• When decision makers need to make difficult choices regarding the allocation of scarce resources and pandemic response, potential strategies include: o Reduction of demand for healthcare services through medical countermeasures (e.g., mass vaccination) or nonpharmaceutical interventions (e.g., social distancing); o Optimization of existing resources (e.g., central command structure); o Augmenting resources (e.g., adding temporary facilities or personnel); and o Implementation of crisis standards of care. • We identified 201 studies from January 2011 through May 2020 evaluating resource allocation and pandemic response strategies addressed in infectious disease threats, natural disasters, terrorism, and other mass casualty events for decision makers. Research designs were observational and critical appraisal results varied. • Most research exists to reduce care demand (n=149); higher quality studies reported benefits of contact tracing, school closures, travel restrictions, port of entry screening, and mass vaccination. We identified 28 evaluations of strategies to augment resources; higher quality studies reported the effectiveness of establishing temporary facilities, using volunteers, and using decision support software. Strategies to optimize existing resources have been assessed in 23 studies; higher quality studies reported successfully expanding scope of work and building on existing agreements between agencies. Few studies have evaluated crisis standards of care strategies. Coronavirus disease 2019 (COVID-19) research is emerging; four higher quality studies evaluated combinations of interventions; one reported the benefit of communitywide mask policies. • Most existing research is based on simulations, and future research is needed to test strategies in practice. Key Question What strategies are available to policymakers to optimize allocation of scarce resources during mass casualty events, especially pandemics? Methods We searched the databases PubMed, Web of Science, and Cochrane Database of Systematic Reviews on May 4, 2020. The search strategy is documented in Appendix A. We included empirical studies and simulations evaluating strategies for policymakers to allocate scarce resources. Studies had to address the allocation of scarce resources (broadly defined, as described above and in the appendix), and interventions had to be aimed at policymakers such as local public health officials rather than individual clinicians. The detailed eligibility criteria are documented in the appendix. For the update, literature reviewers screened citations and full-text articles in duplicate, one reviewer abstracted and appraised the data, and a systematic reviewer checked the work using online data abstraction software. The screening process is outlined in Appendix B. We documented the included studies in an evidence table (Appendix C). We used the same critical appraisal used in the previous report. For the synthesis, studies were stratified by strategy category and higher quality studies (critical appraisal scor...