Quantifying Structural Selection Bias in Observational Cohort Data: A Ponderation Analysis of Age- Specific Incidence Rates to Inform Vaccine Safety Verification

Abstract

Background: A recent nationwide cohort study reported an unadjusted Hazard Ratio (HR) of 2.714 for vitiligo incidence following COVID-19 vaccination, indicating a major safety concern. This finding was based on cohorts with an ≈ 11-year age difference, immediately raising critical concerns regarding extreme structural selection and detection bias.

Objectives: We hypothesize that this extreme association is an artifact of a fatal methodological flaw, challenging the study's internal validity and subsequent external validity. We aim to quantitatively separate the HR attributable to the structural age imbalance (HR Structural) from the residual HR (HR Residual) which measures the uncorrected methodological failure.

Methods: We performed a stratified ponderation analysis using the age distribution of the scrutinized study’s cohorts (Vaccinated, mean age=56.32 years vs Non-Vaccinated, mean age=45.51 years) and applied established national age-specific vitiligo incidence rates (IR) from external epidemiology. This allowed us to quantitatively separate the HR attributable to the structural age imbalance (HR Structural) from the residual HR (HR Residual), which measures the uncorrected methodological failure.

Results: The HR Structural was calculated to be 1.2821. This robust correction demonstrates that the structural age difference explains only 16.43% of the observed excess risk. The remaining HR Residual (2.1168) is the exact measure of the methodological failure caused by the double distortion in cohort design (lowering the baseline risk in the NonVaccine group while maximizing the detection risk in the Vaccine group.

Discussion: The HR=2.714 of the scrutinized study is an unstable statistical artifact. The overwhelming majority of the observed association is a consequence of a fatal design flaw, not a biological risk, resulting in a severe lack of internal and external validity.