30-year trends in admission rates for encephalitis in children in England and effect of improved diagnostics and measles-mumps-rubella vaccination: a population-based observational study

Background: Encephalitis is a serious neurological disorder, yet data on admission rates for all-cause childhood encephalitis in England are scarce. We aimed to estimate admission rates for childhood encephalitis in England over 33 years (1979–2011), to describe trends in admission rates, and to observe how these rates have varied with the introduction of vaccines and improved diagnostics. Methods: We did a retrospective analysis of hospital admission statistics for encephalitis for individuals aged 0–19 years using national data from the Hospital Inpatient Enquiry (HIPE, 1979–85) and Hospital Episode Statistics (HES, 1990–2011). We analysed annual age-specific and age-standardised admission rates in single calendar years and admission rate trends for specified aetiologies in relation to introduction of PCR testing and measles-mumps-rubella (MMR) vaccination. We compared admission rates between the two International Classification of Diseases (ICD) periods, ICD9 (1979–94) and ICD10 (1995–2011). Findings: We found 16 571 encephalitis hospital admissions in the period 1979–2011, with a mean hospital admission rate of 5·97 per 100 000 per year (95% CI 5·52–6·41). Hospital admission rates declined from 1979 to 1994 (ICD9; annual percentage change [APC] −3·30%; 95% CI −2·88 to −3·66; p<0·0001) and increased between 1995 and 2011 (ICD10; APC 3·30%; 2·75–3·85; p<0·0001). Admissions for measles decreased by 97% (from 0·32 to 0·009) and admissions for mumps encephalitis decreased by 98% (from 0·60 to 0·01) after the introduction of the two-dose MMR vaccine. Hospital admission rates for encephalitis of unknown aetiology have increased by 37% since the introduction of PCR testing. Interpretation: Hospital admission rates for all-cause childhood encephalitis in England are increasing. Admissions for measles and mumps encephalitis have decreased substantially. The numbers of encephalitis admissions without a specific diagnosis are increasing despite availability of PCR testing, indicating the need for strategies to improve aetiological diagnosis in children with encephalitis.