cNo previous studies have assessed the persistence of immune responses in individuals with diabetes. We conducted this study to evaluate the long-term immunogenicity and safety of an influenza vaccine in type 2 diabetic subjects compared with nondiabetic controls. A randomized and controlled study was conducted at two university hospitals during the 2012-2013 influenza season. The study vaccine was a standard-dose trivalent subunit inactivated intramuscular vaccine. Serum hemagglutination-inhibiting (HI) antibodies were measured at the time of vaccination and 1 month and 6 months after vaccination. Local and systemic reactions were recorded for 7 days. A total of 105 diabetic patients and 108 controls were included in the analysis. One month after vaccination, both the diabetic and nondiabetic groups satisfied all of the criteria of the Committee for Medical Products for Human Use (CHMP), and the immunogenicity profiles were statistically similar between the two groups. Although the vaccine was well tolerated, and all adverse reactions were mild to moderate, there was a tendency toward a reduced incidence of local reactions in the diabetic group. All values in the long-term immunogenicity profiles were statistically similar between the two groups, except for the seroprotection rate for the A/H1N1 influenza virus strain, which was significantly lower in the elderly diabetic group than that in the elderly nondiabetic group. However, in multivariate analysis, long-term immunogenicity was associated with age and prevaccination titer, regardless of diabetes status. (This study has been registered at CRIS [https://cris.nih.go .kr/cris/en/] under registration no. KCT0001423.)A lthough the available data are limited, diabetic individuals may be more susceptible to influenza infections than nondiabetic individuals (1). In addition, individuals with diabetes are at increased risk of severe influenza virus infection and its complications compared to nondiabetic persons (2-4). Such phenomena are thought to be mediated by impairments in cellular and humoral immunity, which include reduced T cell responses, decreased neutrophil function, and B cell disorders (5). For this reason, annual influenza vaccination is universally recommended for patients with diabetes.To achieve protection against influenza virus infection, vaccinations should elicit a sufficient antibody response. Many studies have shown that diabetic individuals have an immune response to influenza vaccination similar to that of healthy controls, while a few studies have reported suboptimal responses in diabetic subjects (6-12). Immunogenicity should be maintained throughout the entire seasonal epidemic; therefore, an evaluation of longterm immunogenicity is essential before the current conventional vaccination program can be recommended. However, no study has assessed long-term immunogenicity in individuals with diabetes.If the immune responses and safety profiles prove to be unsatisfactory with the conventional influenza vaccine, immunogenicity-enhancing strateg...