It is unlikely the prime minister will have been admitted unless doctors have real concerns

Most people recover from Covid-19 within a week and cannot even be certain they had it, as they probably won’t be tested. The advice is to stay home, rest and take paracetamol. In 80% of cases, that is the end of it.

But NHS advice is that if the symptoms – mainly the dry cough, temperature and fatigue – have not gone by the end of a week, or they get worse, people should seek medical help.

Unlike Matt Hancock, the health secretary, who revealed he had Covid-19 on the same day as the prime minister, Boris Johnson has not recovered within the first week. He is said to have been admitted to hospital for tests, which may include scans of his lungs to check for pneumonia, as well as blood tests. He had a diagnostic test for Covid-19, so doctors will be looking for progression of the disease and to establish that he has not entered the second phase, where the immune system goes into overdrive.

Given the increasing pressure on hospitals at the moment, it is unlikely he will have been admitted unless doctors have real concerns. Minor tests could be carried out in Downing Street.

PM's Covid-19 timeline: from 'mild symptoms' to hospital admission Read more

Early guidelines from the National Institute for Health and Care Excellence (Nice) suggest the following symptoms may help a doctor to decide whether a patient with Covid-19 should go to hospital:

Severe shortness of breath at rest or difficulty breathing.

Coughing up blood.

Blue lips or face.

Feeling cold and clammy with pale or mottled skin.

Collapse or fainting (syncope).

New confusion.

Becoming difficult to rouse.

Little or no urine output.

In the first week, people who are fit and healthy, with a robust immune system, will usually fight off the virus. But the problems come for some people in the second week, when their immune system overreacts to the virus and ends up attacking the body’s own organs. That is why the most seriously ill can end up on life support machines with organ failure.

Chinese data showed that 20% of patients went to hospital. Some 15% had severe disease, which involved breathing difficulties and hypoxia, where some of the tissues of the body are not getting an adequate oxygen supply. That can manifest in anxiety, confusion and restlessness.

Only 5% ended up in critical care, with such severe illness that they needed organ support. Ventilators can take over the patient’s breathing, to allow the lungs time to recover. But patients could also need support for their heart, liver and kidneys – although many of those needing that sort of mechanical help have underlying conditions which make them particularly vulnerable.