Effectiveness of the influenza vaccine at preventing hospitalization due to acute exacerbation of cardiopulmonary disease in Korea from 2011 to 2012

There is a lack of targeted studies to validate the effectiveness of influenza vaccination on the reduction in influenza-related hospitalizations among patients with co-morbidities. In this study, we estimate the effectiveness of influenza vaccination on preventing hospitalizations in persons with cardiopulmonary disease and establish an evidence base for recommendations on influenza vaccination in this population. During the influenza epidemic in 2011-2012, we performed a multicenter, retrospective case-control study. Cases were patients hospitalized due to acute exacerbation of asthma, COPD, ischemic heart disease (IHD), and congestive heart failure (CHF). Controls were selected from outpatients who visited study hospitals but who were not hospitalized. Cases and controls were matched 1:1 based on age, gender, and date of hospital visit. Conditional logistic regression analyses were used to determine the effectiveness of vaccination. Between 25 December 2011 and 5 May 2012, 828 of each hospitalized and control subjects were identified. The influenza vaccination rate of the hospitalized and non-hospitalized patients was 54.2% and 60.4%, respectively (P = 0.006). The overall vaccine effectiveness for preventing hospitalization was 33.7% (95% confidence interval [CI] 14.0-49.0%; P = 0.002). Conditional logistic regression analysis showed that influenza vaccination significantly reduced the risk of hospitalization, especially due to acute exacerbation of IHD and CHF, in patients aged 65 y and older. The estimated vaccine effectiveness in these patients was 56.0% (95% CI 32.1-71.4%, P = 0.002). Influenza vaccination was associated with a reduction in the risk of hospitalization due to acute exacerbation of cardiopulmonary disease. We recommend the vaccine be given primarily to patients with underlying cardiovascular disease, particularly those 65 y of age and older.