ROME, Italy — Pope Francis thanked volunteers Sunday for the “miracle of tenderness” they offer the sick, as he continued his treatment for double pneumonia and doctors reported some positive news.

After more than three weeks in the hospital, the 88-year-old pope is responding well to treatment and has shown a “gradual, slight improvement” in recent days.

Francis on Sunday followed the opening day of a weeklong spiritual retreat in the Vatican for Holy See officials by video conference. He could see the officials, but they couldn’t see him in the Gemelli hospital, the Vatican press office said. Francis also participated in Mass at the hospital’s private chapel.

For the fourth consecutive Sunday, Francis didn’t appear for his weekly noon blessing, but the Vatican distributed the text he would have delivered if he were well enough. In it, the Argentine pope thanked all those who were caring for him and others who are sick and experiencing a “night of pain.”

“Brothers and sisters, during my prolonged hospitalization here, I too experience the thoughtfulness of service and the tenderness of care, in particular from the doctors and health care workers, whom I thank from the bottom of my heart,” read the message from Gemelli hospital.

“And while I am here, I think of the many people who in various ways are close to the sick, and who are for them a sign of the Lord’s presence. We need this, the ‘miracle of tenderness’ which accompanies those who are in adversity, bringing a little light into the night of pain,” he wrote.

The pontiff, who has chronic lung disease and had part of one lung removed as a young man, has remained in stable condition, with no fever and good oxygen levels in his blood for several days, doctors reported.

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

But they kept his prognosis as “guarded,” meaning that he’s not out of danger. The Vatican said that the medical update from Saturday remained valid Sunday.

In his absence, the Vatican’s day-to-day operations continued alongside celebrations of its Holy Year, the once every quarter-century Jubilee that brings millions of pilgrims to Rome. On Sunday, Canadian Cardinal Michael Czerny, who is close to Francis, celebrated the Holy Year Mass for volunteers that Francis was supposed to have celebrated.

During the Mass in St. Peter’s Square, the giant banner bearing Francis’ papal coat of arms fluttered from the loggia of the basilica above. Even while in the hospital, Francis is very much still the pope and in charge of the Catholic Church.

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

Francis was hospitalized Feb. 14 for what was then just a bad case of bronchitis. The infection progressed into a complex respiratory tract infection and double pneumonia that has sidelined Francis for the longest period of his 12-year papacy and raised questions about the future.

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