AstraZeneca and the University of Oxford, front-runners in the quest to deliver a Covid-19 vaccine, are facing questions about their trial results after acknowledging a manufacturing error. On Wednesday, Astra Zeneca and Oxford acknowledged a manufacturing error that impacted the preliminary results of their experimental Covid-19 vaccine. Astra Zeneca and Oxford said their vaccine was 90% effective when a half-dose was given before a full-dose booster, and that two full doses showed an efficacy of 62%. Combined, the drugmakers said the vaccine appeared to be 70% effective. The error comes days after the company and the university described the shots as highly effective.