Association between residential greenness and the risk of inflammatory bowel disease in patients with COVID-19: A Korean nationwide cohort study

Background: Residential greenness has been associated with reduced risk of inflammatory bowel disease (IBD), but its association in the context of COVID-19, a condition associated with increased IBD risk, remains unclear. We evaluated whether the protective association between greenness and IBD persisted among individuals with SARS-CoV-2 infection. Methods: We conducted a nationwide cohort study of 6,900,341 individuals with confirmed COVID-19 in South Korea, using linked data from the National Health Insurance Service, Korea Disease Control and Prevention Agency, and satellite-derived normalized difference vegetation index (NDVI) from NASA’s Terra MODIS (MOD13A3, version 6.1). Individuals were categorized into low (0.2 to <0.4), moderate (0.4 to <0.6), or high (≥0.6) NDVI groups. The primary outcome was incident IBD (Crohn’s disease and ulcerative colitis) ≥30 days after infection, defined using ICD‑10 codes. Inverse probability of treatment weighting balanced covariates, and weighted Cox models estimated adjusted hazard ratios (aHRs) with 95% confidence intervals (CIs). Results: Higher residential greenness was associated with lower risk of developing IBD following COVID-19 infection, with aHRs of 0.93 (95% CI, 0.90–0.97) for moderate and 0.71 (0.68–0.74) for high NDVI compared with low. Subtype analyses showed more consistent associations for ulcerative colitis (aHR 0.91 [95% CI, 0.87–0.96] for moderate; aHR 0.60 [0.57–0.63] for high NDVI). For Crohn’s disease, associations were weaker and limited to individuals exposed to high NDVI (aHR, 0.90 [95% CI, 0.85–0.96]). Associations were more apparent among individuals with mild COVID-19 and were stronger among females, adults aged 40 to 59 years, and those with pre-existing comorbidities. Conclusions: Higher residential greenness was associated with a lower incidence of IBD after COVID-19. Maintaining access to greenness during periods of restricted mobility may help counteract infection-related inflammation.