"I'm sorry, but I can't go on. Merry Christmas and God bless," the 75- year-old Pontiff said hurriedly as he retired inside to be sick. Twenty minutes later he reappeared, to applause from the crowd, but was unable to continue with the greeting he normally reads out in more than 50 languages.
A Vatican spokesman insisted that His Holiness was suffering from no more than a nasty bout of flu, but the episode highlighted the frailty of a man who has undergone major surgery twice in the last three years and lost much of the driving energy that has characterised his 17 years as Pope.
It was the first time since his election in 1978 that John Paul had cut short his heavy Christmas programme of services and addresses. He had appeared to be in fine form until Midnight Mass on Christmas Eve, when he gradually turned pale in the heat of St Peter's, filled with around 12,000 worshippers. When he returned to the sacristy, his vestments were bathed in sweat.
By yesterday morning, he was running a temperature and had to bow out of Christmas Mass, which was taken instead by the vicar-general of Vatican City, Cardinal Virgilio Noe. Traditionally, the Pope gives his Urbi et Orbi address, to the city of Rome and the world, from the balcony of St Peter's. Under the circumstances, however, he decided to deliver it from his private apartments.
He had just given his Christmas greeting in Italian and French when he was forced to stop. "Even the Pope has his weaknesses, but I'm trying to resist," he said.
This time last year, the Vatican was abuzz with speculation about the health of the Pope and possible candidates for the succession. Following a painful recovery from hip-replacement surgery, he looked pale and weak in public and walked only with the help of a cane.
In 1995 he has remained subdued, but much stronger, allowing Vatican insiders to predict with some confidence that he will fulfil his dream of seeing the Catholic Church through the millennium.
Reaction in Rome yesterday was noticeably free of panic. "There's been a bug going round and several of my friends have had temperatures of 38 or 40 degrees," said Paolo Vannini, a physicist with connections in the Church. "Why should the Vatican walls offer any special protection?"
The Pope himself was expected to spend the rest of the year at his country residence, Castel Gandolfo, where he has recuperated from illnesses in the past. As for his Christ- mas message and greeting, the Vatican announced they would be available for the first time on the Internet - address code http://www.vatican.va.