Overview Background: In recent years, the epidemic of respiratory syncytial virus (RSV) has been seen in the summer. In daily clinical practice, we noted that RSV infection tends to become more severe and its treatment tends to be longer, particularly in patients hospitalized in the summer. Thus, we investigated factors associated with the progression to severe condition and the prolongation of treatment in RSV infection. Methods: Subjects were pediatric patients diagnosed with RSV infection

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... hospitalized for treatment during the 5 year s from April 2014 to March 2019. Information on age, sex, season of hospitalization, gestational age, and bronchial asthma of each subject was collected from inpatient medical records. Duration of oxygen administration was used as an index for treatment duration and the use of devices for severe cases as an index for severity. Multiple linear regression analysis and logistic regression analysis were performed to examine the variables associated with duration of oxygen administration and use of a device for severe cases. Results: Data from 298 patients were analyzed. Duration of oxygen administration was significantly associated with bronchial asthma (partial regression coefficient: 0.804, p = 0.010). Hospitalization in summer was significantly associated with use of a device for severe condition (adjusted odds ratio: 5.89, 95% confidence interval: 1.72-20.18). Conclusion: The present study showed that bronchial asthma is a risk factor for prolongation of treatment and infection in summer is a risk factor for progression to severe condition in cases of RSV infection. These findings suggested that children with bronchial asthma and infection in summer need to be treated carefully regardless of their age, sex, and gestational age. Background 4 Respiratory syncytial virus (RSV) is an RNA virus that belongs to the pneumovirus genus, Paramyxoviridae family, and causes respiratory infection in infants [1]. Infection is established and symptoms appear in early infancy when there are maternal antibodies. Half of infants develop symptoms by the age of 1 year, and almost 100% by the age of 2. The symptoms become severe in 1% to 3% of patients, requiring hospitalization [2]. Only upper respiratory tract infection results from RSV infection in most infants [3]. However, when lower respiratory tract infection including bronchiolitis and pneumonia occurs, expiratory stridor, prolonged expiration, polypnea, and other symptoms are observed and the condition sometimes becomes severe [2]. It is commonly believed that the epidemic occurs in winter in temperate zones, and mainly in December in Japan, while epidemic levels have recently been seen in summer [4]. The same tendency was observed in Tokyo [5]. In our daily clinical practice in recent years, RSV infection has apparently required longer hospitalization and has been more severe in summer than in winter. This tendency was especially prominent in children whose underlying disease was bronchial asthma. Currently, premature infants or children with congenital heart diseases are considered to be at an increased risk for progression of RSV infection to a severe condition and therefore prophylactic palivizumab, an anti-RSV antibody, is indicated for these children in many countries [6] . In response to the recent epidemic of RSV in summer, timing of the administration of palivizumab has been examined in many studies [7, 8]. However, other risk factors for progression of RSV infection to a severe condition have been investigated in only a small number of studies, and it is difficult to predict how hard the treatment would be in daily clinical practice. In this study, we hypothesized that the timing of hospitalization and the presence or absence of bronchial asthma affects the treatment of RSV infection, and retrospectively examined the children whose RSV infection required KO, MS, TT, MK, MS, MK, and KM planned the study. HO and AK checked the study design and examined the statistical validity. KO and HO were in charge of manuscript writing. All authors read and approved the final manuscript. Funding None.