Effectiveness of inactivated influenza vaccine against laboratory-confirmed influenza pneumonia among adults aged ≥65 years in Japan

BACKGROUND: The effectiveness of inactivated influenza vaccine (IIV) against laboratory-confirmed influenza pneumonia in older adults remains to be established. METHODS: Pneumonia patients aged ≥65 years who visited a study hospital in Chiba, Japan, were prospectively enrolled from February 2012 to January 2014. Sputum samples were collected from participants and tested for influenza virus by polymerase chain reaction assays. Influenza vaccine effectiveness (IVE) against laboratory-confirmed influenza pneumonia was estimated by a test-negative design. RESULTS: Among a total of 814 pneumonia patients, 42 (5.2%) tested positive for influenza: 40 were positive for influenza A virus, and two were positive for influenza B virus. The IVE against laboratory-confirmed influenza pneumonia was 58.3% (95% confidence interval, 28.8–75.6%). The IVE against influenza pneumonia hospital admission, severe pneumonia, and death was 60.2% (95% CI, 22.8–79.4%), 65.5% (95% CI, 44.3–78.7%), and 71% (95% CI, −62.9% to 94.8%), respectively. In the subgroup analyses, the IVE against influenza pneumonia was higher for patients with immunosuppressive conditions (85.9%; 95% CI, 67.4–93.9%) than for those without (48.7%; 95% CI, 2.7–73%) but did not differ by patients’ statin use status. CONCLUSION: IIV effectively reduces the risk of laboratory-confirmed influenza pneumonia in older adults.