Effectiveness of the 2003–2004 influenza vaccine among U.S. military basic trainees: a year of suboptimal match between vaccine and circulating strain

Abstract

Effectiveness of the 2003–2004 influenza vaccine was evaluated at five military basic training centers throughout the United States. Data from surveillance conducted in December and January 2003–2004 in this highly vaccinated population were evaluated. During this period, 10.6% (37/350) of specimens were positive for influenza A. A 14-day period after vaccination was considered the period prior to immune protection; vaccine effectiveness (VE) was calculated based on febrile respiratory illness presentation and laboratory confirmation of influenza before or after this 14-day period. Thirty-two cases presented within 14 days of vaccination, and five cases presented beyond 14 days from vaccination. VE in this population was estimated to be 94.4% for laboratory-confirmed influenza. In contrast, VE was only 13.9% for influenza-like illness (ILI) without a laboratory confirmation.