Vatican City - Pope Francis sat out mass and his traditional Angelus prayer from hospital on Sunday, with a Vatican health official saying the 88-year-old pontiff needed to preserve his strength for the Church's busy Jubilee year.

Francis was admitted Friday to Rome's Gemelli hospital with bronchitis, one of several bouts of flu or bronchitis he has caught in recent winters.

The Vatican said Saturday that doctors prescribed "absolute rest," although they said tests had indicated some improvements in his condition.

Unable to attend Sunday mass at St. Peter's Basilica, the pope sent a written address to a group of artists and other cultural figures in attendance.

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"I would have liked to be among you but, as you know, I am here at the Gemelli Hospital because I still need some treatment for my bronchitis," Francis wrote.

During the mass – which is usually presided over by the pope – Cardinal Jose Tolentino de Mendonca read Francis' homily.

"We live in a time when new walls are being erected, when differences become a pretext for division rather than an opportunity for mutual enrichment," the cardinal read. "But you, men and women of the world of culture, are called to build bridges."

The head of the health office for the Italian Bishops' Conference, Massimo Angelelli, told AFPTV it was "absolutely necessary for him to recover his strength" as "the Jubilee is still long."

The Catholic Church has designated 2025 a "Jubilee Year" that is expected to draw more than 30 million visitors to Rome.

Occurring every 25 years, the Jubilee is intended as a period of reflection and penance, and is marked by a long list of cultural and religious events – many of them presided over by the pope.

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Despite his health issues – which include knee pain and his reliance on a wheelchair – the pope keeps a very busy schedule and has said he has no current plans to slow down.

But in the days ahead of his hospitalization, Francis – who had part of one of his lungs removed as a young man – several times asked aides to read his public speeches aloud on his behalf.

On Saturday, the Vatican said in a statement that tests had confirmed the pope was suffering from a "respiratory tract infection" but was not exhibiting signs of fever, and showed "improvement in some values."