Child neurodevelopment

Autism: what the WHO and CDC say, which signs show up early, and why vaccines have nothing to do with it

According to WHO estimates, about 1 person in 127 had autism spectrum disorder in 2021. A brief guide to the definition, early signs, screening, risk factors and the vaccine myth.

Neuro Hub editorial teamOctober 27, 20263 min read

In brief

According to Global Burden of Disease data for 2021, about 1 person in 127 had autism; estimates differ markedly between studies.
At two years, an experienced professional’s diagnosis can be considered very reliable; signs are sometimes noticeable even earlier.
The MMR vaccine does not cause autism: the study that claimed otherwise turned out to be fraudulent and was withdrawn.

What autism is

The WHO defines autism (autism spectrum disorder) as a diverse group of conditions of brain development. It is marked by some difficulty with social interaction and communication, and by atypical patterns of activity and behaviour. The abilities and needs of autistic people vary widely and can change over time.

How many people have autism

According to the 2021 Global Burden of Disease (GBD) database, which the WHO cites, worldwide about 1 person in 127 had autism. The WHO cautions: reported prevalence varies markedly between studies, and in many low- and middle-income countries it is unknown.

Which signs can be noticed early

The CDC list (May 2024) by age; a child may show not all of these signs, and sometimes none:

avoids eye contact or does not maintain it;
9 months: does not respond to their name, shows few facial expressions;
12 months: does not play simple back-and-forth games, uses few or no gestures such as waving;
15 months: does not share interests, for example does not show a favourite toy;
18 months: does not point to something interesting;
24 months: does not notice when others are hurt or upset;
36 months: does not join other children in play;
48 months: does not take part in pretend play;
60 months: does not sing, dance or perform for others.

The CDC lists among restricted and repetitive behaviours: lining up objects and becoming upset when the order is changed; repeating words or phrases; playing with toys the same way every time; focusing on parts of objects; strong distress over minor changes; intense interests and insistence on routines; repetitive movements (hand-flapping, spinning); unusual reactions to sounds, smells, tastes, looks or textures.

Screening and diagnosis

According to the American Academy of Pediatrics (as cited by the CDC), autism-specific screening is done at visits at 18 and 24 months. One well-known tool is the M-CHAT parent questionnaire.
Screening does not make a diagnosis. According to the CDC, its tools “do not provide conclusive evidence of developmental delays and do not result in diagnoses”; a positive result is grounds for a more thorough assessment.
Research has shown that autism can sometimes be detected at 18 months or younger. By age 2, a diagnosis by an experienced professional can be considered very reliable.

What is linked to a higher likelihood of autism

According to the WHO, both environmental and genetic factors likely make autism more probable. Associated factors include: advanced parental age, maternal diabetes during pregnancy, prenatal exposure to certain air pollutants or heavy metals, prematurity, severe birth complications and low birth weight. Prenatal exposure to valproate and carbamazepine also appears more often in children with autism.

The vaccine myth

The measles, mumps and rubella (MMR) vaccine does not cause autism. The study that suggested a link was later shown to be wrong and fraudulent, and it was withdrawn. Other childhood vaccines do not increase the risk of autism; studies of the preservative thiomersal and of aluminium as an additive also found no increase in risk.

Co-occurring conditions and support

People with autism often have epilepsy, depression, anxiety or ADHD. Intellectual functioning varies widely, from profound impairment to superior levels. The WHO stresses that timely access to early evidence-based psychosocial interventions can improve a child’s ability to communicate and interact, and that a child’s development should be monitored as part of routine maternal and child health care.

If you have concerns about your child’s development, see a paediatrician or family doctor and ask about developmental screening. Do not wait for the child to “grow out of it”.

Sources

This material is for information only and does not replace consultation with a doctor or psychologist. Self-medication can be harmful to your health.