Background: Few studies have directly compared the clinical impact of different types and subtypes of influenza viruses in children.

Methods: In a prospective study of respiratory infections in preenrolled cohorts of children <=13 years of age, we compared the clinical features and the overall burden of illness between outpatient children with A/H1N1, A/H3N2 and B infections. The data were derived from structured medical records filled out by study physicians and from daily symptom diaries filled out by the parents throughout the follow-up period.

Results: Of 358 children included in the analyses, 203 (57%) had influenza A/H1N1, 96 (27%) had A/H3N2, and 59 (16%) had influenza B infection. Children with influenza A/H3N2 were significantly younger (median, 3.2 years) than those with A/H1N1 (median, 4.8 years) or B (median, 5.1 years) infections (P < 0.0001). When adjusted for age, children 3–6 years of age with A/H3N2 infection had a higher frequency of fever >=39.0°C (67% vs. 38%; P = 0.002), longer duration of fever (median, 4 vs. 3 days; P = 0.02) and more antibiotic treatments (43% vs. 20%; P = 0.004) than did children with A/H1N1 infections. Overall, the clinical presentation, duration of illness, frequency of complications, children’s absenteeism from day care or school and parental work absenteeism were comparable between children with A/H1N1, A/H3N2 and B infections.

Conclusions: Adjusted for age, the clinical manifestations and the burden of illness are largely comparable between children with influenza A/H1N1, A/H3N2 and B infections.