Implementation of universal influenza immunization recommendations for healthy young children: results of a randomized, controlled trial with registry-based recall.

BACKGROUND: An Advisory Committee on Immunization Practices policy of encouraging influenza vaccination for healthy 6- to 23-month-old children was in effect during the 2003-2004 influenza season, which was unusually severe in Colorado. We collaborated with 5 pediatric practices to attempt universal influenza immunization in this age group.

OBJECTIVES: The objectives were (1) to assess the maximal influenza immunization rates that could be achieved for healthy young children in private practice settings, (2) to evaluate the efficacy of registry-based reminder/recall for influenza vaccination, and (3) to describe methods used by private practices to implement the recommendations.

METHODS: The study was conducted in 5 private pediatric practices in Denver, Colorado, with a common billing system and immunization registry. Although recommendations by the Advisory Committee on Immunization Practices included children who were 6 to 23 months of age at any point during the influenza season, our practices chose not to recall children 22 to 23 months of age, because they would have become >24 months of age during the study period. Therefore, our study population consisted of all healthy children 6 to 21 months of age from the 5 practices (N = 5193), who were randomized to intervention groups (n = 2595) that received up to 3 reminder/recall letters or to control groups (n = 2598) that received usual care. The primary outcome was receipt of >or=1 influenza immunization, as noted either in the immunization registry or in billing data.

RESULTS: Immunization rates for >or=1 dose of influenza vaccine for the intervention groups in the 5 practices were 75.9%, 75.4%, 68.1%, 55.6%, and 44.3% at the end of the season. Overall, 62.4% of children in the intervention groups and 58.0% of children in the control groups were immunized (4.4% absolute difference), with absolute differences, compared with control values, ranging from 1.0% to 9.1% according to practice. However, before intensive media coverage of the influenza outbreak began (November 15, 2003), absolute differences, compared with control values, ranged from 5.1% to 15.3% and were 9.6% overall. Before November 15, significant effects of recall were seen for children in the intervention groups, in both the 12- to 21-month age category (10.4% increase over control) and the 6- to 11-month category (8.1% increase over control); at the end of the season, however, significant effects of recall were seen only for the older age group (6.2% increase over control). The rates of receipt of 2 vaccine doses >or=1 month apart for eligible children ranged from 21% to 48% among the practices. Four of the 5 practices held influenza immunization clinics during office hours, evenings, or weekends, and these clinics achieved higher coverage rates.

CONCLUSIONS: These results demonstrated that, in an epidemic influenza year, private practices were able to immunize the majority of 6- to 21-month-old children in a timely manner. Although media coverage regarding the epidemic blunted the effect of registry-based recall, recall was effective in increasing rates early in the epidemic, especially for children between 1 and 2 years of age. The practices that achieved the highest immunization rates were proactive in planning influenza clinics to handle the extra volume of immunizations required.