INTRODUCTIONMany care homes across the globe implemented strict “no visitor” and/or cohorting policies and curtailed group activities as part of their infection prevention and control strategies. Although there have been several modelling studies of the impacts of non-pharmaceutical interventions on COVID-19 epidemics, few have examined shielding [1van Bunnik BA Morgan AL Bessell P et al.Segmentation and shielding of the most vulnerable members of the population as elements of an exit strategy from COVID-19 lockdown., 2Van Zandvoort K Jarvis C Pearson C. Response strategies for COVID-19 epidemics in African settings: a mathematical modelling study., 3Evaluation of" stratify and shield" as a policy option for ending the COVID-19 lockdown in the UK., 4Neufeld Z Khataee H Czirok A. Targeted adaptive isolation strategy for Covid-19 pandemic., 5Weitz J Beckett S Coenen A Demory D Dominguez-Mirazo M Dushoff J. Intervention Serology and Interaction Substitution: Modeling the Role of'Shield Immunity'in Reducing COVID-19 Epidemic Spread. medRxiv.]. These studies have modelled shielding strategies targeting vulnerable groups in the general population and provided different views on how such strategies could be ended. None of them have explicitly considered shielding care home residents to our knowledge.Although visitation restrictions to shield residents have been suggested as an intervention to partially prevent the introduction of COVID-19 into care homes, experts and advocates are increasingly concerned that such practice may cause substantial unintended harms to the health and wellbeing of residents [6Stall NM Johnstone J McGeer AJ Dhuper M Dunning J Sinha SK. Finding the Right Balance: An Evidence-Informed Guidance Document to Support the Re-Opening of Canadian Nursing Homes to Family Caregivers and Visitors during the COVID-19 Pandemic.]. A recent survey conducted in English care homes reported that the deprivation of visitation from and physical contact with loved ones have predominantly contributed to lowering residents’ mood, exacerbating irritability, agitation, and anxiety among residents and the symptoms of their dermentia, and reducing oral intake [7Learning from the impacts of COVID-19 on care homes: a pilot survey.]. A more sustainable and balanced approach that both allows needed contact with family visitors, but also prevents the introduction and spread of COVID-19 in care homes may be needed. Understanding to what extent these visiting policy interventions protect residents is important to inform decisions about how to balance the risk of COVID-19 and care home residents’ well-being.Cohorting is considered a common and effective infection control measure in acute care settings such as hospitals and some studies showed the association between the presence of an outbreak and the care home resident population [8Lee T Jordan NN Sanchez JL Gaydos JC. Selected nonvaccine interventions to prevent infectious acute respiratory disease., 9Burton JK Bayne G Evans C et al.Evolution and impact of COVID-19 outbreaks in care homes: population analysis in 189 care homes in one geographic region., 10Abrams HR Loomer L Gandhi A Grabowski DC. Characteristics of U.S. Nursing Homes with COVID-19 Cases.]. However, the impact of this intervention in care home has not been well studied. As healthcare systems are likely to bear additional costs for staffing, equipment, and support to implement cohorting in care homes, evaluating the effectiveness of this intervention is important.

To address these issues, we developed an agent-based model to investigate the impacts of visitation and cohorting policies as well as care home population size upon the transmission dynamics of COVID-19 in care homes. The model simulates the transmission dynamics of COVID-19 via contacts between individuals, including residents, staff members, and visitors.

DISCUSSIONS

This study proposes an agent-based model to study halting or restricting visitation and cohorting in care homes in response to COVID-19, interventions included in the UK national guidance and implemented in numerous care homes across the world. These intensive interventions have led to growing concerns about their negative impacts upon the well-being of residents and burdens to healthcare systems. However, the effectiveness of these intervention strategies has not been well investigated. Our modelling study helped address this gap of understanding the effectiveness of visitation and cohorting policies in controlling the ingress of COVID-19 into, and its spread in, this setting.

