Encephalitis after influenza in Sweden 1987-1998: a rare complication of a common infection

Eur Neurol. 2009;61(5):289-94. doi: 10.1159/000206854. Epub 2009 Mar 17.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Aged, 80 and over
  • Child
  • Encephalitis, Viral / epidemiology*
  • Encephalitis, Viral / etiology*
  • Encephalitis, Viral / mortality
  • Female
  • Humans
  • Incidence
  • Influenza, Human / complications*
  • Influenza, Human / epidemiology*
  • Influenza, Human / mortality
  • Male
  • Middle Aged
  • Registries
  • Sweden / epidemiology
  • Young Adult