OBJECTIVES: Some people are thought to be highly susceptible to influenza viruses. However, in Japanese real-world practice, whether or not these susceptible individuals truly exist and the factors associated with such susceptibility have not been fully elucidated. The objective of this study is to identify susceptible individuals and explore the factors associated with their susceptibility to influenza viruses in Japan.

METHODS: We analyzed a Japanese health insurance claims database (January 2005-April 2019) of more than 5.0 million insured persons. Firstly, the frequency of influenza events (F) was defined as the number of prescriptions of anti-influenza drugs per observation period (OP). We defined people with F (times/years) ≥1 as susceptible individuals, F = 0 as non-susceptible individuals, and 0<F<1 as the others. Next, among individuals with OP ≥5 years, susceptible individuals were identified as cases. For each case, 60 non-susceptible individuals were randomly selected as controls from the population to be matched for sex and age. The prevalence of comorbid diseases during OP was investigated using the ICD-10 code, and the differences between the two groups were analyzed using a Z-test.

RESULTS: The total numbers of susceptible individuals, non-susceptible individuals, and others were 5,028, 1,080,567 and 826,589, and the mean ages were 13.7, 42.1 and 30.2, respectively. A total of 429 cases were matched to 25,740 controls. Among the top 20 comorbid diseases identified, in ascending order of p-value, 19 were higher proportion in cases than controls, and eight were related to respiratory diseases (J209, J304, J459, J029, J189, J069, J039, and J46-).

CONCLUSIONS: Susceptible individuals accounted for less than 0.3% of the total and were younger than those in other groups. They had a higher prevalence of respiratory diseases than non-susceptible individuals. Further studies are needed to elucidate the causal relationship between the diseases and influenza virus infections.