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 to the UK’s Northern Ireland.

European Commission President Ursula von der Leyen, center left, and European Commissioner for Health Stella Kyriakides, center right, pass by the media as they arrive for a debate on the united EU approach to COVID-19 vaccinations at the European Parliament in Brussels, on Feb. 10, 2021. Francisco Seco/AP Photo