BRUSSELS—The European Union’s top official expressed regret for creating a row with Britain last month when the bloc briefly considered applying an emergency restriction on exports of COVID-19 vaccines also to the UK’s Northern Ireland. Amid a dispute with Anglo-Swedish drugmaker AstraZeneca, the EU introduced tighter rules on exports of COVID-19 vaccines that could hit shipments to nations like the United Kingdom. To implement its plan, EU officials thought about also introducing controls on exports to Northern Ireland from Ireland, which is part of the EU. That would have created a hard border. And since the Brexit deal guarantees that goods flow freely between the EU and Northern Ireland to protect the Irish peace process, the plans sparked concerns and outrage in Britain, Northern Ireland, and Ireland. Amid the outcry, the EU reversed its decision. Speaking at the European Parliament on Wednesday, European Commission president Ursula von der Leyen said …