Boris Johnson's move to intensive care means he could need a ventilator. (PA Images)

Boris Johnson is “extremely sick” and is likely to need a ventilator after his coronavirus symptoms worsened, an expert has said.

Derek Hill, professor of medical imaging at University College London, has outlined what sort of treatment medics have available to treat the prime minister, who was admitted to hospital on Sunday with his coronavirus symptoms persisting.

He was moved to the critical unit at St Thomas’ Hospital on Monday evening, leaving Dominic Raab to deputise in his place. Johnson was taken there in case he requires ventilation.

Prof Hill said the prime minister could be given a breathing aid called continuous positive airway pressure, which sends air and oxygen into the mouth at a steady rate, boosting the amount of oxygen entering the lungs.

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It bridges the gap between an oxygen mask and full ventilation, but Prof Hill said many COVID-19 patients “progress to invasive ventilation”, which is used for people who develop such a severe illness that they struggle to, or cannot, breathe for themselves.

The patient is heavily sedated and a tube is guided through the mouth into the windpipe. They can be fed through a tube which goes into the stomach through their nose.

Prof Hill said: “One of the features of COVID-19 in all countries seems to be that many more men become seriously ill than women – especially in the over 40 age group.

“Also we know that people under about 60 seem to have a higher chance of making a recovery from critical illness with COVID-19 than older people.

Foreign secretary Dominic Raab is deputising for Boris Johnson while he is in intensive care. (PA Images)

“But there is no doubt this turn of events means Boris Johnson is extremely sick.”

He said it outlined three “important healthcare needs” of COVID-19 – the shortage of mechanical ventilators, especially high quality intensive care models, the amount of oxygen needed by patients to help them breathe, which could potentially become short in supply, and the skilled staff needed to look after people who need intensive care.

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The prime minister has been inundated with messages of support as he continues to be treated for the illness.

The World Health Organization says about 14% of patients with COVID-19 will become so ill they need oxygen therapy, with 5% needing intensive care.

Read more: Boris Johnson in intensive care

Doctors have said Johnson is likely to have undergone several tests to check on his heart, lungs, oxygen levels, white blood cell count and liver and kidney functions.

Data from the Intensive Care National Audit and Research Centre has found the death rate of people suffering from COVID-19 who are admitted to intensive care in the UK is just above than 50%.

Its sample is based on 2,249 patients, of which 690 have known outcomes. 346 died will 344 were discharged.

The rest remain in critical care.

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