*Article* **Association of Consecutive Influenza Vaccinations and Pneumonia: A Population-Based Case-Control Study**

**Liang-Tsai Yeh 1,2,†, Chi-Ho Chan 3,†, Shun-Fa Yang 1,4, Han-Wei Yeh 5, Ying-Tung Yeh 6,7,8, Yu-Hsun Wang 4, Ming-Chih Chou 1, Chao-Bin Yeh 1,9,10 and Ying-Hock Teng 9,10,11,\***


Received: 14 February 2019; Accepted: 23 March 2019; Published: 26 March 2019

**Abstract:** The purpose of this study was to investigate whether individuals receiving influenza vaccines have a lower risk of pneumonia. A nationwide population-based case-control study was conducted using data from the National Health Insurance Research Database in Taiwan. We enrolled 7565 patients each in pneumonia and non-pneumonia groups after diagnosis of patients with chronic pulmonary disease, and these patients were individually age and sex matched in a 1:1 ratio. Using conditional logistic regression analysis, adjusted odds ratios (aORs) were estimated in patients who received influenza vaccination and those who had not previously had pneumonia. Moreover, we also analyzed the interval between vaccination and the onset of pneumonia and the number of vaccinations received by patients. This was compared with patients who never received influenza vaccination. Patients who had received influenza vaccination and had been vaccinated for two consecutive years (aOR = 0.85, confidence interval (CI) = 0.79–0.93 and aOR = 0.75, CI = 0.67–0.85, respectively) showed lower rates of pneumonia occurrence by 15–25%. In conclusion, influenza vaccination significantly reduces the occurrence of pneumonia, especially in individuals who receive vaccination in consecutive years.

**Keywords:** influenza vaccination; pneumonia; population-based case-control study
