The vaccine being developed with Oxford University was 90% effective when a half-dose was given before a full-dose booster, the partners said on Monday. Most of the people in the trial received a placebo or the regimen of two full doses, which was 62% effective. However, that regime was administered to participants in a group whose age was capped at 55, Warp Speed’s Moncef Slaoui said. Researchers have been wondering why a smaller dose of the vaccine might have appeared to be more effective than a larger one.