Natural history of pandemic H1N1 2009 influenza infection in healthy pediatric outpatients.

Objective: The pandemic influenza H1N1 2009 (pH1N1) virus is expected to remain a prominent circulating strain in the current and subsequent influenza seasons. The objective of this study was to compare the clinical course of infection with laboratory-confirmed pH1N1 and seasonal influenza A and B in a cohort of previously healthy children managed in the outpatient setting without antiviral therapy.

Methods: Previously healthy children 17 years of age or younger were prospectively enrolled during the first wave of the 2009 pandemic (May-July 2009) and the 2 preceding influenza seasons from a single primary care physician office and a tertiary children's hospital emergency department. Inclusion criteria were: age ≤17 years; laboratory-confirmed influenza; and not receiving antiviral agents. Follow-up telephone interviews were conducted approximately 2 days and 14 days after presentation to assess symptom duration.

Results: A total of 251 patients (101 with pH1N1, 90 with seasonal influenza A, 60 with seasonal influenza B) were included. Presenting symptoms and duration of symptoms associated with pH1N1 were similar to those with seasonal influenza. Median fever duration in pH1N1 was 3 days and had resolved within 8 days in 95% of patients. Among patients with pH1N1, abnormalities on chest auscultation (10 of 101), the hospitalization rate (2 of 101), the proportion of children receiving intravenous fluid therapy (1 of 101) or supplemental oxygen (1 of 101) were not significantly different from patients with seasonal influenza.

Conclusions: In most children without underlying risk factors for severe influenza, pH1N1 virus causes an uncomplicated respiratory tract illness, no more severe than seasonal influenza.