**1. Introduction**

Pneumonia is a common clinical presentation after a respiratory infection. Since 2016, pneumonia has been the third most common cause of death in Taiwan. The majority of pneumonia cases can

be classified as either community-acquired, hospital-acquired, or acquired after traveling to foreign countries. Bacterial pneumonia is usually a complication of influenza virus infection [1]. The American Thoracic Society and Infectious Diseases Society of America have classified pneumonia into three types, namely community-acquired pneumonia (CAP), hospital-acquired (or nosocomial) pneumonia, and ventilator-associated pneumonia, according to the epidemiology, pathogenesis, and risk factors for infection in patients with pneumonia [2,3]. The etiology of pneumonia includes bacteria, viruses, fungi, and protozoa. Generally, pathogens that potentially cause pneumonia subsist of "typical" bacteria and "atypical" organisms, including *Mycoplasma pneumoniae*, and respiratory viruses, such as influenza viruses. However, 11–20% of pneumonia cases are polymicrobial, and the etiology usually consists of a combination of typical and atypical pathogens [4]. *Streptococcus pneumoniae* has been reported as the major pathogen in secondary infection after individuals were infected by influenza viruses, and it increases the mortality risk of patients [5–8]. Previous studies such as Tessmer et al. [9] show that prior influenza vaccination is associated with a less severe clinical course and improved long-term survival in patients with CAP, especially during the influenza epidemic season. However, Shinjoh et al. [10] reported that children who were immunized for two consecutive seasons experienced decreased vaccine effectiveness and were more likely to acquire influenza and that this might be associated with immunity against influenza infection in the previous season. Therefore, the aims of this study are also to clarify this.

Among the pathogens, influenza is one of the primary causes of pneumonia and influential respiratory diseases [11,12]. In general, human influenza viruses include A/H1N1, A/H3N2, and B viruses. Because of the antigenic drift of viruses, epidemics of influenza are reported every year in Taiwan. To effectively prevent the disease, large influenza vaccination programs have been held by the health authority of the Taiwanese government, and the trivalent influenza vaccine containing influenza A/H1N1, A/H3N2, and B viruses is employed annually. To increase the influenza vaccination coverage, free-of-charge vaccination programs have been held since 1988 for several groups, including young children under 6 months old, students from primary school to senior high school, and adults with high influenza risk (i.e., people aged over 50 years and patients of any age with a chronic illness like diabetes, chronic hepatitis, cardiovascular disease, chronic pulmonary disease, chronic renal disease, etc.) [13]. In fact, a characteristic of influenza infection in the elderly is a high frequency of pneumonia complications. Therefore, inoculation with the influenza vaccine is critical for both preventing influenza infection and lowering the risk of post-influenza pneumonia development in the elderly [14]. Influenza vaccination has been reported to be associated not only with prevention of influenza epidemics but also reduced risk of several diseases, such as acute kidney injury, diabetes, cardiovascular disease, and respiratory failure in chronic obstructive pulmonary disease, especially in elderly people [15–20]. However, the relationship between pneumonia and influenza vaccination remains unclear. This study investigated whether individuals receiving influenza vaccines have decreased risk of developing pneumonia.
