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Although acute bronchiolitis is primarily caused by viral infections, antibiotics are often administered to children with acute bronchiolitis. Due to concerns about serious bacterial infections (SBI), neonates and young infants are particularly prone to antibiotic overuse. This study aimed to identify the clinical characteristics and prevalence of SBI in neonates and infants aged < 90 days with acute bronchiolitis. We retrospectively reviewed the medical records of 651 neonates and infants aged < 90 days hospitalized with acute bronchiolitis between September 2015 and August 2024. Demographic and clinical data were analyzed to assess their clinical characteristics and the prevalence of SBI. Of the 651 infants, 230 (35.3%) had fever, and 485 (74.5%) received antibiotics. Blood cultures were performed in 646 (99.2%) infants; 52 (8.0%) yielded bacterial growth, but 50 were skin contaminants and one (Enterococcus faecium from an afebrile infant) was considered clinically insignificant. Only one (0.2%) infant had a probable true bacteremia caused by methicillin-susceptible Staphylococcus aureus. Among 621 (95.4%) infants who underwent urine cultures, seven (1.1%) had both bacteriuria and pyuria. Among them, only one (0.2%) infant had fever, which was deemed to have clinically significant urinary tract infection. Cerebrospinal fluid cultures in 31 infants were all negative.

SBI were rare in neonates and infants aged < 90 days hospitalized for acute bronchiolitis. This suggests that routine sepsis workups and empirical antibiotic therapy are not necessary for most of these patients. A selective approach to bacterial testing and antibiotic therapy is strongly justified.

•Empirical antibiotics are commonly used in infants aged ‹90 days with acute bronchiolitis due to concerns about concurrent serious bacterial infections.

•Serious bacterial infections are extremely rare in infants aged ‹90 days hospitalized with acute bronchiolitis, regardless of fever.