Assessment of the effectiveness of the 2003-04 influenza vaccine among children and adults--Colorado, 2003

The 2003-04 influenza season was characterized by the early onset of influenza activity, reports of severe illness, particularly in children, and predominant circulation of an influenza A (H3N2) virus strain that was antigenically different from the influenza A (H3N2) vaccine strain. In 2003, a retrospective cohort study among children and a case-control study among adults in Colorado were conducted to provide preliminary data on the effectiveness of the 2003-04 influenza vaccine. This report summarizes the results of those studies, which indicated vaccine effectiveness (VE) among both adults and children, differing from results of a previous study that did not indicate effectiveness among adults.