Trends in the incidence of asthma in adults: A Korean population-based study

Background Current population-based data on the incidence of asthma in Korean adults are scarce, although the cross-sectional prevalence study had been conducted. Moreover, estimates of asthma incidence in elderly are rare. Objective We aimed to investigate the annual incidence of asthma among Korean adults and identify differences in the asthma incidence according to demographic and socioeconomic characteristics over the study period 2004–2012. Methods A population-based cohort study was conducted using the 2002–2014 National Health Insurance Service–National Sample Cohort (NHIS-NSC). It consisted of 1,025,340 randomly selected participants, comprising 2.2% of the total eligible Korean population in 2002. Among these, 746,816 adults older than 20 years receiving National Health Insurance were included in the study. The cohort consisted of 746,816 adults aged over 20 years with National Health Insurance (NHI). We defined cases as a person with two or more physician claims with a primary diagnosis of asthma (ICD 10 code J45 or J46) and asthma medications within at least 1-year interval. Age- and sex-specific incidence rates were determined by dividing the total number of adults newly diagnosed with asthma by the total number of person-years (PY) accumulated. The incidence rates were age-adjusted to according to the 2005 Korean population census. Poisson regression analysis was used to calculate sex- and age-adjusted incidence rates and to test the difference in incidence trends according to age, sex, and income groups. Trends in incidence over time were assessed using Joinpoint Regression Software. We analyzed the trend of each line segment using the annual percent change (APC), and the overall trend for the whole study period (2004–2012), using the average APC (AAPC). Results The incidence rate was significantly increasing until 2008(APC 13.8), and from 2008 to 2012, it showed decreasing trend although not significant (APC −2.9), The crude incidence rate per 1000 person-years was 4.53 in 2004, 8.04 in 2008, 7.08 in 2012, respectively. Age and sex adjustment did not change this trend. Incidence for women was higher than for men and the difference was constant according to the year. Incidence rates increased with age and this difference increased over calendar time. There were no significant differences in the incidence of asthma by five income groups. Conclusion Our population-based cohort study observed that the incidence of asthma in Korean adults (2004–2012) has not increased from 2008 onward, directly indicating the stabilization of asthma incidence as suggested in previous studies. Further studies are needed to investigate the decrease in asthma incidence rate and high incidence in susceptible groups.