Effectiveness of the Influenza Vaccine in Preventing Hospitalizations of Patients with Influenza Community-Acquired Pneumonia

Introduction: Influenza vaccination is the primary strategy for prevention of influenza infection. Influenza infection can vary from mild or even asymptomatic illness to severe community-acquired pneumonia (CAP). Although many national and international investigators and organizations report annual estimates of influenza vaccine effectiveness for prevention of influenza infection in the community, few studies report estimates for the prevention of hospitalizations due to influenza CAP, the most severe form of the infection. The objective of this study is to determine the effectiveness of the influenza vaccine for prevention of hospitalization in patients with influenza-associated CAP. Methods: This was a test-negative study using data from the first two years of the University of Louisville Pneumonia Study, a prospective, observational study of all hospitalized patients with pneumonia in Louisville, Kentucky from 6/1/2014 – 5/31/2016. Univariate and multivariate logistic models were used to evaluate the association between vaccine status and influenza-associated/non-influenza-associated CAP hospitalization. Unadjusted and adjusted vaccine effectiveness estimates were calculated. Results: A total of 1951 hospitalized patients with CAP were included in the analysis, and 831 (43%) reported having received the influenza vaccination for the influenza season by the time they were hospitalized. A total of 152 (8%) cases of influenza-CAP were confirmed in the study population, with 63 (8%) cases confirmed in vaccinated individuals. The unadjusted vaccine effectiveness was not significant, with a point estimate of 5% (95% CI: -33%, 32%). After adjusting for potential cofounders, vaccine effectiveness was also found to not be significant with a point estimate of 8% (95% CI: -30%, 35%). Conclusions: In conclusion, we found that, over the 2014/2015 and 2015/2016 influenza seasons, influenza vaccine was not effective for prevention of hospitalization with CAP due to influenza. More effective vaccines are necessary to prevent the most serious forms of influenza