Methodological Considerations on the External Validity of the Kim HJ et al. COVID-19 Vaccination Study (Biomark Res 13:114, 2025): A Quantitative Analysis

Abstract

Background/Objectives: The Kim HJ et al. (2025) cohort study reported a surprising finding: a significantly higher 1-year cancer incidence risk in the COVID-19 vaccinated group. Given the vast public health implications, this result requires immediate and rigorous examination. This analysis provides the first formal external critique, integrating computational reviews to evaluate the methodology and test the external validity of the cohort.

Methods: Aggregated data from the Kim HJ et al. matched cohort (n=2,975,035) were used to calculate the overall Crude Incidence Rate (CR) of cancer. This study tested the resulting Crude Incidence Rate (CR) of the cohort against the established official national average CR for South Korea for the reference period (2020–2022). A secondary analysis of the final cohort's size ratio was performed to hypothesize a potential flaw in the 1:4 Propensity Score Matching (PSM) procedure.

Results: The cohort's overall CR was found to be 40.78 per 10,000, representing a substantial 22.26% downward deviation from the national average (52.46 per 10,000; SD 2.97). This discrepancy establishes a pronounced epidemiological paradox, strongly suggesting a lack of external validity for the cohort. Based on the exact 4:1 ratio of the final matched groups, the analysis proposes that the PSM procedure was likely inverted or misapplied, with the smaller unvaccinated group being used as the base cohort '1' for matching against the vaccinated group '4', which may have contributed to the suppression of the overall CR.

Conclusions: The reliability of the statistical associations reported by Kim HJ et al. is challenged by a possible lack of external validity and the hypothesized methodological ambiguities concerning the PSM. We conclude that independent validation is mandatory and reiterate the call for public access to the underlying Korean National Health Insurance database to resolve these contradictions