During the influenza A/H1N1 epidemic a total of 94 children with PCR-confirmed PIA were hospitalized in the three hospitals in Wuerzburg, Northern Bavaria, which cover the pediatric population of the city of Wuerzburg and its surroundings. The clinical course was mostly benign with cough (80%), fever (78%), and rhinorrhea (51%) as predominant symptoms. Only six percent of PIAH were admitted to an ICU; which is about three times lower than reported from Argentina, Canada and USA (17-19%) [9–11] but similar to 8% found in a UK hospital  and identical to 6% of ICU admissions reported from a large hospital in Hamburg (Northern Germany) . In Germany, both in our study and in Hamburg, a clearly lower percentage of patients with PIA received treatment with oseltamivir (25% and 28%, respectively) and antibiotics (30% and 25%, respectively) , when compared to antiviral (46-99%) [9–11] and antibiotic (74-86%) treatment in other PIAH studies [9–11]. In Argentina, oxygen was supplemented five times more often (82%) than was to be observed in our study (17%) . Mechanical ventilation was required six times more often (17%) than documented in our study (3%), whereas data from USA and Canada revealed a similar frequency (6%) for hospitalized patients [9, 10]. The low rates of severe influenza cases in the present study correspond with the results of an earlier study on severe seasonal influenza in Germany . Hence, on the one hand, the higher primary and secondary complication rates among hospitalized patients in other countries may reflect a real increase in the complication rate, due to delayed treatment with a limited access to primary health care. On the other hand, observed heterogeneity in the severity of hospitalized patients may result from differences in hospitalization access, with a higher threshold for hospitalization in countries with lower socio-economic status or limited health insurance (as in the USA) compared to Germany .
Underlying diseases were documented in 43% of PIAH, with asthma (25% of all children with predisposing factors) reported as one of the most frequent conditions. These results are comparable to the 32-40% of cases with underlying diseases (predominantly led by asthma) reported by most other surveys on pediatric PIAH [8, 11, 12, 14]. Of 40 patients with underlying diseases, 10% received ICU treatment, in contrast to only 4% of 54 previously healthy patients, indicating a more severe course of disease in risk group children. However, the majority of children with underlying disease had an uncomplicated course of disease. It may be assumed that at least in part they were hospitalized for pre-emptive treatment and monitoring of possible complications. In contrast, a recent study from seven Austrian hospitals on PIA patients seeking emergency medical care reported underlying chronic conditions only in 13% of PIA patients <18 years of age .
In October 2009, the German Advisory Board on Immunization (STIKO) recommended vaccination against PIA for selected risk groups. For children, it was recommended that primarily children above six months of age with underlying diseases, such as chronic diseases of the air ways, cardiovascular system, liver or kidneys, should be vaccinated. Secondarily, healthy children should be vaccinated as well . The first PIA vaccine was available in Germany at the end of October 2009. In our study, 40 (43%) children suffered from underlying diseases and, hence, ideally should have been vaccinated against influenza. Of these 40 children, two children were younger than six months and 16 children became ill before the PIA vaccine was available. Of the remaining 22 children with underlying diseases only two (9%) had received a PIA vaccination. Only for one child (2%) out of 51 children without predisposing factors aged above six months a PIA vaccination was reported. The low PIA vaccination coverage found in our study is confirmed by results from cross-sectional surveys in children <14 years of age in Germany (8% coverage) , and from a German surveillance study on severe PIA cases <15 years of age admitted to intensive care units (9% coverage) .
Potential limitations may result from the differences in criteria for inpatient treatment and use of diagnostic methods depending on the individual decision by the admitting physician. The number of hospitalizations corresponded to a conservative incidence estimate of at least 118 PIAH per 100,000 children <18 years of age. However, this may considerably underestimate the true pediatric PIAH incidence as only patients with laboratory-confirmed PIA were included; children hospitalized with respiratory symptoms or influenza-like illness without being tested for influenza were not captured in this study.