A Critical Note on Contradictions in South Korean Cancer Incidence Rates: The Paradox of Crude Rates Derived from the Kim HJ et al. Cohort (Biomark Res, 13:114, 2025) Showing Concurrent Increases in the Vaccinated and Overall Decrease

This note examines a pronounced epidemiological paradox that emerges when calculating the crude incidence rates (CRs) using the raw figures provided in the study, "1-year risks of cancers associated with COVID-19 vaccination: a large population-based cohort study in South Korea" (Kim HJ et al., Biomark Res, 13:114, 2025). Sourced from the Korean National Health Insurance database, the Kim et al. study suggests a higher rate of new cancer cases among the vaccinated population compared to the unvaccinated. However, when the raw figures (12,133 total cancer cases among 2,975,035 individuals) are used to calculate the overall Crude Incidence Rate (CR) for the study cohort, the estimated rate is 40.78 per 10,000 population. This value deviates downwards by over 22% from the official national average CR of 52.46 per 10,000 for all cancers recorded by the Korean Central Cancer Registry in the years immediately preceding the vaccination campaign (2020–2022). This fundamental discrepancy creates a paradox: the study simultaneously suggests a rising risk within the majority group (vaccinated CR: 42.63/10,000) while yielding an overall CR significantly lower than the established national baseline. We conclude that this pronounced lack of representativeness in the cohort's crude incidence rate likely introduced unidentified confounding factors, potentially biasing the final association results. To resolve this dilemma and enable comprehensive independent validation of the observed statistical association, the underlying Korean National Health Insurance database must be made publicly accessible to the international scientific community.