Increased risk of developmental neurologic impairment after high exposure to thimerosal-containing vaccine in first month of life.
Division o f Epidemiology and Surveillance, V accine Safety and Development Branch, Mailstop E-61, 770-639-8327.
Background: Concern has risen on the presence ofthe ethylmercury containing preservative thimerosal in vaccines. We assessed the risk for neurologic and renal impairment associated with past exposure to thimerosal-containing vaccine using automated data from the Vaccine Safety Datalink (VSD). VSD is a large linked database from four health maintenance organizations in Washington, Oregon and California, containing immunization, medical visit and demographic data on over 400,000 infants born between '91 and '97.
Methods:· We categorized the cumulative ethylmercury exposure from thimerosal containing vaccines after onemonth of life and assessed the subsequent risk of degenerative and developmental neurologic disorders and renal disorders before the age of six. We applied proportional hazard models adjusting for HMO,year of birth, and gender, excluding premature babies. Results: We identified 286 children with degenerative and 3702 with developmental neurologic disorders, and 310 with renal disorders. The relative risk (RR) of developing a neurologic development disorder was 1.8 ( 95% confidence intervals [CI] ::: 1.1-2.8) when comparing the highest exposure group at 1 month of age (cumulative dose> 25 ug) to the unexposed group. Within this group we also found an elevated risk for the following disorders: autism (RR 7.6, 95% Cl = 1.8-31.5), nonorganic sleep disorders (RR 5.0, 95% Cl = 1.6-15.9}, and speech disorders (RR 2.1, 95% (1=1.1-4.0). For the neurologic degenerative and renal disorders group we found no significantly increased risk or a decreased risk.
Conclusion: This analysis suggests that high exposure to ethylmercury from thimerosal-containing vaccines in the first month of life increases the risk of subsequent development of neurologic development impairment, but not of neurologic degenerative or renal impairment. Further confirmatory studies are needed.