National Surveillance of Influenza-Associated Encephalopathy in Japan over Six Years, before and during the 2009–2010 Influenza Pandemic

Abstract

Influenza-associated encephalopathy (IAE) is a serious complication of influenza and is reported most frequently in Japan. This paper presents an assessment of the epidemiological characteristics of influenza A (H1N1) 2009-associated encephalopathy in comparison to seasonal IAE, based on Japanese national surveillance data of influenza-like illness (ILI) and IAE during flu seasons from 2004–2005 through 2009–2010. In each season before the pandemic, 34–55 IAE cases (mean 47.8; 95% confidence interval: 36.1–59.4) were reported, and these cases increased drastically to 331 during the 2009 pandemic (6.9-fold the previous seasons). Of the 331 IAE cases, 322 cases were reported as influenza A (H1N1) 2009-associated encephalopathy. The peaks of IAE were consistent with the peaks of the influenza epidemics and pandemics. A total of 570 cases of IAE (seasonal A, 170; seasonal B, 50; influenza A (H1N1) 2009, 322; unknown, 28) were reported over six seasons. The case fatality rate (CFR) ranged from 4.8 to 18.2% before the pandemic seasons and 3.6% in the 2009 pandemic season. The CFR of pandemic-IAE was 3.7%, which is lower than that of influenza A−/B-associated encephalopathy (12.9%, p<0.001; 14.0%, p = 0.002; respectively). The median age of IAE was 7 years during the pandemic, which is higher than that of influenza A−/B-associated encephalopathy (4, p<0.001; 4.5, p = 0.006; respectively). However, the number of pandemic-IAE cases per estimated ILI outpatients peaked in the 0–4-year age group and data both before and during the pandemic season showed a U-shape pattern. This suggests that the high incidence of influenza infection in the 0–4 year age group may lead to a high incidence of IAE in the same age group in a future influenza season. Further studies should include epidemiologic case definitions and clinical details of IAE to gain a more accurate understanding of the epidemiologic status of IAE.

Japanese Surveillance Systems and Treatment for Influenza

“…Of the 331 IAE cases reported in the year, 322 (97.2 %) were linked to influenza A(H1N1)pdm09 infection. The case fatality rate of IAE was 3.7 %, which was lower than that of influenza A- and B-associated encephalopathy reported in the previous seasons (12.9 and 14.0 %, respectively) [28]. …”

“…The number of IAE cases increased by 6.9-fold during the 2009 pandemic compared to the previous seasons [28]. Of the 331 IAE cases reported in the year, 322 (97.2 %) were linked to influenza A(H1N1)pdm09 infection.…”

Paediatric Acute Encephalitis: Infection and Inflammation

“…Of note, and similar to previous reports from Australia [7], none of the children or adults had been previously vaccinated against influenza, including 32 % who had indications for this reinforcing that these syndromes are potentially preventable. Japanese surveillance for influenzaassociated encephalopathy (IAE) (2009-2011) noted that attack rates were approximately 10-fold higher during the H1N1 Influenza A pandemic than from seasonal influenza, and again were highest in young children [36]. Case fatality rates of approximately 5-18 % were reported in this series.…”

Neurologic Complications of Influenza B Virus Infection in Adults, Romania

“…In the United States in 2010–11, influenza B viruses were involved in 38% of deaths in children caused by influenza ( 23 ). In Japan, a 6-year national surveillance identified 50 patients (median age 4.5 years) with of influenza B virus–associated encephalopathy/encephalitis, of whom 7 (14%) died ( 24 ). …”