Autism-Vaccine Link: Evidence Doesn't Dispel Doubts
Many major medical groups say vaccines don't cause autism. Many parents say they do. So who's right?
Vaccine-Autism Disconnect: The Mercury Question continued...
"I certainly wouldn't argue that the only source of mercury and the only
source of harm is the mercury in vaccines," Bernard says. She says she
believes children continue to be harmed by vaccines that still contain
But while Harvard's McCormick agrees that mercury is a dangerous substance,
the claim that mercury poisoning and autism are the same doesn't hold up under
scrutiny, she says. "It's based on a very superficial similarity."
Symptoms of mercury poisoning can
include irritability, depression, anxiety, visual problems, speech problems,
and sensory nerve problems. In autism, there can be findings of delayed speech,
increase or decrease in response to sensory stimuli, and avoidance of human eye
Vaccine-Autism Disconnect: Meet Ethyl and Methyl
Recently, basic assumptions
about thimerosal have come into question that may further confuse any links to
Thimerosal contains ethyl mercury. The government's assessments of health
risks for thimerosal are based on what is known about another chemical form of
mercury, called methyl mercury.
The health assessments for methyl mercury are based on exposure by eating or
drinking it. Warnings about eating certain kinds of fish, for example, are all
about fish contaminated with methyl mercury.
Thimerosal, on the other hand, isn't eaten; When experts called for reducing
and removing thimerosal from vaccines in 1999, they assumed that methyl mercury
and ethyl mercury were pretty much the same, and the health risks may be the
same even though the way children are exposed is different. A study
published in the journal Pediatrics in January 2008 shows that ethyl
mercury leaves children's blood faster than methyl mercury does, so the health
risks may not be the same.
However, while it left the blood quickly, it doesn't necessarily mean it
left the body. While the study did show that the mercury did not damage the
kidneys, for instance, it didn't look at other organs.
Vaccines and Autism: Other Studies Under Way
Sue Swedo, MD, chief
of the pediatric and developmental neuropsychiatry branch of the National
Institute of Mental Health, says federal researchers have not closed the door
to looking at whether vaccination might, in rare cases, be linked to
The strongest case
for a link comes from children with regressive autism -- children who seem to
be developing normally, but who then lose the social and language skills they
had developed and slide into autism. To parents, such children seem to have
been the victims of some environmental toxin. As this regression occurs at the
same time children receive multiple vaccines, many wonder whether vaccines
might carry such a toxin.
"Our studies of
regressive autism are taking a very shotgun approach to environmental factors
in autism," Swedo tells WebMD. "We are saying we don't really know
right now whether such factors might be involved."