ABSTRACT
There is growing scientific interest in immunity mandates/passports (IMP) for viral diseases in light of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. IMP isolate those who remain nonimmune from various settings to reduce nonhousehold transmissions from the nonimmune and reduce severe/critical illness among the nonimmune. A major limitation in the scientific literature is that there are currently no methods to quantify how many nonimmune individuals need to be isolated to achieve these purported benefits. This paper develops a procedure for estimating the benefits of IMP using a novel variant of the number needed to treat which we call the number needed to isolate (NNI). We use data from the SARS-CoV-2 pandemic to demonstrate the properties and utility of the NNI and to inform the debate about IMP. We focus on data from the European Union, United Kingdom, United States, Canada, Australia, and Israel during the fall 2021 when the Delta (B.1.617.2) variant predominated.