The prime minister, who has been in self-isolation after testing positive for COVID-19 10 days ago, was taken as a “precautionary step” on the advice of his doctor.
He is understood to be in an NHS hospital in London, where he will stay for “as long as needed”.
On Friday, he said he was “feeling better”, but that he would remain in self-isolation until his temperature dropped.
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Downing Street has not revealed details about his condition but one doctor said his lungs would likely be the focus of attention and that Johnson most likely had “moderate” symptoms.
Dr Sarah Jarvis, a GP and fellow of the Royal College of General Practitioners, told the BBC it was likely the PM had “moderate” symptoms of coronavirus.
“The majority of people who develop coronavirus will have mild symptoms – that’s probably 80% or so,” she said. “There is a severe stage, a moderate stage and a critical stage.”
Dr Jarvis, who is not Johnson’s doctor, added: “Given he is staying in charge of the government, that suggests to me that he probably has moderate disease but that, as a precaution, he is being taken in to check the oxygen levels in his blood, to do X-rays and probably scans of his chest.
“And to do blood tests to see, for instance, what his white cell counts look like and what his liver functions look like.”
“The main focus is going to be on his lungs.
“The majority of people are going to progress to have inflammation of the lungs, and that inflammation can result in damage to lung tissue, but also importantly can prevent oxygen being transferred into the bloodstream.”
She said the tests themselves are very rapid.
“You can clamp a monitor on to somebody’s finger and that will give you a test very quickly. In fact, in general practice we have systems set up where people who have been discharged from hospital are having those tests delivered to them and they just pop the monitor on to their finger and literally within 30 seconds it will give a result.
“The X-ray will take a little longer and the scan will need to be assessed and looked at by a consultant – we are certainly talking a couple of hours I would say.”
Prof Derek Hill, Professor of Medical Imaging Science, Department of Medical Physics & Biomedical Engineering, UCL, said: “Many people attending hospital with Covid-19 have difficulty breathing. One test performed on these people is imaging of the lung, with ultrasound or CT scans, to get an indication of how badly their lungs are affected.
“Some people are rapidly discharged. Some others can quickly deteriorate and need help breathing. We have no reason to believe the PM needs such help. But there are various types of breathing help, depending on the breathing difficulties.
“Some patients can manage with the help of oxygen, others with more serious breathing problems end up being put on a ventilator that breathes for them. This can involve heavy sedation and sometimes the patient needs to be paralysed.”
Johnson has shared several video updates from his Number 11 flat since his diagnosis, and joined the nationwide clap for NHS staff on Thursday evening.
Another quick update from me on our campaign against #coronavirus.— Boris Johnson #StayHomeSaveLives (@BorisJohnson) April 3, 2020
You are saving lives by staying at home, so I urge you to stick with it this weekend, even if we do have some fine weather.#StayHomeSaveLives pic.twitter.com/4GHmJhxXQ0
He has not been seen publicly since, although he spoke to new Labour leader Sir Keir Starmer on Saturday afternoon.
In his social media post on Friday, Johnson looked and sounded tired as he said: “Although I’m feeling better and I’ve done my seven days of isolation, alas I still have one of the symptoms, a minor symptom: I still have a temperature.
“So, in accordance with government advice, I must continue my self-isolation until that symptom itself goes.”