Clinical and socioeconomic impact of moderate-to-severe versus mild influenza in children.

Some studies have assessed the efficacy of influenza vaccination in children separately for moderate-to-severe and any influenza, but the definition used for identifying children with moderate-to-severe illness has not been validated. We analyzed clinical and socioeconomic data from two prospective cohort studies of respiratory infections among children aged ≤13 years (four influenza seasons, 3,416 child-seasons of follow-up). We categorized children with laboratory-confirmed influenza into two mutually exclusive groups of moderate-to-severe and mild influenza using the previously proposed criteria. We obtained the data for the analyses from structured medical records filled out by the study physicians and from daily symptom cards filled out by the parents. Of 434 cases of influenza, 217 (50 %) were classified as moderate-to-severe and 217 (50 %) as mild. The mean duration of fever was 4.0 days in children with moderate-to-severe influenza and 3.1 days in those with milder illness (P < 0.0001). Antibiotics were prescribed to 111 (51 %) children with moderate-to-severe and to ten (5 %) children with mild influenza (P < 0.0001). The rates of parental work absenteeism were 184 days per 100 children with moderate-to-severe influenza and 135 days per 100 children with mild influenza (P = 0.02). The corresponding rates of children's own absenteeism from day care or school were 297 and 233 days respectively per 100 children (P = 0.006). Categorization of children into groups with moderate-to-severe and mild influenza is meaningful, and it identifies children in whom the clinical and socioeconomic impact of influenza is highest. Illness severity should be considered when assessing influenza vaccine effectiveness in children.