When the community infection prevalence where staff live is above approximately one-third of the prevalence where visitors come from, reducing the number of visitors allowed had little impact on the ingress of COVID-19 into, and its spread in, the care home. Residents can still acquire the infection from staff members who interact with several other individuals in the care home and are likely to spread the virus, which affects the likelihood and size of an outbreak more than the effect of the visiting policy. Current evidence from care homes in England has highlighted that staff, particularly bank and agency staff, have been an unwitting source of infection [23Hodgson K Grimm F Vestesson E Brine R Deeny S. Briefing: Adult social care and COVID-19.,24Ladhani SN Chow JY Janarthanan R et al.Increased risk of SARS-CoV-2 infection in staff working across different care homes: enhanced CoVID-19 outbreak investigations in London care Homes.]. If indeed staff live near the care home and provided local transmission is not very low compared to the rest of the population, the finding suggests that care homes can relax their visitation policy to a level for which they are able to ensure that all visitors strictly adhere to infection control measures. An early warning system that estimates the relative community prevalence of COVID-19 in a local area and the whole region/country could help care homes decide when they should halt visitation to protect their residents and staff.Our findings suggest that shielding residents in care homes will not be as effective as reported in a number of studies, which have considered shielding vulnerable populations more broadly [1van Bunnik BA Morgan AL Bessell P et al.Segmentation and shielding of the most vulnerable members of the population as elements of an exit strategy from COVID-19 lockdown., 2Van Zandvoort K Jarvis C Pearson C. Response strategies for COVID-19 epidemics in African settings: a mathematical modelling study., 3Evaluation of" stratify and shield" as a policy option for ending the COVID-19 lockdown in the UK., 4Neufeld Z Khataee H Czirok A. Targeted adaptive isolation strategy for Covid-19 pandemic., 5Weitz J Beckett S Coenen A Demory D Dominguez-Mirazo M Dushoff J. Intervention Serology and Interaction Substitution: Modeling the Role of'Shield Immunity'in Reducing COVID-19 Epidemic Spread. medRxiv.]. These studies used age-stratified compartmental meta-population models that assume homogeneous mixing within a compartment. Although such models incorporated different transmission rates between compartments representing age-specific populations or shielders/non-shielders, they did not account for contact patterns at an individual level that we accounted for in our model. In particular, if staff and visitors could introduce COVID-19 into a care home in equal probabilities (i.e. equal prevalence in the communities where staff and visitors live and the same probability of infection per contact), staff are more likely to spread the virus than visitors. Staff come into contact with several residents and other staff members. Therefore, they can acquire the infection from an individual in the care home and transmit it to another, further spreading the virus. By contrast, visitors are less likely to mediate transmissions between residents as they only interact with a very limited number of staff and residents (e.g., a resident whom they come to visit and staff members looking after this resident). Thus, shielding by stopping visiting is not very effective in most circumstances as long as staff and their close contacts outside the care home are not also shielded from the community, which seems unlikely. We did not investigate the effect of shielding care home residents from visitors on the spread of COVID-19 in the community while other models examined the effects of shielding interventions on the overall population. There may be a risk that visitors can acquire COVID-19 from staff and residents in care homes and spread it to others in the community. Furthermore, while vulnerable groups in other models were shielded from the rest of the population, our model only considered shielding residents from visitors.The modelling results on the risk of outbreak occurrence in care homes with different population sizes aligned with the reported data in Scottish care homes. US data also indicated significant associations between the presence of an outbreak and care home size [10Abrams HR Loomer L Gandhi A Grabowski DC. Characteristics of U.S. Nursing Homes with COVID-19 Cases.]. As the number of staff members and visitors are generally proportional to the number of residents in a care home, the likelihood that COVID-19 is introduced into the facility by these individuals increases as its size increases. Moreover, in care homes with different capacities but similar structures (i.e. same number of units, staff pooling systems, and residents-to-staff ratio), an individual can come into contact with a greater number of other different individuals, leading to a higher probability of interacting with an infected individual and, therefore, acquiring the infection.

Although cohorting of residents and staff did not affect either the elapsed time until the first resident is infected or the risk of outbreak occurrence, this intervention reduced the impact of an outbreak once it occurs. This is because the number of staff members and visitors who can introduce the virus into the facility was the same for all cohorting scenarios. Cohorting reduced the probability of having an outbreak in each unit but the overall probability for the entire facility did not decline (i.e. when an outbreak occurs in at least one of its cohorts). Nevertheless, cohorting disrupted the spread of COVID-19 and reduced the extent of an outbreak as infected individuals came into contact with fewer other individuals, and mostly ones from within their cohort.

Although care home size cannot be altered without losing places for existing and potential residents, cohorting residents and staff into smaller, discrete units could potentially alleviate the extent of an outbreak once it occurs. The cohorting intervention is more impactful in circumstances when the risk of transmission per contact is high, such as when PPE provision is inadequate, compliance to hand hygiene and wearing PPE is low, and/or maintaining social distancing is difficult. Reshaping the structure of care homes, however, requires the care home's efforts to recruit and train additional staff as well as outside support to accommodate sufficient levels of staff within each unit to maintain safe care. Staff illness and absence during COVID-19 outbreaks could further complicate the cohorting situation.

The study is subject to a number of limitations. We have not incorporated changes in individuals’ behaviours as a result of implementing the shielding and/or cohorting interventions into the model. Therefore, we have not captured how such changes would affect the outcomes. As the changes in behaviour in the presence of interventions and the relationships between behavioural changes and risks of transmission are difficult to predict [25Jarvis CI Van Zandvoort K Gimma A et al.Quantifying the impact of physical distance measures on the transmission of COVID-19 in the UK.], it is essential to continue to closely monitor outbreaks in care homes. Furthermore, as our model has assumed that visitors only come into contact with the resident whom they visit and do not interact with other residents, the effect of loosening visiting policy may be underestimated. However, relaxing this assumption will lead to the same impact as increasing the number of visitors allowed. Also, interactions between visitors and residents other than the one whom they visit are unlikely to happen amidst the ongoing pandemic.