mercury contained in some vaccines is processed
differently in the body and is possibly less toxic
to children than mercury found in pollution and
fish, U.S. researchers reported on Thursday.
Tests in monkeys showed that the ethyl mercury
contained in the vaccine preservative thimerosal
is cleared quickly by the body, while methyl mercury
persists much longer.
This suggests that current Environmental Protection
Agency guidelines on mercury exposure should not
apply to the type of mercury in vaccines - and
could help answer doubts about the safety of some
vaccines, the researchers wrote in this week's
issue of Environmental Health Perspectives.
"The current debate linking the use of thimerosal
in vaccines to autism and other developmental
disorders has led many families to question whether
the potential risks associated with early childhood
immunizations may outweigh the benefits,"
Thomas Burbacher of the University of Washington
in Seattle and colleagues wrote in their report.
The Institute of Medicine, an independent body
that advises the federal government, has said
there is no evidence of any link between vaccines
and autism. It has advised researchers looking
for the causes of autism to look elsewhere.
This has enraged autism activists, who fear a
Burbacher's team said it would make sense to study
more closely the effects of thimerosal, a mercury-based
preservative once widely used in vaccines. It
has been removed from most childhood vaccines
because of the furor.
Thimerosal remains in the influenza vaccine and
in some vaccines used outside the United States.
Current government advice on vaccine exposure
limits are based on studies done on people who
were exposed to methyl mercury from industrial
accidents. It can clearly cause long-term brain
and nervous system damage.
An earlier study calculated that children receive
187.5 micrograms of ethyl mercury from thimerosal-containing
vaccines given over the first 14 weeks of life.
This can exceed EPA guidelines for methyl mercury
exposure during pregnancy.
Some experts have argued that thimerosal breaks
down into a different form of mercury in the body
-- ethyl mercury -- and that this is cleared more
Tests on human infants suggest this does happen,
but their brains cannot be directly examined.
Burbacher's team tested 41 newborn monkeys, feeding
them either methyl mercury or giving them shots
of thimerosal in doses comparable to those given
vaccinated human infants. The mercury from the
vaccines was cleared out of the body much more
quickly than was methyl mercury, they said.
It took just over eight days to completely clear
mercury from thimerosal, while it took 21 days
to clear methyl mercury from the blood, they found.
"Brain concentrations of total mercury were
significantly lower by about three-fold for the
thimerosal-exposed infants when compared to the
methyl mercury infants," they wrote.
The researchers said this does not mean thimerosal
is harmless and urged more research.
"This information is critical if we are to
respond to public concerns regarding the safety
of childhood immunizations," they wrote.
"This study emphasizes that thimerosal and
methyl mercury behave differently in the body.
Given that we routinely inject thimerosal into
millions of infants, the study authors' call for
more in-depth research on the subject is the right
way to go," said Dr. Jim Burkhart, science
editor for the journal.