Almost half of the children with severe complications associated with seasonal influenza were previously healthy children, for whom there is currently no recommendation to vaccinate against influenza. Fifty% were children with chronic diseases for whom vaccination had been recommended but had not been followed. In these children the severe course of disease might have been prevented by vaccination. Oseltamivir treatment was often started several days after start of influenza symptoms or PICU admission, indicating that influenza is often diagnosed too late for effective antiviral patient treatment. The general incidence of influenza-associated severe disease in German PICUs was very low, but under-diagnosis and under-reporting during these prepandemic influenza seasons may be likely. Therefore prospective studies providing influenza subtype screening on PICU admissions of children with respiratory disease are needed to assess more accurately the burden of severe influenza-associated complications. These studies are necessary to provide data and evidence for the discussion on influenza prevention by general routine vaccination in children.  Epidemiological and clinical characteristics of childhood pandemic 2009 H1N1 virus infection: an observational cohort study BACKGROUND: There was a pandemic influenza around the world in 2009 including South Korea since last pandemic occurred four decades ago. We aimed to evaluate the epidemiological and clinical characteristics of this infection in childhood. METHODS: We evaluated the epidemiologic characteristics of all the subjects infected with the 2009 H1N1 influenza A virus (2,971 patients, ≤ 15 years of age), and the clinical and laboratory findings of the inpatients (217 patients, 80 had pneumonia) between 1 September 2009 and 31 January 2010 in a single hospital throughout the epidemic. RESULTS: The age distribution of all the subjects was relatively even. Over 90% of cases occurred during a two-month period. Two hundred and five patients (94.5%) received oseltamivir within 48 h of fever onset, and 97% of inpatients defervesced within 48 h of medication. The group with pneumonia included more males than females, and had higher leukocytes counts with lower lymphocyte differentials than the group without pneumonia. The white blood cell count and lymphocyte differential were associated with the severity of pneumonia. Corticosteroid treatment for severe pneumonia patients was highly effective in preventing disease progression. CONCLUSION: Children of all ages affected with even rates of infection, but males were predominant in pneumonia patients. Pneumonia patients showed lymphopenia and its severity was associated with the severity of illness. Our results suggest that the mechanism of lung injury in 2009 H1N1 virus infection may be associated with the host immune response. Although influenza virus infection has been a major global concern since the pandemic 1918 'Spanish flu', there have been no pandemic influenzas for near four decades after the 1968 'Hong Kong flu'. The pandemic 2009 H1N1 influenza A (2009 H1N1) virus infection was reported first in Mexico in February 2009, and then the virus spread rapidly worldwide, including in South Korea [1] . It has been reported that the majority of H1N1 patients were children and young adults and the mortality rate was not higher than that of seasonal influenza [2] [3] [4] . The majority of patients affected by the 2009 H1N1 virus infection recovered uneventfully, but some previously healthy patients developed a rapidly progressive pneumonia, leading to acute respiratory distress syndrome (ARDS), multi-organ failure, and death [5] [6] [7] . With this enigma, the pathogenesis of acute lung injury (pneumonia) in influenza infections remains unknown [1, 8, 9] .