Thimerosal and Autism
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[edit] Definition
Thimerosal (C9H9HgNaO2S) is a disinfectant added to vaccines and some other substances as a preservative against bacterial contamination.
Autism is a neurodevelopmental disorder marked by antisocial behavior, lack of interest in ones surroundings, and problems with communication. Some have theorized that mercury poisoning might be among the causes of autism, but no conclusive data currently exist which establish this link.
The autism-thimerosal hypothesis holds that, since thimerosal contains mercury, inoculation with vaccines preserved with thimerosal can cause autism.
[edit] Discussion
[edit] California (2005): Autism numbers not declining1
Proponents of the autism-thimerosal theory have claimed the rate of autism is decreasing, most notably in California. They believe this because the total number of new cases in the California DDS (Department of Developmental Services) with an autism code has declined over the last two years. Because thimerosal started to be removed from vaccines in the 1999-2000 timeframe, their belief is that such removal has resulted in the decline, since children in the 2000-2001 timeframe would've received less (if not no) mercury in their vaccines. This claim has been made in several places, most notably in many newspaper articles and David Kirby in this post.
However, numerous papers like this one have illustrated that using numbers from sources like the DDS are fraught with peril. Basically, you can't use special services numbers to track the cases of autism. There are too many variables - not everyone with the diagnosis seeks government services, numbers don't account for population shifts and diagnosis changes, etc. In fact, the DDS itself says that you can't use the DDS numbers in this manner.
And keep in mind that declines and gains of this magnitude may or may not be statistically significant. It is very possible that the increase or decrease in new cases is within the bounds of what you'd expect to happen by chance.
In any event, the anti-thimerosal lobby's entire premise is that the total number of cases of autism is declining, therefore the removal of thimerosal should take some credit. And using California numbers, that is partially true. These are the "trailing" numbers of new cases for the previous twelve months. These numbers were calculated from the DDS Quarterly Client Characteristic Report. Basically, you just subtract the number of cases with an autism code in Jun-2004 from the number of cases with an autism code in Jun-2005 to get the "trailing 12 month" number. You can repeat this for each time period and get these same numbers.
| Total | |
| 1-Jun-2005 | 3026 |
| 1-Mar-2005 | 3015 |
| 1-Dec-2004 | 3074 |
| 1-Sep-2004 | 2943 |
| 1-Jun-2004 | 2980 |
| 1-Mar-2004 | 3088 |
| 1-Dec-2003 | 3125 |
| 1-Sep-2003 | 3177 |
What is obvious from this chart is that you see a substantial drop during the middle of 2004 in total new autism cases, followed by what appears to be a slight increase. In fact the number of new cases for June '04-'05 has actually increased slightly over the same period a year ago.
Now for the hypothesis proposed to be true, the total number of cases in children 5 and under should be declining at a steady rate. Remember, the total number of cases of autism they cite have to do with the entire population, not just young children. Below is the same analysis of the numbers, except this time limiting the search to children under the age of 5.
| 0-5 | |
| 1-Jun-2005 | 554 |
| 1-Mar-2005 | 519 |
| 1-Dec-2004 | 537 |
| 1-Sep-2004 | 431 |
| 1-Jun-2004 | 450 |
| 1-Mar-2004 | 567 |
| 1-Dec-2003 | 586 |
| 1-Sep-2003 | 643 |
What you see is a similar trend to the total number of autism cases - a decline early in 2004, followed by a significant uptick. Again, for children ages 0-5, the number of new cases in the June '04-'05 time frame is up substantially (over 20 percent) over the same period a year ago.
Now look at the numbers for children aged 6-9 and 10-13:
| 6-9 | 10-13 | |
| 1-Jun-2005 | 642 | 727 |
| 1-Mar-2005 | 701 | 695 |
| 1-Dec-2004 | 699 | 758 |
| 1-Sep-2004 | 708 | 769 |
| 1-Jun-2004 | 713 | 813 |
| 1-Mar-2004 | 691 | 836 |
| 1-Dec-2003 | 786 | 813 |
| 1-Sep-2003 | 792 | 802 |
This is very interesting, because what you see is fairly substantial decline in the number of cases in older children. For example, among 6-9 year olds, the number of new cases in the June '04-'05 time frame dropped from approximately 10 percent from the year prior. A similar trend is noticable among 10-13 year olds. That is counter to what you'd expect if the removal of thimerosal had an impact on autism caseloads - one would expect increases in the number of cases in these age groups, not declines. There are actual increases in some age that offset a portion of these declines, but the reality is that the decline in autism caseloads in California is not solely attributable to a decline in just children aged 5 and under.
So there's no way that the decline in the number of cases of autism can realistically be attributed to the removal of thimerosal, thus there must be some other factor.
One possible factor is this - in the later part of 2003, the eligibility standards for receiving services were tightened to comply with federal law. The number of "functional areas" a child had to have substantial limitation in was increased to three. Now, this didn't necessarily affect children currently in the system, although the law does allow for "redetermination" of a child's condition and some of that could have taken place.
Another possible factor was budget pressure - caseloads in general have been growing, putting pressure (especially on regional centers from which this data is culled from) on the DDS system in general. From this analysis:
- Analyst's Recommendation: We recommend that the Legislature direct DDS to report at budget hearings on the feasibility of achieving a 10 percent reduction in state expenditures for contracts, programs, and projects included in the RC operations budget as "pass-through" items. The DDS would identify the savings that could be obtained within particular pass-through items, the steps necessary to reduce costs, and the effect, if any, on the quality of services provided directly to RC clients.
As well, there has been a flattening in population growth in California, from well over 2 percent earlier in the decade to about 1.5 percent today. Probably a minor factor, but again, something that could partially explain both the total new caseload decline as well as the number of new cases of autism coming through the system.
And as stated above, the DDS caseloads are constantly changing and in-flux.
The bottom line is that - the oft-repeated claim of anti-thimerosal groups that the California DDS numbers are indicative of the removal of mercury from vaccines are not telling the whole story, nor are they even considering other factors that could result in the decline of this numbers. And in fact, they're using numbers to make a claim the agency itself says can't be used for that purpose.
It is incumbent on media sources and others to do additional research to understand the true nature of these declines, rather than take at face value the claims of those with a vested interest in making the case that thimerosal causes autism.
[edit] Related Topics
[edit] External Links
Wikipedia article on thimerosal.
