Some people have wondered whether vaccinations cause attention-deficit/ hyperactivity disorder (ADHD). The diseases that vaccines prevent do not cause this disorder, so it is not likely that vaccines to prevent them would. However, concerns were related to thimerosal, a mercury-containing preservative, which was in several vaccines commonly given during the first few years of life.

The detrimental health effects caused by mercury are dependent on a variety of factors including the form and dose of mercury and the developmental age of the person being exposed. Most of the data from mercury exposures has resulted from the type commonly ingested when we eat fish, known as methyl mercury. The type of mercury in vaccines is called thimerosal, which is made of ethyl mercury. Ethyl mercury is less likely to accumulate and cause ill effects compared to methyl mercury because it is eliminated from the body much more quickly. However, when concerns about an association between mercury exposure from vaccines and conditions related to attention deficit disorders arose, researchers tested this hypothesis.

Three large studies provided assurance that vaccines didn’t cause attention deficit disorders:

• Verstraeten and colleagues compared thimerosal exposure and a variety of neurodevelopmental delays, including attention deficit disorders, in more than 124,000 infants from two larger HMO databases and an additional 16,700 children in a third HMO. Children who received thimerosal-containing vaccines were not at greater risk of attention deficit disorders than those who didn’t receive these vaccines.
• Andrews and colleagues compared thimerosal exposure from DTP/DT vaccines in the first six months of life in about 110,000 children. Again, receipt of thimerosal-containing vaccines was not associated with the development of attention deficit disorders.
• Another concern was related to children getting “too many vaccines too soon,” so Smith and Woods compared children who got vaccines according to the recommended schedule (the timely group) with those whose parents had chosen to delay or withhold vaccines (the untimely group). The study included more than 1,000 children. The researchers used statistical analyses to compare these two groups for a variety of outcomes, some of which were attention deficit-related characteristics. They found no association between timely receipt of immunizations and developmental delays.
• Therefore, parents can be reassured that vaccines do not cause ADHD or related conditions.

References

• Andrews N, Miller E, Grant A, et. al. Thimerosal exposure in infants and developmental disorders: A retrospective cohort study in the United Kingdom does not support a causal association. Pediatrics. 2004, 114(3): 584-91.
• Smith MJ and Woods CR. On-time vaccine receipt in the first year does not adversely affect neuropsychological outcomes. Pediatrics. 2010, 125(6): 1134-41.
• Verstraeten T, Davis RL, DeStefano F, et. al. Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases. Pediatrics. 2003, 112(5): 1039-48.
• World Health Organization. Mercury and Health Fact Sheet. 2017, March 31.