**3. Results**

In total, 90,491 patients diagnosed with chronic pulmonary disease were initially recruited from 2010 to 2012. Next, 8010 of the 90,491 patients were selected according to a new diagnosis of pneumonia after chronic pulmonary disease diagnosis. Another 56,233 patients who were never diagnosed with pneumonia were also selected. After this, we performed a 1:1 age and sex match, and 7565 patients were enrolled in each of the two groups: pneumonia and non-pneumonia (Figure 1). The distributions of age and sex in the groups were not significant (Table 1).


**Table 1.** Demographic characteristics of pneumonia and non-pneumonia.

### *Int. J. Environ. Res. Public Health* **2019**, *16*, 1078


**Table 1.** *Cont.*

\* *p* < 0.05, \*\* *p* < 0.01.

**Figure 1.** Flow chart for patient selection.

Next, we analyzed the risk of pneumonia and the selected comorbidities. Patients who had received an influenza vaccination showed a 15% decreased risk of pneumonia (adjusted odds ratio (aOR) = 0.85, confidence interval (CI) = 0.79–0.93). Furthermore, patients had an increased risk of pneumonia with following comorbidities: hypertension, diabetes, cerebrovascular disease, renal disease, and liver disease (Table 2).



† Adjusted for hypertension, diabetes, cerebrovascular disease, renal disease, liver disease, ischemic heart disease, dementia, alcohol-related disorder, and malignancy. OR: odds ratio; CI: confidence interval; \*\* *p* < 0.01.

We also analyzed the interval between the onset of pneumonia and receiving the influenza vaccination, as well as the number of vaccinations. We compared these results with patients who had never received an influenza vaccination. Patients that received the vaccine one year prior to the study showed a 13% reduction in the risk of developing pneumonia (aOR = 0.87, CI = 0.78–0.98). Patients vaccinated for two consecutive years prior to the study showed a 25% decreased risk of developing pneumonia (aOR = 0.75, CI = 0.67–0.85). Furthermore, patients vaccinated for three consecutive years compared with those who had never received influenza vaccination showed a 44% decreased risk of developing pneumonia (aOR = 0.56, CI = 0.45–0.69) (Table 3).


**Table 3.** Conditional logistic regression of the frequency of receiving an influenza vaccination.

† Adjusted for hypertension, diabetes, cerebrovascular disease, renal disease, liver disease, ischemic heart disease, dementia, alcohol-related disorder, and malignancy. 1 yr: receive influenza vaccine in the first year before the pneumonia diagnosis. 2 yr: receive influenza vaccine in 1 to 2 years before the pneumonia diagnosis. \* *p* < 0.05, \*\* *p* < 0.01.

Finally, we performed a subgroup analysis by age and sex for the vaccination and non-vaccination groups. After we subdivided the patients by age into three subgroups (<40, 40–65, and ≥65), we observed that only those patients aged ≥ 65 had an obvious reduced risk of developing pneumonia (aOR = 0.78, CI = 0.71–0.86). Male patients also showed a lower risk of pneumonia than the female patients. However, vaccination did not reduce the risk of pneumonia in patients aged 40–65 years (Table 4).


**Table 4.** Subgroup analysis of the conditional logistic regression between vaccination and nonvaccination groups.

a Adjusted for hypertension, diabetes, cerebrovascular disease, renal disease, liver disease, ischemic heart disease, and malignancy. b Adjusted for hypertension, diabetes, cerebrovascular disease, renal disease, liver disease, ischemic heart disease, dementia, alcohol-related disorder, and malignancy. \*\* *p* < 0.01.
