Long stays in intensive care and being ventilated are thought to increase the risk, the researchers say.

Doctors should look out for depression, anxiety and post-traumatic stress disorder (PTSD) after recovery.

Most patients, particularly those with mild symptoms, will not be affected by mental health problems.

The evidence is based on studies of patients with severe acute respiratory syndrome (Sars) and Middle-East respiratory syndrome (Mers), as well early data on Covid-19 patients.

The researchers, from the UK and Italy, found evidence of confusion and agitation in more than 60% of intensive care patients with Covid in a small number of studies, mostly from China.

They warned that PTSD could become an issue in some patients, based on the fact that 33% of survivors of Sars and Mers experienced post-traumatic stress more than two years after they were seriously ill.

There were also frequent reports from these patients of recalling traumatic memories, memory problems, fatigue, insomnia and low mood, during their recovery.

But it is still not clear how the current pandemic will affect people's mental health without more research, the study concludes.

'Ultra-high risk'

"Our analysis of more than 3,550 coronavirus cases suggests that most people will not suffer from mental health problems following coronavirus infection", says Dr Jonathan Rogers from University College London, who co-led the research.

But he warned that delirium - which can cover everything from patients hallucinating and being agitated to sitting completely still - could affect some patients.

Confusion is not uncommon among patients in intensive care, he said, but it could be hitting older patients, who are already vulnerable, hardest.

Image copyright Getty Images

And the longer they stay in hospital without any contact with relatives, the worse the confusion can become.

A UK study, not yet peer-reviewed, found that around 20% of people admitted to hospital with severe Covid-19 had confusion.

"Monitoring for the development of symptoms should be a routine part of the care we provide," Dr Rogers said.

Commenting on the study, Dr Iris Sommer from the University Medical Centre, Groningen, in the Netherlands, said patients with Covid-19 who needed to be treated in ICU were "an ultra high-risk group for developing acute psychiatric disorders, especially delirium".

Unlike Sars and Mers survivors, she said, they were returning to a society "in deep economic crisis" with some countries "still in lockdown and enforcing physical isolation".

She said this would keep stress levels high after recovery and could increase the risk of anxiety and depression.