ROME (AP) — Former Italian Premier Silvio Berlusconi is responding “optimally” to COVID-19 treatment but is the most vulnerable type of patient and is in “the most delicate phase” of the virus, his personal doctor said Sunday.

Dr. Alberto Zangrillo repeated Sunday that he nevertheless remained “cautiously optimistic” about Berlusconi’s recovery.

The 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 the San Raffaele hospital in Milan early Friday after testing positive for the virus earlier in the week. At the time he had the early stages of a lung infection.

“The patient is responding optimally to treatment," Zangrillo told reporters outside the hospital. “This doesn’t mean we can claim victory because, as you know, he belongs to the most fragile category,” given his age.

Data from Italy’s Superior Institute of Health indicates that men aged 80-87 have the highest COVID-19 death rate among all cases in Italy, at 47%.

Zangrillo suggested Berlusconi wouldn't be released anytime soon, recalling that the virus “requires adequate treatment and takes its time.”

Berlusconi spent some of his summer vacation 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 vacationed in Sardinia.

Zangrillo, Berlusconi's longtime doctor, is head of intensive care at San Raffaele. To date, Berlusconi is believed to still be in a VIP ward of the clinic.

Zangrillo has been criticized for having asserted at the end of May, when Italy's lockdown had greatly slowed its rate of new infections, that “clinically speaking the virus doesn't exist anymore." Zangrillo has since acknowledged that statement was too strong and “off-key" and was based merely on observing that fewer patients required intensive care at the time.