After days of a cautious optimism and two weeks in a hospital with pneumonia in both lungs, Pope Francis on Friday suffered another respiratory crisis, renewing concerns about the prognosis for the leader of the Roman Catholic Church.

The Vatican said on Friday night that Francis, who is 88 and has a history of respiratory ailments, suffered a bronchial spasm that caused him to inhale his vomit after a coughing fit. That, in turn, caused a “worsening of the respiratory picture,” and required aspiration.

It said that the crisis occurred early Friday afternoon, after a morning spent of respiratory physiotherapy and prayer in the chapel, and he required “noninvasive” ventilation — meaning he was not intubated, which requires sedation, and that he remained “alert and conscious at all times.”

The Vatican added that the pope’s doctors said it would take 24 to 48 hours to determine whether the crisis had worsened the pope’s condition.

Francis is now using a mask covering his mouth and nose to help him breathe, as he has at other points in stay at Gemelli Hospital in Rome. The Vatican said Francis was cooperating “with therapeutic maneuvers,” and was in “good spirits.”

The pope was admitted to the hospital with bronchitis and later, pneumonia was diagnosed. He had a respiratory crisis last Saturday that he appeared to have overcome, and medical bulletins in recent days were cautiously upbeat, though doctors had said he was not out of danger. In recent days doctors had said that Francis was no longer in “critical condition,” but that his situation was “complex.”

“It’s a setback,” Austen Ivereigh, Pope Francis’ biographer, wrote on X, adding, “I was getting too confident. Please keep up prayers.”

Others shared his concern.

“Given his age, and given that he is in such a state, we must be worried,” Father Felicien Abengalo, a priest in Rome, said on Friday morning. “His illness is not a minor illness.”

At St. Peter’s Square on Friday evening, Cardinal Víctor Manuel Fernández, the leader of the Vatican’s office on church teaching, and a close aide to the pope, led thousands of the faithful in saying the rosary to pray for Pope Francis’ health.

“It’s very emotional to be here in the square, I think, knowing that he’s taken a turn for the worse, so we are all going to unite our prayers with his and ask for God’s grace for the whole church and for Pope Francis,” said Sophie Taylor, a high school religious teacher from Durham, England.

“When anyone is sick, we have no control over that and he doesn’t have control over that, only God does,” she added. “All we can do is pray.”

People joined in prayer, framed by the vast square’s colonnades, with the soaring domed basilica brightly lighted behind rows of cardinals and clerics.

“Certainly it is important to the Holy Father that our prayer is not only for him, but also for all those who in this particular, dramatic and distressing moment in the world are bearing the weight of war, poverty and sickness,” Cardinal Fernández said at the start of the rosary. Francis “joins in that prayer,” he said.

The Rev. Jordi Pujol, a Spanish priest who works in the church communications department for the Pontifical University of the Holy Cross in Rome, pointed to a window high up on the Apostolic Palace where Pope Francis gives his Sunday Angelus blessing and prayer.

“The pope from that window there always asks us to pray for him, and I see this as our response to 12 years” of his asking, he said. “I think it’s very beautiful that people are coming spontaneously.”