3.1 Patient characteristics
A total of 965 patients were registered between January 2015 and August 2019. The numbers of patients for each season were 167 in 2014-2015, 217 in 2015-2016, 179 in 2016-2017, 298 in 2017-2018, and 105 in 2018-2019. Thirty patients were excluded because their admissions were not associated with influenza. Another eleven patients also were excluded because the causes of admission were not described. Thus, 924 patients were analyzed. The patient characteristics are shown in Table 1. The median age was 78 years (IQR, 67-84). The age distribution of each season is indicated in Figure 1. The median ages of each season were 78 years (IQR, 66.5-85.5) in 2014-2015, 70 (55-80) in 2015-2016, 80.5 (73-86) in 2016-2017, 79 (70-86) in 2017-2018, and 74.5 (63.5-81) in 2018-2019. The patients in the 2015-2016 season were significantly younger than those in other seasons (p <0.05).
A total of 684 patients (74.0%) had influenza A, and 237 patients (25.6%) had influenza B. Twenty-five patients (2.7%) were positive for both influenza A and B, and the influenza type was unknown in 28 patients (3.0%) who were diagnosed on clinical basis. Two hundred and twenty-two (24.0%) and 95 patients (10.3%) had been immunized with an influenza vaccine or the 23-valent pneumococcal polysaccharide vaccines, respectively.