Aims: To estimate excess morbidity during periods of influenza and respiratory syncytial virus (RSV) activity. Methods: Retrospective analysis of a sentinel practice network database in active and non-active virus periods. Main outcome measures: clinical diagnoses of new episodes of influenza-like illness (ILI), acute bronchitis, asthma, and otitismedia. Results: The clinical diagnosis of ILI was consistent with influenza virus activity and acute bronchitis with RSV. During periods of virus activity, estimates of excess morbidity in children aged 1-4 and 5-14 years diagnosed as having acute otitis media exceeded those diagnosed with each of the other three conditions; in children < 1 year estimates for acute bronchitis were highest. Using a broad definition of virus activity and summarising the data for all children diagnosed with ILI, 60%was attributable to influenza (40%RSV) as were 37%of episodes diagnosed as ocute bronchitis, 9%of those with asthma and 48%of those with otitis media. Using a narrow definition, corresponding proportions were: for ILI diagnoses 77%(23%RSV), acute bronchitis 32%, asthma zero, and otitis media 45%. Acute bronchitis was diagnosed twice as frequently in association with RSV as with influenza in all age groups: excess asthma episodes were only evident in RSV active periods. Conclusions: Except in relation to ILI, RSV caused more illness than the influenza virus in the respiratory diagnoses examined, emphasising the need for RSV prevention and treatment. Influenza was not associated with excess asthma episodes.
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