The COVID-19 pandemic, commonly referred to as "coronavirus", first began in the city of Wuhan, China in December 2019. This virus has since spread globally, with infections reported in nearly every country. COVID-19 targets the body's respiratory system, where infections can be found in the nose, throat and lungs. The effect of COVID-19 infection is very variable, where many people might not know that they have been infected and have recovered from COVID-19. However, COVID-19 infection can cause people to have difficulty breathing. This can be severe enough to require hospitalisation and potentially intensive care treatment.

While they are being treated in hospital, COVID-19 infected patients can be found to have inflamed tissue in their lungs (referred to medically as "pneumonitis"). This inflammation is thought to be caused by their body's immune systems overacting to the infection rather than the COVID-19 virus itself. By potentially dampening down this overreaction of their immune system, it is hoped that COVID-19 patients with inflamed lungs have better and quicker chance to survive.

Mesenchymal stromal cells (MSCs) have been shown to have anti-inflammatory and healing properties on injured tissue. MSCs have been trialled in various diseases but have not yet been tested on patients with COVID-19. In this study, the investigators will obtain bone marrow from healthy volunteers to develop a cell-based treatment for COVID-19-related pneumonitis. The investigators will also determine whether it is feasible to recruit bone marrow donors in a clinically useful timeframe to treat COVID-19 patients. A future trial, COMET20, will use the bone marrow-derived MSCs (BM-MSCs) manufactured in COMET20d to treat COVID-19 patients suffering with pneumonitis, to determine whether the BMMSCs can reduce the likelihood for mechanical ventilation and reduce hospitalisation.

Inclusion Criteria:

• Have given written informed consent to participate
• Be aged between 18 and 40 years old
• Have Eastern Cooperative Oncology Group (ECOG) performance status 0
• Be negative for mandatory infectious disease markers (IDM) as per World Marrow Donor Association (WMDA) guidelines
• Present a negative SARS-CoV2 screen
• Show absence of clinical symptoms of COVID-19 (Not have been in known COVID-19 contact within the previous 14 days, Adherence to national social distancing guidelines for 14 days)
• Have a BMI <35kg/m^2
• Women of childbearing potential need a negative pregnancy test (urine or blood) within 7 days prior to the marrow collection

Exclusion Criteria:

• Any major disease which would represent a contraindication to bone marrow donation based on WMDA guidelines
• Presence of any previous or active malignancy (other than non-melanoma skin cancer)
• Any other concurrent severe and/or uncontrolled medical condition
• Women who are pregnant or breast-feeding
• Any acute or chronic back complaint
• Presence of anaesthetic risk factors

Healthy adults aged 18-40 years who are negative for mandatory infectious disease markers including current COVID-19 infection.