One hypothesis for the rise in autism is exposure to the chemical preservative Thimerosal used in many childhood immunizations. Thimerosal is a mercury-containing preservative used in some vaccines and other products since the 1930's. During the 1970s and 1980s, the number of mandated vaccines gradually increased from 8 in 1980 to 22 in 2000, to cover a wider range of potential diseases.  

In 1998, British gastroenterologist Dr. Andrew Wakefield published a paper in the journal the Lancet in which he described a dozen young patients who were suffering from both autism-like symptoms and intestinal problems including inflammation, pain and bloating. Eight of the kids reportedly began exhibiting signs of autism days after receiving the vaccine against measles, mumps and rubella (MMR). While Wakefield and his co-authors were careful not to suggest that these cases proved a connection between vaccines and autism, they did imply that exposure to the measles virus could be a contributing factor to the children's autism. A subsequent investigation by reporter Brian Deer largely discredited Wakefield's work, which was retracted by the Lancet in Feb., 2010.

In October 1999, Cable News Network (CNN) aired a program on which the parents of a three-year-old boy stated that he had developed autism two weeks after receiving the measles, mumps and rubella (MMR) vaccine. An American Academy of Pediatrics official on the program explained why there was no reason to believe that a link exists between autism and vaccination, but dramatic before-and-after videotapes of the child may have persuaded some parents to think twice about having their children vaccinated. The high visibility of the CNN report helped bring the possibility of a link between immunizations and autism into public debate, with the controversy continuing to this day.

Many conflicting studies and discussions about the spread of autism have failed to yield a clear consensus on the cause, with those who suspect an immunizaton link accusing big business and the government of a coverup, while those who doubt a link dismissing the suggestion as conspiracy theory. This report looks at one small piece of the puzzle to see what percentage of kindergarterners in California are receiving all of the required immunizations.

The California Department of Public Health conducts the Kindergarten Immunization Assessment each fall to monitor compliance with the California School Immunization Law. Schools assess the immunization status of kindergarten students and report this information to the Immunization Branch. For this report the assessment data for 1991-2008 (by county) was collected in Adobe Acrobat (PDF) format. These PDF files were then exported to Microsoft Word with the essential data now embedded in tables, which were then copied and pasted individually into Excel. This resulted in Excel files with some formatting issues including extra blank columns and mis-aligned headers, which were manually corrected for each file for all of the available data.

The same data was also downloaded in map form for the years 2003 and 2007 from the web site Kidswatch.org, for the Figures and Maps section of this report.

Data conversion resulted in three Excel files for each year under consideration:

• Number of schools reporting by county and type (public vs. private)
• Total enrollment and student immunization status by county
• Number and percent of students receiving each of several key vaccines

A partial example of the data for number of students with all required immunizations (for 1998) is provided below:

Kindergarteners With All Required Immunizations by County

The above chart was prepared using samples of the Excel data from every fourth year (1991-2007) for visual simplicity. Note that most counties have immunization rates above 80% throughout the study period, with only a few values below that.

As shown visually in the above comparison maps, immuization rates don't change much in most counties over time, and all rates are above 75%.

Immunization rates have been high (>75%) in all counties throughout the time period studied, with no radical changes apparent. Some variation can be observed, but nothing which would correspond with the rapid rise in autism diagnosis in California.

The original premise of this report that rising autism rates might correspond to the percentage of children being immunized is not supported by the data reviewed. However, since immunization rates have been consistently high throughout the study period, a reported increase in the number of shots being administered to each child suggests greater total exposure to the chemicals being used as preservatives in the immunizations. Hence a good follow-up study would be to assess the number of shots being administered in childhood, the amount and type(s) of preservatives used in each shot, and the overall resulting exposure to preservative compounds.