By Crispian Balmer and Alvise Armellini

VATICAN CITY (Reuters) -Three weeks to the day after being admitted to Rome's Gemelli hospital, Pope Francis is still struggling to shake off the double pneumonia that has battered his already fragile health.

The 88-year-old pontiff has not been seen in public since entering hospital, and the Vatican has given no indication of when he might come out, or when he could resume anything like his usually demanding work schedule.

The road to recovery is likely to be long and fraught with danger, said medical experts who are not involved in his care and were speaking in general terms about his condition.

"I've seen patients who have spent months in hospital in this sort of situation. Of course he can recover, but the chances for a negative outcome are high," said Professor Christoph Lange, Secretary General of the International Union Against Tuberculosis and Lung Disease.

"The best advice would be to let him rest, let him take whatever time he needs to get well again," he told Reuters.

Underscoring his diminished condition, the pope released a brief audio message on Thursday night giving thanks to all those praying for him. His voice was broken, breathless and hard to understand.

The Vatican has released daily updates from the pope's medical team, breaking previous Church taboos by providing a detailed, real-time assessment of the pontiff's health.

In what was seen as a positive development, his doctors said on Thursday that given his clinical stability, they would provide their next bulletin on Saturday, skipping Friday.

However, they continued to caution that his overall prognosis remained guarded, as they have each day since February 22, meaning he is not yet out of danger.

Loredana Sarmati, professor of infectious diseases at Rome's Tor Vergata University, said it would be possible to start talking about recovery only once the guarded prognosis was lifted.

"Pneumonia of this magnitude is challenging even for a young person. Here we are talking about an elderly man and above all a man who already in previous months had respiratory problems," she said. "Pneumonia can take weeks and weeks to heal."

The current illness is the most serious of several respiratory ailments the pope has had in recent years. He had part of a lung removed as a young man after suffering pleurisy.

UPS AND DOWNS

The Vatican has said the pope has suffered four attacks of breathlessness since February 22. The last was on March 3, caused by a significant accumulation of mucus in his lungs, suggesting he did not have the muscle strength to expel the secretions.

Doctors observing the situation drew comfort from the fact he has not had a repeat episode since Monday. Likewise, they said it was reassuring he has not had any reported fever since February 14 and that, according to the Vatican, his blood tests have remained stable.

However, the fact that his overall condition was stationary after three weeks of intensive treatment, including antibiotics, was not reassuring, said a senior physician who recently retired from Gemelli and declined to give his name because of his ties to the hospital that is caring for the pope.

The physician, who has not seen Francis' medical record, said doctors would normally hope to see an improvement after so many days and added that each breathing crisis would have put a huge strain on the pope's heart. The Vatican has repeatedly said parameters measuring his circulatory system remain stable.

Francis has been receiving almost constant supplemental oxygen for the past two weeks, either through a breathing mask or a small hose under his nose.

If his condition did deteriorate, the next step could involve sedating him and placing an oxygen tube into his throat.

However, this would be a last resort for such an elderly man, medical experts said. It would also need the pope's authorisation, raising questions about what level of treatment he would be prepared to accept.

It is not known if Francis has given any instructions about his future care. In a message to the World Medical Association in 2017 he addressed end-of-life issues, saying it was morally legitimate to withdraw "overzealous treatment".

The Catholic Church teaches that it is obligatory to give patients life-saving care, but it also allows for a patient or their carers to reject "burdensome" or "disproportionate" procedures to keep a terminally ill person alive.

PHYSIOTHERAPY

As a step towards his rehabilitation, the Vatican has said the pope has been getting respiratory therapy to improve his lung function since at least February 26. He is also receiving physical therapy to try to overcome the effects of his longest hospital stay since he became pope 12 years ago.

"It is a race against time for the loss of muscle structure," said Lange.

While the Vatican says Francis has spent long spells in an armchair, it is not clear if he can walk or stand unaided.

If and when he is able to return to the Vatican, doctors cautioned he might never regain his former strength.

"He can be healed, but it is possible that some damage will remain in the lungs, so maybe there won't be a 100% recovery," said Carlo Vancheri, professor of respiratory diseases at the University of Catania in Sicily.

(Reporting by Crispian Balmer, Alvise Armellini and Emilio ParodiEditing by Frances Kerry)