Background: Respiratory syncytial virus (RSV) is the most important viral pathogen for lower respiratory tract infection (LRI) in infants and children. An RSV-specific monoclonal antibody has been developed to provide prophylaxis against RSV associated LRI (RSV-LRI). The objective of this study was to determine the impact of RSV as a cause of LRI in children younger than 3 years of age to provide data to aide in the implementation of forthcoming prophylaxis against RSV. Methods: We analyzed the viral etiology of LRI in hospitalized Japanese children younger than 3 years of age admitted to Shizuoka Red Cross Hospital from July, 1997 to June, 2000. Results: A total of 535 patients younger than 3 years of age were hospitalized with LRI at Shizuoka Red Cross Hospital from July 1, 1997 to June 30, 2000. Of these, a positive diagnosis of RSV infection was made in 168 patients (31.4%). Most of the patients with RSV infection had been well and had had no underlying disease that was defined as risk factor of RSV infection (94.0%). The peak incidence of LRI was observed in the winter each year and the number of LRI was strongly associated with the epidemic of RSV (r=0.700, P<0.0001). The number of patients with LRI younger than 6 months of age was 116 (21.7%). Of these 116 patients younger than 6 months with LRI, 55 patients (47.4%) were confirmed to have RSV infection. The proportions of RSV infection to total LRI was greatest in early infants younger than 6 months (P < 0.0001). The number of patients with which RSV infection was detected in LRI patients younger than 3 years was highest during the first five months of life and there was a dramatic decrease in incidence of RSV infection with increasing age thereafter. Conclusions: The incidence of LRI hospitalization is highly affected by RSV infection epidemic. The proportion of RSV infections among early infants younger than 6 months is greater than that of older patients. The prophylaxis against RSV will be needed to be toward early infants.
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