Britain has taken a major step toward rolling out the AstraZeneca vaccine to its population—despite questions being raised by U.S. experts about some of its trial data. Earlier this week, some scientists said they were pretty confused by 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. While the success rate was higher in the sub-group, at 90 percent, some experts said the relatively small number of participants made it hard to trust the findings. Despite that, Britain gave the vaccine a vote of confidence on Friday when it asked its regulator to assess it for a rollout. “This letter is an important step towards deploying a vaccine as quickly as safely possible,” said Health Secretary Matt Hancock, adding that the rollout should begin before Christmas.