Verifiability of “Reducing Contact by 80%â€

This paper discusses issues related to the verification of the "reducing contact by 80%" policy implemented during the first state of emergency declaration, utilizing a logic model to show the relationship between the policy (output) and its outcome. The focus of this paper is not on verifying the effect itself, but on examining the structure that makes it difficult to validate the mathematical model on which the 80% contact reduction was based.This difficulty stems from the unclear and unobservable concept of what "contact" refers to in the real world. This structure simultaneously leads to difficulties in implementing contact reduction measures in practice. Since contact cannot be directly observed, the inconvenience arises that, if the mathematical model is assumed correct, a failure to reduce new infections by 80% could be interpreted as insufficient effort by the general public. This approach can be described as outcome-based epidemic control.During the state of emergency, an attempt was made to validate the model using human mobility and contact rates with contact frequency as an observable outcome. However, the model could not be validated due to inconsistencies with the original plan. Additionally, the ex-post analysis by Prof. Nishiura's team abandoned the validation of the model because they used a different model than the one on which the contact reduction was based. The recommendation for 80% contact reduction presented the analytical results in which the variables in the original model were mistaken. It would have been possible to correct this and validate the original model, but that path was not taken.