On June 5, researchers at the University of Oxford quietly made a change to a late-stage clinical trial of their Covid-19 vaccine. In an amendment noted in a document marked ‘confidential’, they said they were adding a new group of participants.

The adjustment might seem minor in a large-scale study. But it masked a mistake that would have potentially far-reaching consequences: Many of the United Kingdom trial subjects had inadvertently been given only about a half dose of the vaccine...