Dr. Alan Greene on Flu Vaccinations During Pregnancy

Can the mercury in a flu shot affect an unborn baby?

flu vaccination
Photo: Sherry Yates Young/shutterstock.com


I'm concerned about thimerosal. I had the flu shot during my last two pregnancies. I was surprised to find that even though they have taken thimerosal out of most vaccines, it is still in the flu shot. Can mercury in my system affect an unborn baby?


Mercury comes in several forms: elemental mercury (like that found in old thermometers, blood pressure machines, and old paints), inorganic mercury salts (found in disinfectants, antiseptics, and preservatives -- such as the thimerosal that used to be common in vaccines), and organic mercury, such as that found in fish, which comes from manufacturing pollution. The three are handled by the body very differently. The elemental mercury leaves in the urine and half of it is gone within 60 days (half-life of 60 days). The salts like thimerosal more quickly, with a half-life of less than 40 days. And the organic mercury stays longer -- it is stored in fat.

We know that mercury in a mother's system can affect a baby -- the tragic story of mercury poisoning at Minamata Bay in Japan taught us that. (Before this, the placenta was believed to totally protect the baby.) Those babies had high levels of organic (fat soluble) mercury and low birth weights, small heads, developmental delay, cerebral palsy, blindness, deafness, and seizures. The effects were obvious. We now know that reducing exposure to organic mercury is important for pregnant women, which is why warnings have been issued about how much and what kinds of seafood to eat while pregnant.

The amount of inorganic mercury found in the thimerosal used as a preservative in the flu vaccine has not been shown to cause problems. But because of the real possibility of an undetected risk, and because other alternatives are available, I strongly agree with the plan to remove it from all vaccines. Mercury is a toxin, and there is no amount that is proven to be safe.

However, even though I am opposed to mercury in vaccines, there are some people for whom the risks of the flu outweigh the risks of the thimerosal. About 20,000 people die from the flu each year in the U.S.

The information on this Web site is designed for educational purposes only. It is not intended to be a substitute for informed medical advice or care. You should not use this information to diagnose or treat any health problems or illnesses without consulting your pediatrician or family doctor. Please consult a doctor with any questions or concerns you might have regarding your or your child's condition.

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