Effect of mass immunization against influenza encephalopathy on mortality rates in children

BACKGROUND: Since the Japanese influenza vaccination program for school children was terminated in 1994, a steep rise has been noted in the number of young children reported with fulminant influenza and influenza associated with acute encephalopathy/encephalitis. The purpose of the present study was to interpret and clarify the effect of mass influenza vaccination on mortality of Japanese children, aged <19 years, with influenza and influenza-associated acute encephalopathy/encephalitis. METHODS: The authors examined the distribution of mortality rates of children in Japan aged <19 years with influenza during 1950-2000, influenza associated with central nervous system (CNS) signs in the period 1950-1978, and the estimated cases of influenza associated with acute encephalopathy/encephalitis in the period 1987-2000. RESULTS: Total influenza mortality among children aged <19 years has increased since 1990, with children aged <4 years after 1994 being the worst affected. The mean values of mortality rates of influenza associated with central nervous system signs during 1963-1978 and the estimated mortality of children aged <9 years during 1979-1994 were significantly lower than in some years before 1962, and after 1995 (P < 0.05). The annual mortality rates of influenza-associated acute encephalopathy/encephalitis during 1995-2000 were significantly higher than the expected mortality of influenza associated with central nervous system signs in children aged <14 years (P < 0.05). CONCLUSION: The results of the present study suggest that mass immunization of school-aged children reduced the mortality rate from influenza-associated acute encephalopathy/encephalitis in children less than 9 years of age.