The Pope has admitted he is unsure as to why some Catholic cardinals are reluctant to receive the Covid-19 vaccine.
He made the comments while speaking to reporters onboard the Papal plane as it flew from Slovakia to Italy.
The Pope said one cardinal had spent time in intensive care because of the virus.
"Even in the College of Cardinals there are some vaccine negationists," he said, according to the BBC. "But one of them, poor thing, has been hospitalised with the virus. These are the ironies of life."
In other comments, he said the hesitancy among cardinals was "a bit strange because humanity has a history of friendship with vaccines".
"As children [we were vaccinated] for measles, polio - all the children were vaccinated and no one said anything," he said.
The Pope himself has been vaccinated against Covid. He said that almost everyone in the Vatican had now received the jab.
Reflecting on the public debate surrounding the Covid-19 vaccine, he said there was a need to "clarify things and speak calmly".
Some people have refrained from having the vaccine on religious or pro-life grounds because of the use of cell lines from aborted fetuses in the development and testing stages.
Last year, the Congregation for the Doctrine of the Faith issued a statement saying that it was "morally acceptable" to take vaccines made in this way "when ethically irreproachable Covid-19 vaccines are not available".
In the UK, the Catholic Bishops Conference of England and Wales has encouraged people to get vaccinated.
"Each of us has a duty to protect others from infection with its danger of serious illness, and for some, death. A vaccine is the most effective way to achieve this unless one decides to self-isolate," the bishops said.
The US Conference of Catholic Bishops' Committee on Doctrine issued a similar statement saying that being vaccinated "can be an act of charity that serves the common good," while adding that it would continue to insist that pharmaceutical companies stop using abortion-derived cell lines.