VACCINE STUDY FINDING FUEL DEBATE OVER AUTISM
A new UC Davis study links a mercury-containing preservative once prevalent in many children’s vaccines to immune system dysfunction in mice.
The findings are sure to heighten the debate over whether vaccines play a role in causing autism, especially since there is growing evidence that many autistic children have irregular immune systems.
The researchers released their results with an abundance of caution, stressing that numerous questions remain about thimerosal’s effect on humans and its relationship, if any, to the recent rapid rise in the numbers of autistic children.
The study is a road map for future investigations of the effect mercury from a variety of sources has on developing immune systems, said senior author Isaac Pessah, a UC Davis toxicologist.
"This is a clue, a lead – I hope it will stimulate a lot more research,"said Cindy Lawler, a program administrator for the National Institute of Environmental Health Sciences, which funded the study with the UC Davis M.I.N.D. Institute.
Although no longer used widely in children’s vaccines in the United States, thimerosal remains in some flu vaccines and over-the-counter products.
The Davis researchers removed dendritic cells from mice, then analyzed how thimerosal affected those cells in culture dishes. A dendritic cell acts as a sentry, helping to marshal the body’s immune system against invading viruses or bacteria.
One dendritic cell can activate as many as 300 T-cells, the white blood cells that help find and kill attacking agents.
Among the study’s findings:
- Thimerosal dramatically altered dendritic cells by garbling their signaling system.
- At lower levels, it caused dendritic cells to secrete abnormal levels of cytokine, a substance that initiates inflammatory responses.
- At higher levels, it caused the death of dendritic cells, preventing them from maturing and activating T-cells.
- Even extremely small amounts of thimerosal interfered significantly with dendritic cell function after a few minutes of exposure.
- Immature dendritic cells were particularly sensitive to thimerosal.
Thimerosal has been used as a preservative in some vaccines and other pharmaceutical products since the 1930s.
Manufacturers use it to prevent fungal and bacterial contamination in multidose vials. It isn’t needed in individually packaged doses, a more expensive method.
Manufacturers began to remove thimerosal from children’s vaccines in 1999 when the American Academy of Pediatrics and the U.S. Public Health Service urged them to do so as a precautionary measure.
The recommendation came after the Food and Drug Administration determined that children who received the full set of recommended immunizations could accumulate doses of mercury exceeding Environmental Protection Agency guidelines.
Today, most children’s vaccines in the United States contain only trace amounts of the preservative. The one exception is the flu shot, which is available with and without thimerosal, although the thimerosal version is much more prevalent.
The preservative remains in some ear drops, nose drops, skin creams and cosmetics, as well as adult diphtheria and tetanus vaccines.
Anyone who takes a hard look at thimerosal wades into roiling political waters. While federal health officials insist it is safe, several thousand parents of autistic children have sued pharmaceutical companies, blaming them for causing their children’s disorder.
Congressional leaders have entered the fray, arguing about whether to grant pharmaceutical companies immunity from such lawsuits.
Pessah made sure Tuesday he avoided aligning with either side in the debate. "We’re just trying to do the best science we can,"he said.
Many questions remain, said Pessah, who directs the UC Davis Children’s Center for Environmental Health and Disease Prevention.
"I want to be clear that I’m not saying thimerosal is causing autism,"he said.
The study doesn’t reveal whether thimerosal would have the same effect in a mouse’s body as in a petri dish, or whether it affects human cells the same way it does mice cells, he noted.
It also is not known whether immune system dysfunction leads to autism.
Despite the questions, the findings will intrigue many parents of autistic children.
"It is another step in our understanding,"said Rick Rollens, the father of an autistic boy and a co-founder of the M.I.N.D. Institute. "The thing that’s most exciting to me about this is that science is now listening to us and is pointed in what we believe is the right direction."
The next step for researchers, Pessah said, is to test the effect of thimerosal and other forms of mercury on human dendritic cells.
UC Davis will gather such cells from 350 children enrolled in an ongoing autism study.
A complementary UC Davis study is analyzing whether autism is actually several disorders lumped into one category. Many experts believe it has a variety of causes.
California will ban thimerosal from vaccines given to expectant mothers and children younger than 3 years old beginning this July.
Although some parental groups have urged the CDC to recommend removal of the preservative from the flu vaccine, the CDC has declined to do so.
Manufacturers do not produce enough of the thimerosal-free children’s version for everyone who needs it, said CDC spokeswoman Lola Russell.
Reach Sandy Kleffman at 925-943-8249.
THIMEROSAL AT A GLANCE
- Most health experts recommend full vaccinations for children. Only trace amounts of thimerosal exist in most children’s vaccines today.
- The one exception is the flu shot, which is available with and without the preservative. Parents who want the thimerosal-free version should request it when their children receive shots.
- Thimerosal is in some ear and nose drops, creams and other over-the-counter pharmaceutical products.
- Thimerosal remains in some adult diphtheria and tetanus vaccines.
