Evaluating the effects of the 2017 National Health Insurance coverage expansion on amenable mortality and its disparities between areas in South Korea using Bayesian structural time-series models

To improve the low coverage rate of the National Health Insurance (NHI), South Korea implemented the NHI coverage expansion plan in 2017 to cover medically essential non -covered services and reduce copayment rates. This study aimed to estimate the effects of the 2017 NHI coverage expansion on amenable mortality and its disparities between areas in South Korea under a controlled interrupted time -series design using Bayesian structural time -series models. Age -standardized amenable mortality rates and rate differences (RDs) and rate ratios (RRs) between areas for amenable mortality were calculated monthly between July 2012 and December 2021 and used as the response series. The non-equivalent control series were monthly non -avoidable mortality rates and their regional disparities. After the coverage expansion, amenable mortality rates decreased for both males (-8.8%, 95% credible interval [CrI] -13.4% to -3.9%) and females (-8.3%, 95% CrI -13.4% to -2.4%), with the largest decline in the non -Seoul -Capital metropolitan area (-11.6%, 95% CrI -16.5% to -6.3%) rather than the Seoul Capital Area (-7.5%, 95% CrI -11.9% to -2.5%) and a non -significant reduction in the nonSeoul -Capital non -metropolitan area in females. RDs and RRs between areas for amenable mortality decreased non -significantly (-16.2%, 95% CrI -31.3% to 2.6% for RD and -1.2%, 95% CrI -3.7% to 1.5% for RR), except for a significant decrease in RD in males (-21.8%, 95% CrI -38.0% to -1.5%), and decreased less in females than in males. The coverage expansion was generally effective in reducing amenable mortality rates by area, but had limited effects in closing amenable mortality disparities between areas, favoring males and the non -SeoulCapital metropolitan area. These results implied that additional measures are necessary to improve access to quality health care for females and underserved areas to enhance the effectiveness of the coverage expansion.