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).

The proposed study is a prospective observational cohort design which will be carried out across three different trusts: Barts Health NHS Trust (St Bartholomew's Hospital, The Royal London Hospital, Whipps Cross Hospital and Newham Hospital), Royal Free London NHS Foundation Trust (Royal Free Hospital) and University College London Hospitals NHS Foundation Trust (UCLH).

Participants will be asymptomatic front-facing HCWs who carry out their tasks in different areas of the corresponding hospital: Accident and Emergency, Adult Medical Admissions Unit, Medical and Surgical Wards and Intensive Care Units.

This study substantially uses existing infrastructure: Recruits into this study who are subsequently suspected to have COVID-19 can be co-recruited into ISARIC using ISARIC Ethics Ref: 13/SC/0149 (Oxford C Research Ethics Committee, UK CRN /CPMS ID 14152 IRAS ID126600 for acute samples and data collection. Sampling can be delivered via existing research personnel from furloughed projects (CLRN nurses, research fellows, Barts Bioresource). Convalescent sampling will be via an otherwise inactive Clinical Trials unit. It

Inclusion Criteria:

• Age at least 18 years AND
• Asymptomatic (meaning healthy enough to attend work according to Trust policy at the time) AND
• Work in the designated clinical environments for at least 5 hours for at least one day during the study period.

Participants will be free to withdraw from the study at any point, but collection of these data is considered to be in the public interest and will fall under the scope of a 'Public task' by the GDPR definition. Under these conditions rights to erasure and data portability do not apply, and archiving and further processing for scientific research purposes is compatible with the original purpose; no further participant data from medical records will be collected.

The research team may withdraw a participant from the study in the following situations:

- No longer meets the inclusion/exclusion criteria if participants circumstance change

Healthy asymptomatic healthcare workers attending their usual place of work that can be St Bartholomew's Hospital, Royal Free London or UCLH.

Publication of results in open access journals; sharing of results in scientific databases.

24 months