Fenella McAndrew ^1* Rachel Sacks-Davis ^1,2 Romesh Abeysuriya ^1,2 Dominic Delport ^1,2 Daniel West ³ Indra Parta ³ Suman S. Majumdar ^1,2,4 Margaret Hellard ^1,2,5,6,7 Nick Scott ^1,2

• ¹ Burnet Institute, Melbourne, Australia
• ² Department of Clinical Epidemiology, School of Public Health and Preventive Medicine, Monash University, Malvern, Victoria, Australia
• ³ Department of Health, Victoria, Melbourne, Victoria, Australia
• ⁴ Department of Paediatrics, Melbourne Medical School, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Parkville, Victoria, Australia
• ⁵ Melbourne School of Population and Global Health, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, Carlton, Victoria, Australia
• ⁶ Department of Infectious Diseases, Central Clinical School, Faculty of Medicine, Nursing & Health Sciences, Monash University, Melbourne, Victoria, Australia
• ⁷ Victorian Infectious Diseases Reference Laboratory, Melbourne, Victoria, Australia

Introduction: A disproportionate number of COVID-19 deaths occur in Residential Aged Care Facilities (RACFs), where better evidence is needed to target COVID-19 interventions to prevent mortality. This study used an agent-based model to assess the role of community prevalence, vaccination strategies, and non-pharmaceutical interventions (NPIs) on COVID-19 outcomes in RACFs in Victoria, Australia. Methods: The model simulated outbreaks in RACFs over time, and was calibrated to distributions for outbreak size, outbreak duration, and case fatality rate in Victorian RACFs over 2022. The number of incursions to RACFs per day were estimated to fit total deaths and diagnoses over time and community prevalence. Total infections, diagnoses, and deaths in RACFs were estimated over July 2023-June 2024 under scenarios of different: community epidemic wave assumptions (magnitude and frequency); RACF vaccination strategies (6-monthly, 12-monthly, no further vaccines); additional non-pharmaceutical interventions (10%, 25%, 50% efficacy); and reduction in incursions (30% or 60%). Results: Total RACF outcomes were proportional to cumulative community infections and incursion rates, suggesting potential for strategic visitation/staff policies or community-based interventions to reduce deaths. Recency of vaccination when epidemic waves occurred was critical; compared with 6-monthly boosters, 12-monthly boosters had approximately 1.2 times more deaths and no further boosters had approximately 1.6 times more deaths over July 2023-June 2024. Additional NPIs, even with only 10-25% efficacy, could lead to a 13-31% reduction in deaths in RACFs. Conclusion: Future community epidemic wave patterns are unknown but will be major drivers of outcomes in RACFs. Maintaining high coverage of recent vaccination, minimizing incursions, and increasing NPIs can have a major impact on cumulative infections and deaths.