What is COVID-19?
COVID-19 is a disease caused by the newly emerged SARS-CoV-2 virus, commonly known as coronavirus.
COVID-19 is characterised by symptoms ranging from fever and a dry cough in mild cases, to difficulty breathing and rapid respiratory failure in severe cases. There are currently no treatments for the disease or vaccines to prevent its spread.
The virus explained
SARS-CoV-2 is part of the coronavirus family. Coronaviruses cause disease in humans and other animals, typically affecting the throat, lungs and intestinal tract. Other forms of coronavirus were responsible for the SARS (Severe Acute Respiratory Syndrome) and MERS (Middle East Respiratory Syndrome) outbreaks in 2002-3 and 2012 respectively.
Although viruses in this family vary in terms of disease caused and how they pass from host to host, their basic structure is the same. Coronaviruses are made up of three key parts:
- Genetic information contained in RNA
- A viral envelope
- Spike proteins
Genetic information is stored in the virus using RNA, made up of nucleic acids similar to DNA. Viruses cannot make copies of themselves without a host cell. The viral RNA contains instructions to take over the biological machinery inside human cells, repurposing it for replication.
The viral envelope is a fatty coat that protects the RNA and provides a structure to house the spike proteins. This helps the virus to survive when travelling between host cells. The envelope is disrupted by detergents such as soap or by being dried out, which is why viruses cannot survive on hard surfaces indefinitely.
Spike proteins allow the virus to attach to host cell surface receptors and enter them, which is required for the virus to replicate. The spikes make the virus look like a crown ('corona'), giving the virus its name.
How coronavirus causes disease
Once the virus has entered a host epithelial cell in the throat or lungs, it releases RNA which is 'read' by the cell to generate new proteins, used as building blocks for new virus particles or to avoid detection by the immune system. A single cell may be hijacked to produce millions of new viruses before it eventually dies, shedding the particles into the body. These may infect other people or other cells in the same host.
Infected cells and cellular debris trigger the immune system, which is designed to fight infection. If this is successful, an infected person will only experience mild symptoms before making a full recovery. However, in some cases, more extensive infection can overwhelm the immune system. The immune response then goes into overdrive, destroying healthy cells, damaging the lungs and causing them to fill with fluid. This causes severe breathing difficulties which can result in death if not resolved.
The details of how the novel coronavirus causes disease are still emerging, which means that the research and treatment landscapes are changing rapidly.
From a clinical perspective, the disease pathway consists of three distinct phases with gradually increasing severity of illness.
Stage I - Early infection
This represents the viral replication phase, when the immune system is just starting to respond to infection.
Early infection is associated with fever and a dry cough, with some people also experiencing myalgia, early nausea and diarrhoea, or a sore throat.
The majority of people recover fully from this stage of illness.
Stage II - Pulmonary phase
As the virus replicates further and the immune system response becomes heightened, inflammatory cascades in the lungs result in shortness of breath and a lack of oxygen (hypoxia). This may show up on CT scans or X-ray images as opaque areas in the lung.
Stage III - Hyperinflammation phase
The hyperinflammation phase happens when the immune system is overwhelmed by the virus, damaging healthy tissue and failing to repair damage. At this stage assisted ventilation may be required. Numerous inflammatory markers, including signalling proteins called cytokines, are present in large numbers upon testing.
Further information on COVID-19 and coronavirus is available from UK Research & Innovation (UKRI) and the British Society for Immunology (BSI).