Estimating the risk reduction of isolation on COVID-19 non-household transmission and severe/critical illness in non-immune individuals: September to November 2021

In the fall 2021, immunity mandates/passports for COVID-19 started to be discussed and implemented globally. In addition to increasing vaccination levels, these interventions isolate non-immune individuals from various settings to reduce non-household transmission and severe/critical illness. This is based on the hypothesis that the non-immune are at high absolute risk of these outcomes. However, these absolute risks were not quantified in the literature such that the absolute risk reductions of isolation on these outcomes remain unknown. This study estimated these absolute risks from September to November 2021 prior to the emergence of Omicron (B.1.1.529) using known data on the risk of infection, transmission in non-household settings, and age-stratified severe/critical illness in non-immune individuals for the Delta (B.1.617.2) variant, focusing on the European Union, United Kingdom, United States, Canada, Australia, and Israel. This allowed us to quantify the absolute risk reductions of isolation on (1) non-household transmission from the non-immune and (2) severe/critical illness amongst the non-immune in these regions during this period. We observed that on any given day the absolute risk reductions of isolation were typically small for transmission in most types of non-household settings and severe/critical illness in most age-groups, especially those aged <40. During a wave or sustained higher infection risks, the risk reductions were modest only for transmission in intimate social gatherings and severe/critical illness in adults aged ≥50-60. The limitations of this study and the implications for the expected benefits of isolating non-immune individuals on reducing these outcomes are discussed. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study received no grant from any funding agency, commercial, or not-for-profit sectors. It has also received no support of any kind from any individual or organization. BH is supported by a personal research grant from the University of Wroclaw within the 'Excellence Initiative, Research University' framework and by a scholarship from the Polish Ministry of Education and Science. None of these institutions were involved in this research and did not fund it directly. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes All data are available in the manuscript/supplementary material and at https://github.com/TheNNIforViralTransmission/SARS-CoV-2 and https://covid-app.cloud.forces.gc.ca/map.