COVID-19 | Days after grabbing headlines with its Covid-19 "vaccine for the world", AstraZeneca is facing tricky questions about its success rate that some experts say could hinder its chances of getting speedy US and EU regulatory approval.

Several scientists have raised doubts about the robustness of results showing the shot was 90 percent effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.

"All we have to go on is a limited data release," said Peter Openshaw, a professor of experimental medicine at Imperial College London. "We have to wait for the full data and to see how the regulators view the results," he said, adding that US and European regulators "might possibly take a different view" from each other...