Background: Earlier studies reported that infants <12 months of age were at high risk of hospitalization for 2009 pandemic influenza A H1N1 virus (2009 H1N1) infection, but there was little information about the clinical courses in these patients.
Methods: A retrospective study was performed using the nationwide inpatient Diagnosis Procedure Combination Database in Japan. We searched data on hospitalizations, between July and December 2009, of infants (<12 months) and children 12 to <24 months of age with influenza. We examined their demographic data and hospitalization details, including treatment, complications, and outcomes.
Results: During the study period, 1023 infants were hospitalized for influenza; 98.1% of them were previously healthy. Complications attributed to influenza were found in 181 (17.7%) infants, including 3 critical illnesses and 1 death. Oseltamivir was prescribed to 535 (53.3%) infants. We found no evidence that oseltamivir was associated with either complications or length of hospital stay. During the same period, 579 children 1 year of age were admitted for 2009 H1N1, and 56.5% of them had influenza-related complications.
Conclusions: In Japan, infants with 2009 H1N1 were twice as likely to be hospitalized as children 1 year of age. However, the clinical course of the hospitalized infants was generally uncomplicated.