Body Mass Index, Waist Circumference, Readmission, and Healthcare Costs After Influenza- or Pneumonia-Related Hospitalization: A Sex-Stratified Nationwide Cohort Study in Korea

Background/Objectives: Obesity is associated with respiratory infection risk, but its relationship with post-discharge healthcare burden remains unclear. This study examined the associations of body mass index (BMI) and waist circumference (WC), evaluated separately, with hospitalization and post-discharge outcomes in analyses stratified by sex among South Korean adults with claims-defined hospitalizations involving influenza or pneumonia. Methods: This retrospective cohort study used the 2018 National Health Insurance Service (NHIS) Customized Research Database linked to health-screening data. Adults aged 20–89 years with claims-defined hospitalizations involving influenza or pneumonia were included if BMI and WC measurements were available from the most recent NHIS health-screening examination conducted within two years before the index admission. Length of stay and index hospitalization costs were summarized descriptively. Cox proportional hazards models were used to estimate hazard ratios (HRs) for all-cause and pneumonia-related readmission within two years after discharge, and two-part models were used to estimate readmission-related costs, expressed in US dollars (USD). Results: The cohort included 175,774 patients, of whom 80,257 (45.7%) were male. Category-specific associations varied according to the anthropometric measure, outcome, and sex stratum. In female patients, the highest WC category had the largest estimated all-cause readmission HR (HR 1.195, 95% confidence interval 1.157–1.235), and the highest model-based mean cost of the first all-cause readmission was also observed in this category (USD 1407.4). In male patients, several higher BMI categories had lower estimated readmission HRs than the reference category; however, these estimates should not be interpreted as protective effects because competing mortality was not accounted for using competing-risk methods. Patterns for pneumonia-related readmission and costs were less consistent. Conclusions: BMI and WC showed category-specific associations with readmission, while model-based mean readmission costs varied across anthropometric categories and sex strata. Higher WC categories were associated with higher all-cause readmission HRs in the female stratum; however, the independent or incremental prognostic value of BMI and WC was not evaluated.