The CDC’s latest ADDM report estimated autism prevalence among 8-year-olds at about
1 in 31 nationally, and the California site was
1.6 in 31. The key point is that California’s data came from
one San Diego-area surveillance site, not every child in the entire state.
The CDC report itself gives the explanation for why California is higher: better and earlier identification. California had access to health, education, early intervention, and state developmental-disability service records, and CDC specifically notes California has had the highest prevalence since joining the ADDM network, in part because of early-screening initiatives and regional centers that evaluate and coordinate services for developmental disabilities. In other words, California is finding more cases that other places miss or find later.
That matters because autism “prevalence” here means
identified autism, not necessarily the true biological rate of autism. If one state screens earlier, uses more data sources, has better service access, and diagnoses more children who would have gone undiagnosed elsewhere, its reported prevalence rises even if vaccines have nothing to do with it.
As for vaccinated vs. unvaccinated comparisons: yes, this has been studied repeatedly. One of the strongest studies followed
657,461 children in Denmark and compared MMR-vaccinated children with unvaccinated children. The adjusted autism hazard ratio was
0.93, with a 95% confidence interval of
0.85 to 1.02, meaning no increased autism risk was found. It also found no increased risk among children with autism risk factors or after vaccination timing windows.
Another large Danish population study of
537,303 children also found strong evidence against MMR vaccination causing autism.
The U.S. Institute of Medicine / National Academies reviewed the evidence on MMR, thimerosal-containing vaccines, and autism and concluded that the epidemiologic evidence
favors rejection of a causal relationship between MMR vaccine and autism, and also between thimerosal-containing vaccines and autism.
A 2014 meta-analysis pooling cohort and case-control data also found no increased autism risk from vaccination, MMR, mercury, or thimerosal exposure.