Boris Johnson is in intensive care after his COVID-19 symptoms worsened. Here's what research shows so far about outcomes in the most severe coronavirus cases.

• UK Prime Minister Boris Johnson was moved to an intensive care unit after his coronavirus symptoms worsened on Monday.
• Johnson's spokesman said he was admitted to the hospital on Sunday for "persistent coronavirus symptoms."
• Research that has tracked clinical outcomes for ICU patients doesn't yet give a complete picture of outcomes for the most severe coronavirus cases.
• But in the short term, studies show that most coronavirus patients that enter the ICU do not get discharged from the hospital within the first few weeks.
• One study found that 15% of people admitted to the ICU in the UK for COVID-19 died.
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UK Prime Minister Boris Johnson has been moved into an intensive-care unit after his condition deteriorated on Monday. Johnson was admitted to the hospital on Sunday for "persistent coronavirus symptoms," then put in intensive care at around 7 p.m. local time on Monday evening, his spokesman said.

While 80% of COVID-19 cases are considered mild (meaning patients do not require hospitalization), severe cases can involve pneumonialike symptoms and respiratory failure. In an intensive care unit (ICU), some patients are placed on a ventilator that enables them to breathe. Johnson's spokesman said the 55-year-old prime minister is not on a ventilator, and said claims suggesting he was were "disinformation."

A study from US Centers for Disease Control and Prevention found that coronavirus patients were admitted to the ICU in 4.9% to 11.5% of the 2,449 cases analyzed. About 53% of ICU admissions between February 12 and March 16 were patients older than 65.

Here's what research has found so far about the outcomes for patients admitted to the ICU.

Preliminary studies ended too soon to know longer-term outcomes for ICU patients

Several studies have collected data about patient outcomes in the ICU, but most lasted only a few weeks - and at that point, a majority of the ICU patients in question were still in critical care.

So existing research isn't sufficient to draw conclusions about what happens to ICU patients in the longer term. But in the short term, scientists have found that very few ICU patients get discharged from the hospital within the first few weeks.

One study of 2,249 coronavirus patients admitted to critical care in the UK as of April 3 revealed that 346 of those patients - 15% - died in the ICU. Another 15% of the critical-care patients were discharged, and the remaining 70%, 1,559 patients, were still in critical care as of April 4 when the research ended.

A smaller study looked at 24 patients with COVID-19 who were admitted to the ICU in nine Seattle-area hospitals between February 24 and March 9. The findings showed that the average length of an ICU stay among surviving patients was 14 days.

Half of the patients died after one to 18 days in intensive care. Four of those 12 patients had a do-not-resuscitate order.

"Mortality among these critically ill patients was high," the authors concluded.

Five of the surviving 12 patients were discharged from the hospital, four were discharged from the ICU but remained in the hospital, and three remained in the ICU on ventilators as of March 23.

That data is similar to a study published March 19 in the Journal of the American Medical Association, which looked at 21 patients admitted to the ICU at Washington state's Evergreen Hospital between February 20 and March 5.

As of March 17, 67% of those patients had died, and 24% of patients remained critically ill at the end of the study. Only two patients had been discharged from the ICU.

The trend held true in China, too: Of the 41 coronavirus patients first identified in Wuhan - where the coronavirus pandemic originated in December - 13 were admitted to the ICU by January 4, according to a study published in the journal The Lancet. Five of those ICU patients died by January 22, while seven were discharged from the hospital. The remaining patient was still in intensive care when the research finished.

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