BBC Comment:
Analysis by Emma Vardy, Ireland correspondent
The EU's actions are seen as an audacious move. Unionist politicians are incredulous.
Throughout the Brexit process the EU vehemently insisted there could be no border on the island of Ireland. For a while the entire trade deal hinged on this.
But now, to critics, it appears that principle has vanished over vaccines.
It begs the question, if the EU can decide to push this button on this, what else might they use it for in future?
Those who strongly oppose the Brexit arrangements in Northern Ireland will be pushing even harder now for the UK to use Article 16 to override parts of the deal for its own purposes, too.