The paper claimed the children were consecutively referred, implying an unbiased clinical sample. In reality, several were recruited through anti-vaccine groups or families involved in the lawsuit funding Wakefield, meaning the sample was deliberately cherry-picked to support his predetermined hypothesis.

4) He altered and falsified data

Comparisons between medical records and the published paper revealed extensive falsification:

• symptoms that began before vaccination were rewritten as occurring after MMR
• gastrointestinal findings were exaggerated or invented
• diagnoses were manipulated to fit his fabricated “autistic enterocolitis” syndrome
• normal clinical results were presented as abnormal.

The tragedy in all this is that a fraudulent study that never should have seen the light of day continues, even now, to erode confidence in life-saving vaccines. This has led to reduced vaccination rates, the resurgence of preventable childhood illnesses, and unnecessary deaths.

It has also inflicted immeasurable harm on autistic people and their families by fuelling stigma and misinformation.

Hassan Vally, Associate Professor, Epidemiology, Deakin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.