Younger people are experiencing severe illness and dying from COVID-19, despite perceptions that the disease is only a threat to the elderly, the World Health Organisation has warned.

Key points: The WHO says people in their 30s, 40s and 50s are becoming severely ill and dying from COVID-19

The WHO says people in their 30s, 40s and 50s are becoming severely ill and dying from COVID-19 Some have had underlying conditions, while others have not

Some have had underlying conditions, while others have not Young people are being warned they are not "invincible"

In a press briefing broadcast from Geneva, WHO experts pointed to cases in China, South Korea, Italy and other parts of Europe where people in their 30s, 40s, and 50s have died from the virus.

"We are seeing more and more younger individuals who are experiencing severe disease," epidemiologist and WHO technical lead of COVID-19 response Maria van Kerkhove said.

"Overall most of the people who are experiencing severe disease and ending up in ICU and needing advanced care are older people, and are people with underlying conditions.

"But what we are seeing in some countries is that there are some people in their 30s, who are in their 40s, who are in their 50s, who are in ICU and who have died."

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Dr van Kerkhove said some of the young people who were becoming severely ill had pre-existing conditions, while others did not.

"What we need to better understand … is why are some people progressing from moderate disease to severe disease to critical disease?" she said.

"Why are some people progressing backwards from severe disease to more moderate disease and recovering?"

WHO executive director Michael Ryan suggested the attitude that the disease was not as serious in younger people was perhaps a way of people reassuring themselves they would not be affected.

But he said the evidence that there was a spectrum of severity in younger people, even if they did tend to have a "milder course" of the disease, had "been there all along"

"What we've been saying again and again in this forum — and with our data — is even in [South] Korea where they have managed to control the disease, one in six deaths have been in people under 60," Dr Ryan said.

"In Italy over the last five to six weeks at least 10 to 15 per cent of people in intensive care have been under 50."

He said the fact the virus was not always mild in young people was another reason for them to avoid becoming infected, along with the risk of passing the infection on to more vulnerable populations.

WHO director general Dr Tedros Adhanom Ghebreyesus warned young people were not "invincible" to COVID-19.

"Older people are more vulnerable and people with underlying conditions are more vulnerable, but we have even more evidence now that it effects younger people too," Dr Tedros said.

In Australia, New South Wales Chief Medical Officer Kerry Chant confirmed on Saturday that of the 41 people being treated for COVID-19 in the state's ICU beds, four were in their 30s.

ABC/Reuters