Former Italian prime minister Silvio Berlusconi is responding “optimally” to COVID-19 treatment but belongs to the most vulnerable category of patients and is in “the most delicate phase” of the virus, his doctor says.

Dr Alberto Zangrillo, who is head of intensive care at San Raffaele Hospital in Milan, has repeated that he nevertheless remains “cautiously optimistic” about Mr Berlusconi’s recovery.

“The patient is responding optimally to treatment,” he said on Sunday.

“This doesn’t mean we can claim victory because, as you know, he belongs to the most fragile category” given his age.

Dr Alberto Zangrillo explains Mr Berlusconi’s condition on Sunday. Photo: AAP

Dr Zangrillo also suggested Mr Berlusconi would not be released soon, recalling the virus “requires adequate treatment and takes its time”.

The media tycoon and three-time premier turns 84 in a few weeks and has had a history of heart problems that required being fitted with a pacemaker several years ago.

He checked into San Raffaele on Friday after testing positive for the virus earlier in the week.

Dr Zangrillo said at the time he had the early stages of a lung infection.

Mr Berlusconi spent some of his summer holiday at his seaside villa on Sardinia’s Emerald Coast.

Many of Italy’s recent cases of COVID-19 have been linked to clusters in people who have been on holiday in Sardinia.

Dr Zangrillo, Berlusconi’s long-time doctor, is head of intensive care at San Raffaele.