Author: Hjalmarsson Anders; Blomqvist Paul; Brytting Maria; Linde Annika; Sköldenberg Birgit

Author Affiliation: Infectious Diseases Unit, Department of Medicine Solna, Karolinska Institutet, Stockholm, Sweden. anders.hjalmarsson@ki.se

Keywords: Adolescent; Adult; Aged; Aged, 80 and over; Child; Encephalitis, Viral - epidemiology - etiology - mortality; Female; Humans; Incidence; Influenza, Human - complications - epidemiology - mortality; Male; Middle Aged; Registries; Sweden - epidemiology; Young Adult

Abstract: The aim of this study was to investigate the incidence of influenza-related encephalitis in Sweden during 11.5 years. Studies from Japan report an increased incidence of influenza-related encephalitis/encephalopathy. Few other studies are available. We conducted a retrospective register-based study on the Swedish National Inpatient Register, which covers all Swedish hospitals. In 1987-1998, a total number of 14,250 hospitalized individuals had an influenza diagnosis (population incidence: 137 per million person-years). In-hospital mortality was 4.1%. Using three different approaches, only 21 cases of influenza-related encephalitis were found, corresponding to a rate of 1.5 per 1,000 hospitalized persons with an influenza diagnosis (population incidence 0.21 per million person-years). We conclude that encephalitis following influenza occurs rarely, or is an infrequently recognized, diagnosed or reported complication. The cases we studied in detail have all recovered without sequels.