Fisman et al.’s Psi (Ψ) index is ill-defined and leads to absurd interpretations

CORRELATION has published a new brief report entitled “Fisman et al.’s Psi (Ψ) index is ill-defined and leads to absurd interpretations” authored by Joseph Hickey and Denis G. Rancourt.

The report shows that modeling claims, based on a novel mathematical construction (Ψ), that unvaccinated people disproportionately contribute to the infection risk of vaccinated people are rigorously incorrect.

The incorrect claims were made in the April 2022 article “Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission” by Fisman et al.

Hickey and Rancourt’s previous criticisms of the Fisman et al. article are listed below:

• 2023-10-31: J. Hickey & D.G. Rancourt, “Viral respiratory epidemic modelling of societal segregation based on vaccination status”, medRxiv preprint [See Appendix 3 of Version 1 of the preprint, in particular]
• 2022-04-30: Interview about Fisman et al. paper with Dr. Byram Bridle and Dr. Denis Rancourt, Trish Wood is Critical Podcast
• 2022-04-29: D.G. Rancourt & J. Hickey, “Fisman et al.’s main conclusion does not follow from their model”, e-Letter, CMAJ
• 2022-04-27: D.G. Rancourt & J. Hickey, “OCLA Statement on CMAJ Fisman et al. Article Claiming Disproportionate Infection Risk from Unvaccinated Population, and on Negligent Media Reporting”

Download a copy of the report below: