Methods

This retrospective study was conducted by review of medical charts, laboratory and radiological findings of all children admitted to the Gold Coast Hospital with confirmed pandemic (H1N1) 2009. The study period was from the 25 May 2009 to the 16 August 2009, coinciding with Australia's flu season. During this period, all children admitted into hospital with a febrile or respiratory illness were tested for pandemic (H1N1) 2009 by two Taqman based real-time RT-PCR methods, designed locally in Australia. The two assays, H1-PCR and N1-PCR, were designed targeting the pandemic (H1N1) 2009 virus haemagglutinin and neuraminidase genes, respectively. These RT-PCR methods are found to be sensitive and specific for pandemic (H1N1) 2009 RNA.4 The tests were done at the closest tertiary centre 1 h away and took 2 h to complete. In the peak of the pandemic, due to the overwhelming number of samples, it took 48 h for results to return. Specimens were collected from nasal pharyngeal aspirates or nasal pharyngeal swabs. These specimens were also tested with a multiplex PCR assay for a select respiratory viral panel including influenza A, influenza B, respiratory syncytial virus, parainfluenza types 1, 2 and 3, adenovirus and human metapneumovirus.

Epidemiological characteristics observed were age, gender and pre-existing medical conditions. Pre-existing medical conditions were obtained from the initial history and chart review. These were then categorised according to physiological system. Asthmatic patients were included if they were, at the time of admission, on preventive treatment for asthma. Due to variable documentation of obesity or indigenous background in the medical charts, these factors were not examined in our study.

Clinical features were obtained from findings documented by the treating physician. Based on Queensland Health's pandemic (H1N1) 2009 case definition, children had flu-like illness if they presented with a fever of at least 38°C, or a history of fever, with one symptom of cough, sore throat or rhinorrhoea.5

Diagnosis was based on history and examination findings documented by the admitting physician, laboratory and radiological findings. In our study, respiratory diagnoses were divided into three groups namely, asthma, bronchiolitis and consolidation on chest x ray. Asthma was defined as the presence of wheezing and/or chest retraction in those with a past history of asthma, without evidence of consolidation on chest x ray. Bronchiolitis was defined as the presence of wheezing and/or chest retraction in those <1 year of age without evidence of consolidation on chest x ray. Due to the difficulty in differentiating consolidation caused by pneumonia or asthma, those with bilateral or unilateral consolidation on chest x ray were analysed in one group called consolidation. Febrile convulsion was defined as a seizure associated with a fever, or history of recent fever, with no previous history of afebrile seizures or evidence of central nervous infection or metabolic abnormality. Dehydration was defined according to the admitting physician's initial assessment based on history of fluid loss, or poor oral intake, clinical signs including tachycardia, dry mucous membranes, poor central capillary refill, poor urine output and laboratory findings. Encephalopathy was defined as altered mental status lasting greater than 24 h. Diabetic ketoacidosis was defined by biochemical criteria of a blood glucose greater than 11 mmol/litre and evidence of metabolic acidosis (venous pH <7.3 and/or plasma bicarbonate <15 mmol/litre).

Patients were treated with the antiviral, oseltamivir, at the discretion of the admitting physician. Factors considered when commencing treatment were based on the joint position statement published by the Australasian Society for Infectious Diseases (ASID) and the Thoracic Society of Australia and New Zealand (TSANZ).6 These recommendations included treatment for children with risk factors for severe disease, particularly children <5 years of age, those with immunosuppression or chronic disease like asthma, cardiorespiratory disease, diabetes and renal failure. In children with no risk factors for severe disease, oseltamivir was recommended in those who presented within 48 h of onset of symptoms. Oseltamivir was not recommended in those <1 year of age due to limited safety data. Oseltamivir was given to those hospitalised with severe flu infection even after 48 h of symptom onset.