Incidence of outpatient visits and hospitalizations related to influenza in infants and young children.

OBJECTIVE: The Advisory Committee on Immunization Practices currently encourages influenza vaccination for all children aged 6 to 23 months when feasible, based on studies that have demonstrated that young children have high hospitalization rates attributable to influenza. The Advisory Committee on Immunization Practices recently voted to recommend influenza vaccination for all children beginning during the 2004-2005 influenza season; information on the rate of outpatient visits due to influenza is needed to better evaluate the potential health impact and cost-effectiveness of the recommendation. We estimated the incidence of outpatient visits as well as hospitalizations for specific acute respiratory illnesses and for influenza-associated outpatient-visit and hospitalization rates among healthy infants and children in a Massachusetts health maintenance organization.

DESIGN/METHODS: Surveillance data were used to identify when influenza viruses, respiratory syncytial viruses, and parainfluenza viruses were circulating in the greater Boston area during 1994-2000. Using computerized medical records, we identified outpatient visits and hospitalizations for selected respiratory illnesses. Outpatient-visit rates and hospitalizations attributed to influenza were calculated by subtracting the rate of visits during the periseasonal period from the rate of visits during the influenza period. Rates were stratified by age and risk for complications from influenza.

RESULTS: Between 1994 and 2000, there were 188 139 outpatient visits and 885 hospitalizations for respiratory illnesses in the study population. Among healthy children aged 6 to 23 months, the rate per 100 person-months for outpatient visits during influenza periods was 14.5 (95% confidence interval [CI]: 13.9 to 15.1), and the excess rate that could be attributed to influenza compared with the periseasonal period was 1.8 (95% CI: 1.1 to 2.4). Among healthy children, the rate of hospitalizations for acute respiratory disease was 10.4 per 10 000 person-months (95% CI: 6.0 to 17.0), and the rate that could be attributed to influenza when compared with the periseasonal baseline period was 3.9 (95% CI: -2.0 to 0.0). Among children who were at high risk for complications from influenza, the rate of outpatient visits per 100 person-months was 28.7 (95% CI: 26.6 to 30.9) during influenza periods. The rate of hospitalizations among high-risk children was 44.6 per 10 000 person-months (95% CI: 19.0 to 17.0).

CONCLUSION: Influenza season is associated with a substantial increase in outpatient visits by healthy children. These estimates of outpatient visits for influenza will help quantify the potential health benefits and cost savings from influenza vaccination of healthy children aged 6 to 23 months.