Lack of association between childhood immunizations and encephalitis in California, 1998–2008

ObjectiveA number of new and combination vaccines have been introduced for children in the past two decades. Encephalitis cases occurring within defined time windows following administration of pertussis- or measles-containing vaccines are eligible for compensation by the Vaccine Injury Compensation Program. Due to increased parental concerns about vaccine safety and potential neurologic adverse events following immunization with new and multiple vaccines administered at the same visit, our aim was to determine whether immunizations are associated with an increased risk of encephalitis within defined risk windows.MethodsWe reviewed immunization records from 246 pediatric encephalitis cases referred to the California Encephalitis Project between July 1998 and December 2008. We included data on 110 cases who had been immunized in the year prior to the onset of encephalitis (observation period) and had complete immunization records. We used the case-centered method to test whether cases were more likely to have developed encephalitis in defined risk windows—42, 30 and 21 days after any vaccination, 3 days after pertussis-containing vaccines and 5–15 days after measles-virus containing vaccines—compared with the rest of the observation period.ResultsAll vaccines recommended in the current immunization schedule were represented in our sample. No increased risk of encephalitis was seen following administration of pertussis-containing vaccines, measles-containing vaccines or any number of vaccines administered in a single visit (vaccine episode); the odds ratios and 95% confidence intervals for encephalitis after a vaccine episode were: 1.0 (0.6–1.8) in a 42-day risk window, 0.9 (0.5–1.6) in a 30-day risk window and 1.2 (0.7–2.2) in a 21-day risk window.ConclusionNo association between receipt of currently recommended immunizations and subsequent development of encephalitis was observed in this study.

► 110 California children immunized within a year of encephalitis onset were assessed.
► Case-centered analysis using pre-specified risk windows was performed.
► No association between receipt of any number of vaccines administered in a single visit (vaccine episode) and encephalitis was found.
► No associations between measles or pertussis-containing vaccines and encephalitis were found.