Is thimerosal in vaccines a hidden danger? The debate over mercury and autism risks

Time of India

24-07-2025

Picture this: a little compound humming away inside some vaccine vials since the 1930s, quietly stopping germs from crashing the vaccine party. That's thimerosal—an organomercury compound containing around 49% mercury by weight, used as a preservative in multi-dose vaccine vials to keep bacteria and fungi at bay.
When vaccine manufacturers pierce the vial multiple times, contamination risk spikes—thimerosal prevents that by killing germs before they can cause trouble.
Now let's talk mercury. Unlike methylmercury—which is found in fish and can build up over time—ethylmercury (the type in thimerosal) is processed and expelled quickly by your body. Government agencies and researchers say that at the tiny doses used, it's not harmful—and the only side effects observed are minor, like redness or swelling at the jab site. Reactive allergic responses are rare but possible, and often mild—even though patch testing has identified some sensitivities in 16–18% of cases, without major clinical concern
Does it heighten autism risk?
Back in 1999, as fears around autism and mercury exposure swirled, the CDC, FDA, National Institutes of Health, and pediatric associations recommended removing thimerosal from childhood vaccines as a precaution—not because there was proof it was dangerous.
By about 2001, most vaccines for children under six in the U.S. no longer contained it; today, vaccines like MMR, chickenpox, polio, and pneumococcal never included it at all.
"There is a robust body of peer-reviewed, scientific studies conducted in the United States and countries around the world that support the safety of thimerosal-containing vaccines. The scientific evidence collected over the past 20+ years does not show any evidence of harm, including serious neurodevelopmental disorders, from use of thimerosal in vaccines.
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Specifically, the Institute of Medicine (now known as the National Academy of Medicine), and others have concluded that the evidence favors rejection of a link between thimerosal and autism.
Scientific studies of the risk of other serious neurodevelopmental disorders have failed to support a causal link with thimerosal," a US CDC report says.
Currently, thimerosal isn't in most routine vaccines, but it still pops up in some multi-dose flu vaccine vials, especially those used in clinics or public vaccination drives where single-dose packaging isn't practical.
For flu shots in the U.S., about 90–96% are thimerosal-free, but the multi-dose ones may still have up to ~25 micrograms per dose—while trace amounts might show up during manufacturing in rare vials.
Recently, Robert F. Kennedy Jr.—now heading vaccine policy at HHS—appointed a new advisory panel that has started pushing to remove thimerosal entirely from flu vaccines. That move has alarmed major medical groups, warning it could reduce vaccine access in under-resourced areas and fuel fear over misunderstood risks.
"Although all vaccines routinely recommended for children 6 years of age and younger in the U.S. are available in formulations that do not contain thimerosal, thimerosal has a long record of safe and effective use in preventing bacterial and fungal contamination of vaccines, with no ill effects established other than hypersensitivity and minor local reactions at the site of injection," the CDC report adds.