Epidemiology of patients with severe asthma in Japan: a nationwide descriptive study.

Background: The 2014 European Respiratory Society/American Thoracic Society guidelines defined severe asthma based on treatment intensity and estimated the proportion of severe asthma among all asthma cases to be 5-10%. However, data supporting the estimate and comprehensive and sequential data on asthma cases are scarce. We aimed to estimate the national prevalence and proportion of severe asthma during the last decade. Methods: Using a Japanese national administrative database, which covers ≥99% of the population, we evaluated the prevalence and proportion of severe asthma in 2013, 2015, 2017 and 2019. Additionally, we elucidated the demographic characteristics, treatments and outcomes of patients with asthma. Results: The national prevalence of mild-moderate and severe asthma in 2019 was 800 and 36 per 100 000 persons, respectively. While the prevalence of mild-moderate asthma remained almost constant in the study years, the prevalence of severe asthma decreased, resulting in a reduction in the proportion of severe asthma from 5.6% to 4.3%. Although treatment modalities have evolved, such as the increased use of combination inhalers and asthma biologics, approximately 15% of mild-moderate and 45% of severe asthma cases were still considered "uncontrolled". The number of deaths from asthma decreased in patients with both mild-moderate and severe asthma. Conclusions: This study revealed that the prevalence of severe asthma in Japan decreased during the study period and fell below 5% in the most recent data. Despite treatment evolution, a substantial proportion of patients with both mild-moderate and severe asthma still have poor asthma control.