*2.4. Statistical Analysis*

A comparison of pneumonia and non-pneumonia groups was performed using a Chi-squared test or independent *t*-test, as was appropriate. The association between influenza vaccination and pneumonia was analyzed using conditional logistical regression analysis. The statistical software used was SPSS V.18.0 (SPSS, Chicago, IL, USA), and the significance was defined as *p* < 0.05. On the use of post hoc power analysis of the logistic regression, the alpha error was 0.05 and the beta error was 0.2. The achieved power of the study was 0.83.
