Disentangling the force of infection of SARS-CoV-2 in Dutch long-term care facilities

Background: During the COVID-19 pandemic, residents of long-term care facilities (LTCFs) were disproportionately affected. To inform decision-making around interventions, we quantified the SARS-CoV-2 infection risk for residents and the relative contribution of different infection sources. We estimated the force of infection (FOI) experienced by Dutch LTCF residents over time and quantified the contributions of residents, LTCF healthcare workers (HCWs), and the general population. Methods & findings: Case data were obtained by Municipal Health Services as part of the Dutch national surveillance program. During the study period (1 October 2020 to 10 November 2021), testing policies included symptom-based testing, exposure-based testing, and facility-wide serial testing. We used a data augmentation approach to include uncertainty in the timing of infection, while taking account of different testing policies. We constructed a Bayesian generalized linear model to estimate group-specific transmission rate parameters and contributions to the FOI experienced by residents. During the study period 36,877 cases were registered among residents and 19,676 among HCWs. The total daily FOI towards residents was highest in December 2020 (1.7*10⁻³, 95% CI: 1.5*10⁻³ – 1.9*10⁻³) and lowest in June 2021 (1.1*10⁻⁵ 95%CI: 7.6*10⁻⁶ – 1.7*10⁻⁵). Transmission rate parameters and FOI declined as COVID-19 vaccination rollout started in residents, HCWs, and the older general population (February-May 2021). Most resident infections in spring and summer 2021 (April to August) were attributable to infections in the general population. The relative contribution of the general population to the FOI decreased in July 2021 when vaccination was available population-wide. In October-November 2021, transmission rate parameters and FOI increased again. We observed an increase in residents’ susceptibility to infection in this period, which was only partially explained by the emergence of the Delta variant. Conclusions: Relative contributions to the FOI in LTCF residents varied substantially over time. COVID-19 vaccination appears to have been effective in reducing SARS-CoV-2 transmission towards residents, although other factors such as seasonality or non-pharmaceutical interventions may also have contributed to this. Such estimates of temporal trends in contributions to the FOI in LTCF residents can help inform future intervention efforts.