Failure to address crucial biases other than the healthy vaccinee effect

Abstract

This response critiques Fürst et al. for overlooking two key confounding effects in observational studies on COVID-19 vaccine efficacy: indication bias (IB) and high risk for death (HRD). While the authors correctly identify the healthy vaccinee effect (HVE), they fail to account for IB—shown to be present in Czech nationwide data—and HRD, which reflects clinical practice wherein patients at imminent risk of death were not vaccinated. These effects vary across age groups and, when considered together with HVE, can explain the “paradoxical” patterns the authors highlight. The critique additionally questions the study’s reliance on data from only two health insurers and its interpretation of mortality rates without addressing time-lag effects in COVID-19 deaths.