The scare campaigns claiming, against all evidence, that vaccinations cause autism have contributed the return of diseases once banished from places with advanced medical systems. However, it turns out children with autism are actually less likely to be fully vaccinated than their counterparts.

If we were as quick to jump to conclusions – and as slow to test them – as the anti-vaccination movement, we might conclude vaccinating prevents autism. That's not the case, however. Instead, parents whose children are diagnosed with autism are more susceptible to anti-vaxx scare campaigns and avoid later injections.

Moreover, the younger siblings of autistic children are also less likely to be vaccinated, again apparently because parents distressed by the diagnosis get scared there is something to the myths. Several partially or completely unvaccinated children living together greatly raises the risk of disease outbreaks, and the situation could be even worse if the parents influence friends who live nearby.

The findings come from a study published in JAMA Pediatrics using vaccination records for American children born between 1995 and 2010, and their younger siblings, born between 1997 and 2014. Dr Ousseny Zerbo of the Kaiser Permanente Vaccine Study Center and colleagues found 3,729 children diagnosed with autism spectrum disorder (ASD) and 592,907 with no such diagnosis in the sample.

Children with ASD were 13 percent less likely to be vaccinated than their counterparts, but the effect may be larger for vaccines scheduled after the diagnosis.

The younger siblings of children with ASD were 14 percent less likely to be up to date with their immunizations at the age of one. Encouragingly, however, the difference was just 4 percent at ages 11-12, suggesting many parents who are initially scared off vaccinating their children after an autism diagnosis for the first child subsequently get over these concerns, perhaps after consulting pediatricians.

Previous studies of the same topic had produced conflicting results. However, these used tiny sample sizes compared to Zerbo's. Nevertheless, these studies provided other information, sometimes asking parents about their motivation, confirming some parents blamed their child's ASD on vaccines, causing them to delay or abandon vaccination for younger siblings.

The human brain has a tendency to assume that if events are close together in time the former must have caused the later, which the Romans called “Post hoc, ergo propter hoc”. With autism, symptoms often first becoming clear at similar ages to vaccination schedules, it is unsurprising parents will jump to conclusions, particularly when deluged with false claims. In this context, it is perhaps a relief that vaccinations don't drop off more in families with an ASD diagnosis, but the work may assist health care educators in identifying who to prioritize for discussions of vaccinations' benefits and the true nature of risk.