Do the unvaccinated disproportionately harm the vaccinated in a respiratory pandemic?

TL;DR

This paper critiques the use of a recently introduced parameter in epidemiological models, showing it is incorrectly defined and that its conclusions about disproportionate risks to vaccinated individuals are invalid, potentially misleading public health policies.

Contribution

The paper identifies errors in the definition and application of the parameter ψ, challenging prior claims of disproportionate risk to vaccinated individuals during respiratory pandemics.

Findings

The parameter ψ is incorrectly defined and applied.

Claims of increased risk with segregation, coverage, and efficacy are invalid.

Misuse of ψ could lead to unnecessary public health measures.

Abstract

A parameter $\psi$ was recently defined and introduced into the epidemiological modelling scientific literature, and is being accepted. The said parameter was used to argue that there was a disproportionate risk of infection incurred by vaccinated persons due to contacts with unvaccinated persons during the declared COVID-19 pandemic. Opposing published results show that, in general, there is virtually never a disproportionate risk to the vaccinated from the unvaccinated during a respiratory pandemic. Here, we show that the newly introduced vaccinology parameter $\psi$ is incorrectly defined and that the conclusions of disproportionate risk are not valid. Specifically, we prove that the originating authors Fisman et al. (2022, 2024) incorrectly defined and applied the parameter $\psi$. Their application would imply that the said risk increases with increasing segregation from the unvaccinated (up to complete segregation), increases with increasing vaccination coverage (up to complete coverage) and increases with increasing vaccine efficacy (up to perfect vaccine efficacy), which is impossible. Use of the erroneous parameter $\psi$ has the potential to encourage unnecessarily aggressive public health policies and interventions.