In a 10 year study of viral encephalitis in England, 1,419 patients had HSE and 141 died. HSE had the highest case-fatality rate in adults, which was 12.5 per 100 cases and the mean length of hospital stay was 26.6 days (Davison K, Crowcroft N, Ramsay ME et al. Viral encephalitis in England 1989-1998: what did we miss? Emerging Inf Dis 2003; 9: 234-240).

Age, duration of disease and level of consciousness at the start of antiviral treatment have been reported as major determinants of clinical outcome.

Patients younger than 30 years of age and with a more normal level of consciousness as judged by the GCS, for example lethargic compared with comatose, are more likely to return to normal function than older patients. Children and adolescents made up 90% of those younger than 30 years of age.

Management of suspected viral encephalitis in adults - Association of British Neurologists and British Infection Association National Guidelines
Management of suspected viral encephalitis in children - Association of British Neurologists and British Infection Association National Guidelines