Modelling repurposed from pandemic influenza is currently informing all strategies for SARS-CoV-2 and the disease COVID-19. A customized disease specific understanding will be important to understand subsequent disease waves, vaccine development and therapeutics. For this reason, ISARIC (the International Severe Acute Respiratory and Emerging Infection Consortium) was set up in advance. This focuses on hospitalised and convalescent serum samples to understand severe illness and associated immune response. However, many subjects are seroconverting with mild or even subclinical disease. Information is needed about subclinical infection, the significance of baseline immune status and the earliest immune changes that may occur in mild disease to compare with those of SARS-CoV-2. There is also a need to understand the vulnerability and response to COVID-19 of the NHS workforce of healthcare workers (HCWs). HCW present a cohort with likely higher exposure and seroconversion rates than the general population, but who can be followed up with potential for serial testing enabling an insight into early disease and markers of risk for disease severity. We have set up "COVID-19: Healthcare worker Bioresource: Immune Protection and Pathogenesis in SARS-CoV-2". This urgent fieldwork aims to secure significant (n=400) sampling of healthcare workers (demographics, swabs, blood sampling) at baseline, and weekly whilst they are well and attending work, with acute sampling (if hospitalised, via ISARIC, if their admission hospital is part of the ISARIC network) and convalescent samples post illness. These will be used to address specific questions around the impact of baseline immune function, the earliest immune responses to infection, and the biology of those who get non-hospitalized disease for local research and as a national resource. The proposal links directly with other ongoing ISARIC and community COVID projects sampling in children and the older age population. Reasonable estimates suggest the usable window for baseline sampling of NHS HCW is closing fast (e.g. baseline sampling within 3 weeks).