Influenza A in Young Children with Suspected Respiratory Syncytial Virus Infection

OBJECTIVES: To determine the prevalence of influenza A in young children suspected of having respiratory syncytial virus (RSV) infection and to compare the clinical presentation of these patients with those who have proven RSV infection. METHODS: Children younger than or at 36 months of age who presented to a pediatric emergency department (ED) with suspected RSV infection during the influenza A season of 2001-2002 were eligible. Eligible children had an RSV antigen test ordered as part of their initial clinical management. A consecutive sample of children was enrolled for prospective observational analysis. The main outcome measure was the prevalence of influenza A in young children with suspected RSV infection. The secondary outcome measure was a comparison of the clinical presentations, of the two groups. RESULTS: During the study period, 420 patients presented for evaluation of respiratory illness. RSV tests were ordered on 251 patients. Of 197 eligible patients, 124 (63%) tested positive for RSV and 33 (17%) for influenza A. Influenza A patients were more likely to have temperatures at or above 39 degrees C than RSV patients (36% vs. 15%; p = 0.01). RSV patients were more tachypneic (54 vs. 43 breaths/minute; p < 0.0001) and more often had wheezing (90% vs. 8%; p < 0.0001). Twenty influenza patients (61%) were hospitalized. CONCLUSIONS: This study found a high prevalence of influenza A in young children suspected of having RSV infection. Clinicians should consider influenza A in young febrile children presenting with respiratory illnesses.