evidence: Ray P, Hayward J, Michelson D, Lewis E, Schwalbe J, et al. Encephalopathy after whole-cell pertussis or measles vaccination: lack of evidence for a causal association in a retrospective case-control study. Pediatr Infect Dis J 2006;25:768-773. The authors investigated the possible relationship between whole-cell pertussis (DTP) or measles (MMR) vaccination and encephalopathy, encephalitis, and Reye syndrome by evaluating 15 years of health records from four health maintenance organizations in the United States, which encompassed nearly 2.2 million children. DTP and MMR vaccines were not associated with an increased risk of encephalopathy, encephalitis, or Reye syndrome after vaccination. Additionally, a clinically distinctive pertussis vaccine-induced encephalopathy was not detected, which was consistent with other studies. [...] Griffin MR, Ray WA, Mortimer EA, Fenchel GM, Schaffner W. Risk of seizures and encephalopathy after immunization with the diphtheria-tetanus-pertussis vaccine. JAMA 1990;263:1641-1645.