Autism, Mercury and the California Numbers

07/13/2005 10:52 pm ET Updated May 25, 2011

For months now, a mantra of the thimerosal defenders has been as follows: “Mercury was removed from vaccines years ago, and we have not seen a drop in autism rates.”

It looks like they might have to find a new slogan.

Freshly reported numbers out of California show that new cases entering that state’s disability system (children who are three-to-four years old and newly diagnosed with autism) have indeed dropped since 2002, marking the first decline in new autism cases since California began tracking the mysterious disorder.

We now know that 2002 was the peak year for new autism diagnoses in the state, with 3,259 cases. That number fell to 3,125 in 2003, and dropped to 3,074 in 2004. For the first half of 2005, there were 1,470 new cases, compared to 1,518 in the same period in 2004. A similar downward tick has been reported in Indiana, and other states should begin weighing in soon.

The Golden State, however, is said to operate the gold standard of autism epidemiology, having always tracked “full-blown” autism only, as defined by the DSM-IV manual. In other words, children with milder forms of the disorder, such as PDD and Apserger Syndrome, need not apply for services. This means that nearly two decades of rising cases in California cannot be attributed to wider diagnostic criteria. The autism epidemic is real.

So why is the drop in numbers such a potential bombshell? Because children entering the system today were born in 2001 and 2002, soon after the mercury-based preservative thimerosal began to be phased out of pediatric vaccines in the United States.

In California, fewer children with full-blown autism entered the system in 2003 than in 2002. Most of these kids would have been born in 1999 or 2000, when more mercury-free vaccines began their gradual penetration of the market. In 2004, there was another decline, this time among kids born largely in 2000 or 2001, when total average mercury burden from vaccines presumably would have been reduced further. This year, we are seeing kids born mostly in 2001 and 2002, when mercury levels declined further still.

Is it too early to tell if this is a permanent and meaningful trend? Of course. Could there be other explanations for the drop, such as a budget-crunching reduction in services? Perhaps. But this very decline, at this very moment, has long been predicted by supporters of the thimerosal-autism theory. At the very least, the quivers of their detractors have now been emptied of one arrow, for the time being anyway.

Stay tuned. If the numbers in California and elsewhere continue to drop – and that still is a big if -- the implication of thimerosal in the autism epidemic will be practically undeniable.

David Kirby is author of “Evidence of Harm: Mercury in Vaccines and the Autism Epidemic” (St. Martin’s Press 2005) www.evidenceofharm.com