Marika K. Iwane, PhD MPH¹, Guillermo Herrera, MD¹, Margaret Cortese, MD¹, Carolyn Bridges, MD¹, Cedric Brown, MS¹, Kenneth Gershman, MD², and Alyson K. Shupe, PhD². (1) National Immunization Program, Centers for Disease Control and Prevention, 1600 Clifton Rd, MS E61, Atlanta, GA 30333, 404-639-8769, miwane@cdc.gov, (2) Colorado Department of Public Health and Environment, 4300 Cherry Creek Drive South, Denver, CO 80246

Background: Influenza causes on average 36,000 deaths and 114,000 hospitalizations annually in the US. The 2003-04 US influenza season began early with reports of severe illness and deaths in children and concern about vaccine effectiveness (VE); the vaccine was not well-matched to the predominant circulating strain. An earlier CDC study did not find the 2003-04 vaccine to be effective against influenza-like illness (ILI), which could negatively impact vaccination efforts.

Objective: To evaluate VE of the 2003-04 influenza vaccine against lab-confirmed medically attended influenza among adults.

Methods: We conducted a case-control VE study among persons aged 50-64 years. Cases were obtained from lab-confirmed influenza cases reported to the Colorado Department of Public Health and Environment in November-December 2003 and interviewed by phone. Controls without ILI were recruited through random digit dialing and matched to cases by age, sex, and area code. Logistic regression analyses were used to estimate VE (1-odds ratio).

Results: 330 cases and 1055 controls were interviewed; 50% of cases and 21% of controls had a high-risk medical condition that increases the risk of influenza-related complications. VE was estimated to be 47% (95% CI: 30%, 60%) against lab-confirmed medically attended influenza (38% for high risk; 52% for non-high risk), and 60% (37%, 75%) against influenza-related hospitalization.

Conclusions: This study supports the CDC recommendation to continue influenza vaccination despite suboptimal vaccine match and contrasts with earlier VE estimates. This is the first study to assess VE against lab-confirmed influenza-related hospitalizations. Yearly, prospective studies for the assessment of VE are needed.

Learning Objectives: At the conclusion of the session, the participants in this section will understand

• the importance of evaluating influenza vaccine effectiveness
• the public health need to continue vaccinating, even with a mismatch between the vaccine and circulating influenza strains.

Keywords: Immunizations, Infectious Diseases

Related Web page: www.cdc.gov/nip

Presenting author's disclosure statement:
I do not have any significant financial interest/arrangement or affiliation with any organization/institution whose products or services are being discussed in this session.