Effectiveness of the 2003/2004 Influenza Vaccine Among U.S. Military Basic Trainees: A Year of Suboptimal Match Between Vaccine and Circulating Strain

Effectiveness of the 2003-04 influenza vaccine was evaluated at 5 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 5 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 without a laboratory confirmation.