Impact of Intravesical Bacillus Calmette-Guérin Therapy on COVID-19 Outcomes in Patients with Non-Muscle-Invasive Bladder Cancer during the Pre-Omicron Era (2020–2021): A Nationwide Cohort Study Incorporating Pandemic Temporal Dynamics

This study aims to investigate the impact of intravesical Bacillus Calmette-Guérin (BCG) therapy on COVID-19 outcomes (incidence, severity, and mortality) in patients with non-muscle-invasive bladder cancer (NMIBC). Using the Korean National Health Insurance Service (NHIS) database, a nationwide retrospective cohort of patients newly diagnosed with NMIBC between January 2020 and December 2021 was analyzed. To rigorously address potential confounding variables and selection bias, inverse probability of treatment weighting (IPTW) is applied. Furthermore, the study explicitly stratifies the analysis into two distinct epidemiological periods—the pre-vaccine 2020 (wild-type) era and the mass-vaccination 2021 (Delta variant) era. Through this temporal dynamics approach, the study seeks to evaluate whether the 'trained immunity' induced by BCG provides a consistent, non-specific protective effect against severe respiratory viral threats across different public health contexts.