In the past, vaccines had a big problem: bacteria could grow in them and make people sick. To stop this, a preservative called thimerosal was added to vaccines in the 1930s. It worked well for a long time, and the only issues were minor reactions at the injection site.

But in 1999, health officials asked vaccine makers to remove thimerosal from vaccines. Why? Because thimerosal contains mercury, and there were concerns about whether it could harm the brain, especially in children. Even though there was no proof it was harmful, the decision was made to remove it.

Today, thimerosal is only found in a few vaccines, like some flu shots. Many studies have shown that thimerosal does not cause autism or other developmental issues. In fact, autism rates have continued to rise even after thimerosal was removed from most vaccines.

So why is thimerosal back on the agenda for a meeting of vaccine advisers to the CDC? It's a mystery. Some experts think it's because Robert F. Kennedy Jr. , who leads an anti-vaccine group, wants to create doubt about vaccine safety. One of the new advisers, Lyn Redwood, is also a former leader of that group and will be presenting on the topic.

The decision to remove thimerosal from vaccines had other consequences. At the same time, health officials advised that babies born to mothers without hepatitis B should not receive the hepatitis B shot at birth. But some hospitals stopped giving the shot to all newborns, regardless of risk. This led to the death of a three-month-old child in Michigan.

Looking back, the decision to remove thimerosal was a tough one. The fear was that if it was found to be harmful later, many children could have been affected. But the autism issue did not play a role in the initial decision.

Why Thimerosal in Vaccines is Back on the Table

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