Pope Francis is responding well to treatment and has shown a “gradual, slight improvement” in recent days, the Vatican said Saturday, as he remains in the hospital with double pneumonia.

The 88-year-old pope, who has chronic lung disease and had part of one lung removed as a young man, has remained stable, with no fever and good oxygen levels in his blood for several days, doctors reported in the Vatican’s statement. However, they said his prognosis is guarded, meaning that he’s not out of danger yet.

The doctors said that such stability “as a consequence testifies to a good response to therapy.” It’s the first time doctors have reported Francis responding positively to the treatment for the complex lung infection that was diagnosed after he was hospitalized on Feb. 14.

The pontiff worked and rested during the day on Saturday, as he entered his fourth week at Rome’s Gemelli hospital with his condition stabilized following a few bouts of acute respiratory crises last week.

“In order to record these initial improvements in the coming days as well, his doctors have prudently maintained the prognosis as guarded,” the statement said.

Francis has been using high flows of supplemental oxygen to help him breathe during the day and a noninvasive mechanical ventilation mask at night.

On Thursday night, Francis’ voice was heard by the public for the first time since he was admitted to the hospital. The message, recorded from the pope’s room at the Gemelli Polyclinic hospital in Rome, was played out to those who have been praying for him in St. Peter’s Square at the Vatican.

“I thank you from the bottom of my heart for your prayers for my health from the square,” he said in the message.

That Francis was able to speak to record the message was an encouraging sign, but his frailty could be heard in the recording. Francis’ voice sounded weak and breathless.