Pope Francis is responding well to the treatment for double pneumonia and has shown a "gradual, slight improvement" in recent days, the Vatican said Saturday. But his doctors have decided to keep his prognosis as guarded, meaning that he's not out of danger yet.

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 a Vatican statement.

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.

Francis worked and rested during the day 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.

In his absence, the Vatican's day-to-day operations continued, with Cardinal Pietro Parolin celebrating Mass for an anti-abortion group in St. Peter's Basilica. At the start, Parolin delivered a message from the pope from the hospital on the need to protect life, from birth to natural death.

In the message, dated March 5 and addressed to the Movement for Life, which seeks to provide women with alternatives to abortion, Francis encouraged the faithful to promote anti-abortion activities not just for the unborn, but "for the elderly, no longer independent or the incurably ill."

Later Saturday, another cardinal closely associated with Francis' papacy, Canadian Cardinal Michael Czerny, presides over the nightly recitation of prayers for Francis. Czerny then returns Sunday to celebrate the Holy Year Mass for volunteers that Francis was supposed to have celebrated.

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