But because researchers had used cell lines from a fetus aborted in 1972, the vaccine could present an ethical dilemma for Catholics and others opposed to abortion, said Helen Watt, a senior research fellow with the Anscombe Bioethics Centre, Oxford.

In a briefing paper issued April 27, she said the dilemma is posed by there being “no absolute duty” to boycott such a vaccine and it was not always wrong to use vaccines produced via cell lines derived from aborted fetuses.

"Boycotting a COVID-19 vaccine in the absence of an alternative is a serious action that should be carefully considered because of its potentially grave risks both for the person and for others," she said in the paper published on the website of the center, which serves the Catholic Church in the United Kingdom and Ireland.

She said its use was a matter of individual conscience for Catholics, although they should strive to obtain alternative vaccines, made without fetal cells, once such vaccines arrive on the market.

"Boycotts are often rightly regarded simply as a means of achieving change by highlighting abuses," she added, though "some will feel, whether rightly or wrongly, called to a boycott even if no alternative vaccine is available to them."

When judging the researchers, Watt continued, it was important to remember that "we walk in Rome on paving laid by slaves ... we live in countries that our ancestors unjustly invaded."