Time Trends in United States Autism Prevalence with Co-Occurring Intellectual Disability: Is There a Signature of Thimerosal?
Science, Public Health Policy and the Law, Cynthia Nevison, 06/25/2025, v7.2019-2025
Abstract
The time trend in the cognitive ability of U.S. children identified with autism spectrum disorder (ASD) was systematically tracked for the first time from birth year 1992 to 2014 using data from the Autism and Developmental Disabilities Monitoring (ADDM) Network. The majority of ASD cases over this period had either co-occurring intellectual disability (ID), defined as IQ < 70, or IQ in the borderline range, defined as IQ = 71-85. The fraction of ASD cases with co-occurring ID varied widely among states. The ID fraction was also lower among White children compared to Black, Hispanic, and Asian children, and until recently, among boys compared to girls. The nationwide mean ID fraction initially declined from 48% in birth year 1992 to a low of 31% in birth year 2002, with a particularly steep drop between birth year 2000 and 2002. After 2006, the ID fraction steadily increased back to a pre-2000 value of 40%-41% by birth year 2014. The renewed increase in the ID fraction between birth year 2006 and 2014 fundamentally contradicts the idea that ASD prevalence continues to increase because the ASD diagnosis is expanding to include more mildly affected, high-IQ children. Some of the variations in the ID fraction over time are likely artefacts of the continually changing composition of states in the ADDM surveillance population, especially in the early years, as well as the declining proportion of Whites. However, those factors only explained a small part of the observed variations. The sharp drop in the ID fraction between birth year 2000 and 2002 coincided with the removal of thimerosal, a mercury-containing preservative and neurotoxin linked to autism severity, from most childhood vaccines. Conversely, the renewed increase in the ID fraction after 2006 coincided with the reintroduction of thimerosal via flu shots promoted for infants and pregnant women. Low-income women and children are particularly likely to receive those products in some states, due to health insurance and daycare requirements, which may be contributing to autism severity as well as to the recent strong divergence in ASD prevalence by race/ethnicity.
