The relationships between host factors, virus strain, viral load, and illness severity in respiratory syncytial virus (RSV)-induced bronchiolitis are poorly defined. These relationships were evaluated prospectively in 81 previously healthy infants hospitalized with RSV bronchiolitis. Disease severity was determined by the respiratory rate, the duration of hospitalization, and whether patients during their hospitalization required pediatric intensive care unit admission or mechanical ventilation. RSV typing into subgroup A and B was obtained by RT-PCR-hybridization assay. The nasopharyngeal RSV viral loads were measured by real-time quantitative RT-PCR. Disease severity correlated significantly with the presence of risk factor (estimated gestational age < 37 weeks and/or birth weight < 2,500 g) and with chronologic age

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