The Vatican has played down the importance of Pope Benedict's remarks appearing to temper the opposition of the Roman Catholic Church to condoms.
The Vatican spokesman said the pontiff's comments were not "revolutionary", but added it was the first time Pope Benedict had commented on the issue informally.
The Pope made clear in his view condoms were no answer to the Aids pandemic.
But he said their use could sometimes be justified in exceptional cases.
Vatican spokesman Fr Federico Lombardi said the Pope was speaking about "an exceptional situation" in one of the interviews in the book Light of the World: The Pope, the Church and the Signs of the Times, which is being published on Tuesday.
"The Pope considered an exceptional situation in which the exercise of sexuality is a real danger to the life of another," said Fr Lombardi.
Benedict used the specific example of a male prostitute using a condom to illustrate his apparent shift in position.
"The Pope maintains that condom use to lessen the danger of infection is a 'first assumption of responsibility,'" said Fr Lombardi, quoting from the book.
"In this, the reasoning of the Pope certainly cannot be defined as a revolutionary breakthrough."
The Vatican has long opposed condoms as an artificial form of contraception.
This had drawn heavy criticism, particularly from Aids campaigners, who said condoms were one of the few methods proven to stop the spread of HIV.
The head of the United Nations Aids agency, Michael Sidibe, said the Pope's words were a significant step forward.
They were also welcomed by the Save the Children charity, although a spokesman said the Catholic Church needed to go further in supporting condom use for preventing the spread of Aids.
The new book is based on a series of interviews the Pope gave German Catholic journalist, Peter Seewald, earlier this year.
The Vatican newspaper, L'Osservatore Romano, published excerpts of the interview in its Saturday edition.