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Adult Influenza Vaccine Effectiveness Study


The primary method of preventing complications from community-acquired influenza virus infections among adults is vaccination with trivalent inactivated influenza vaccine (TIV). Since 2000, the Advisory Council on Immunization Practices has recommended that adults aged 50 years and older receive TIV annually. The purpose of this case-control study is to estimate the effectiveness of TIV against influenza-associated hospitalizations among adults aged 50 years and older during several influenza seasons.

Goals & Objectives

  • To evaluate the overall effectiveness of trivalent inactivated influenza vaccine (TIV) among New Haven County residents aged 50 years and older during influenza seasons in preventing hospitalization with a laboratory-confirmed, community-acquired influenza virus infection.
  • To make age-specific estimates of the effectiveness of TIV during several influenza seasons.
  • To evaluate the potential for effect modification of vaccination by the presence of chronic medical conditions.


Influenza viruses are responsible for annual epidemics of respiratory disease that affect all segments of the U.S. population and result in considerable morbidity and mortality. Vaccination with trivalent inactivated influenza vaccine (TIV) has been the major strategy for reducing the effect of influenza virus infections for many years. Though many previous studies of the effectiveness of TIV in adults have been conducted, few have examined the effectiveness of the vaccine against laboratory-confirmed influenza hospitalizations among adults aged 50 years and older. This evaluation is being conducted through the Respiratory Diseases Activity of the Emerging Infections Program Network of the Centers for Disease Control and Prevention, by building upon the existing surveillance system for laboratory-confirmed influenza hospitalizations.

Laboratory-confirmed influenza reports among New Haven County residents are received from the Connecticut Department of Public Health. These laboratory-confirmed cases are followed up by contacting hospital infection control practitioners (ICPs) to identify cases of influenza-related hospitalizations at the ICPs' respective hospitals. For each hospitalized case enrolled, two age-matched, community-dwelling controls who were not hospitalized with an influenza infection are recruited. Cases and controls are contacted by telephone. After obtaining consent for participation, study personnel interview each enrollee regarding influenza vaccine status, chronic medical conditions, and functional status. Additionally, health care providers are contacted for both cases and controls to obtain immunization and medical history. Data collection for this case-control study began with the 2008-09 influenza season.