BRUSSELS/LONDON—EU officials confessed on Saturday to a “blunder” in invoking Northern Ireland Brexit emergency powers during a showdown with Britain over vaccines, and London said it expected its supply of COVID-19 shots would not be interrupted. The European Union has fallen far behind Britain and the United States in the race to vaccinate its public. It announced on Friday it would impose export controls on vaccines, widely seen as a threat to prevent doses from being sent to Britain. But it was forced to reverse part of the announcement within hours, after both Britain and Ireland complained about plans to impose emergency export controls for vaccines across the land border between Ireland and British-ruled Northern Ireland. “They have recognised they have made a mistake and I believe we can now concentrate on making sure that our vaccine programme is successful,” Michael Gove, a senior British cabinet minister told Sky News. …