Longitudinal immune and inflammatory responses in the respiratory mucosa and blood of patients after hospitalisation with COVID-19.

COVID-19 is a disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). It was declared a global pandemic by the World Health Organization on 11th March 2020. In the UK alone there have been over 100,000 deaths to date and many of those who survive may suffer debilitating long lasting effects, termed Long COVID. COVID-19 has also had a devastating impact on economies and societies worldwide due to measures taken to control the spread of the virus, including lockdowns.

A key question which remains unanswered is whether long lasting immunity can develop following infection and how this occurs. Understanding this is critical to predicting the future course of the pandemic and to develop effective vaccines.

There are many possible ways in which the human immune system can protect against reinfection with SARS-CoV-2. Most studies have focused on immune cells and antibodies which circulate in the blood. However, by examining the blood only, we are unable to detect immune responses which may protect against the virus in other areas of the body.

In particular, very little is understood about immune responses which occur within the airways - the connecting passages between the nose, mouth and lungs. This is where the virus is able to first infect human cells and is also the main site of inflammation once infection is established. The airways are therefore likely to be key to protecting against re-infection.

The lining of the nose is easily accessible and provides a representation of the immune response in the airways. We know that for other respiratory viruses, antibodies in the blood do not necessarily provide immunity and having antibodies in the airways may be more important. In fact, we have previously shown that susceptibility to other respiratory infections can be influenced by antibody or cell activity within the nose rather than the blood.

Therefore, it is vital that we examine immune responses to COVID-19 in both the blood and nasal passages if we are to truly understand how the body creates protective immunity against the virus and how long it lasts.

A second, yet equally crucial gap in our understanding of COVID-19 is why some patients suffer from persistent symptoms of COVID-19 after hospitalization. It is essential that we investigate this in order to develop effective treatments. We now understand that severe COVID-19 occurs due to inflammation triggered by our immune system's response to the virus.

However, we do not yet understand how this inflammation changes during recovery. It is possible that symptoms of Long COVID may be explained by ongoing inflammation, despite recovery from the initial infection.

This study aims to understand whether immunity to COVID-19 is associated with long lasting antibody and immune cell responses in the airways. It also aims to understand if Long COVID is associated with persistent inflammation in the body after infection.

This is the first and largest study to longitudinally examine immune responses to COVID-19 in the airways over the course of a year. This study will develop our understanding of how immunity to COVID-19 develops and thus be critical in curbing the progression of the pandemic.