LONDON/BRUSSELS—The European Union on Friday reversed a plan to use emergency Brexit measures to restrict exports of COVID-19 vaccines from crossing the Irish border into the United Kingdom after it sent shockwaves through Northern Ireland, London, and Dublin. In a steep escalation of the EU’s fight to secure vaccine supplies, Brussels had said it would trigger clauses in the Northern Irish Protocol to prevent the vaccines from moving across the open border between EU-member Ireland and the British-run province. Following an outcry in London, Belfast, and Dublin, the EU published a statement just before midnight saying it would ensure that the Northern Ireland Protocol, designed to keep the border open, would not be affected. It warned, however, that should vaccines and active substances move toward third countries and out of the bloc, it would use “all the instruments at its disposal”. Ireland said the EU’s change of heart was welcome …