Vatican City ? In a seismic shift on one of the most profound — and profoundly contentious — Roman Catholic teachings, the Vatican said Tuesday that condoms are the lesser of two evils when used to curb the spread of AIDS, even if their use prevents a pregnancy.

The position was an acknowledgment that the church’s long-held anti-birth control stance against condoms doesn’t justify putting lives at risk.

“This is a game-changer,” declared the Rev. James Martin, a prominent Jesuit writer and editor.

The new stance was staked out as the Vatican explained Pope Benedict XVI’s comments on condoms and HIV in a book that came out Tuesday based on his interview with a German journalist.

The Vatican still holds that condom use is immoral and that church doctrine forbidding artificial birth control remains unchanged. Still, the reassessment on condom use to help prevent disease carries profound significance, particularly in Africa where AIDS is rampant.

“By acknowledging that condoms help prevent the spread of HIV between people in sexual relationships, the pope has completely changed the Catholic discussion on condoms,” said Martin, a liberal-leaning author of several books about spirituality and Catholic teaching.

The development came on a day when U.N. AIDS officials announced that the number of new HIV cases has fallen significantly — thanks to condom use — and a U.S. medical journal published a study showing that a daily pill could help prevent spread of the virus among gay men.

“This is a great day in the fight against AIDS … a major milestone,” said Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition.

Theologians have debated for years whether it could be morally acceptable for HIV-infected people to use condoms to avoid infecting their partners. The Vatican years ago was reportedly preparing a document on the subject, but it never came out.

The groundbreaking shift, coming as it does from the deeply conservative pontiff, would appear likely to restrain any public criticism from Catholic conservatives, who insisted Tuesday that the pope was merely reaffirming the church’s moral teaching.

Conservatives have feared that a comment like this would give support to Catholics who want to challenge the church’s ban on artificial contraception in an environment where they feel they are under siege from a secular, anti-Catholic culture.