COVID-19 in Scottish care homes: A metapopulation model of spread among residents and staff

Care homes in the UK were disproportionately affected by the first wave of the COVID-19 pandemic, accounting for almost half of COVID-19 deaths over the course of the period from 6th March - 15th June 2020. Understanding how infectious diseases establish themselves throughout vulnerable communities is crucial for minimising deaths and lowering the total stress on the National Health Service (NHS Scotland). We model the spread of COVID-19 in the health-board of NHS Lothian, Scotland over the course of the first wave of the pandemic with a compartmental Susceptible - Exposed - Infected reported - Infected unreported - Recovered - Dead (SEIARD), metapopulation model. Care home residents, care home workers and the rest of the population are modelled as subpopulations, interacting on a network describing their mixing habits. We explicitly model the outbreaks reproduction rate and care home visitation level over time for each subpopulation, and execute a data fit and sensitivity analysis, focusing on parameters responsible for intra-subpopulation mixing: staff sharing, staff shift patterns and visitation. The results suggest that hospital discharges were not predominantly responsible for the early outbreak in care homes, and that only a few such cases led to infection seeding in care homes by the 6th of March Sensitivity analysis show the main mode of entry into care homes are infections by staff interacting with the general population. Visitation (before cancellation) and staff sharing were less significant in affecting outbreak size. Focusing on the protection and monitoring of staff, followed by reductions in staff sharing and quick cancellations of visitation can significantly reduce future infection attack rates of COVID-19 in care homes. Author SummaryCOVID-19 has spread throughout care homes in the UK, leading to many deaths of those most vulnerable in our population. This has sparked the need for further understanding of how infectious diseases spread throughout vulnerable communities such as care homes. We developed a model focused on the first wave in the Scottish health board of Lothian, which indicated pathways most likely leading to COVID-19 establishment within care homes. We found that care home visitation and hospital discharges did not significantly affect total COVID-19 cases in care home residents. The most significant route of entry for COVID-19 into care homes was through staff infections from the general population. We suggest to prioritise minimising infections in this pathway to reduce the number of outbreaks in care homes. Our model indicated that care homes were three weeks behind the general population in reducing the reproduction rate of COVID-19. This delay emphasises the need for more planning and support for care homes in organising effective responses to emerging pandemics.