Covid-19 and the associated restrictions on interaction have led to an unprecedented shock to activity and firms’ balance sheets. To assess the impact, this paper applies a new large-scale firm-level simulation model calibrated to the United Kingdom (UK). The paper specifically examines the Coronavirus Job Retention Scheme (CJRS) furlough program and a credit guarantee. The Corporate Sector Agent-Based (CAB) Model (Hillman, Barnes, Wharf and MacDonald, 2021) takes into account: heterogeneity across firms; interactions between firms across a realistic customer-supplier network; and rule-of-thumb behaviour by firms and bankruptcy constraints. The model amplifies the effect of shocks and generates substantial persistence and overshooting, as well as displaying a number of non-linearities. The CAB uses a data-rich approach based on ORBIS firm-level data and the OECD Input-Output tables. Simulations in this paper are calibrated to the observed path of UK output in 2020.