The EU has introduced vaccine export controls that would make it possible for member states to stop exports to the UK.
There's a specific consideration for Northern Ireland, however:
"Member States should take due account of the need for supply, in Northern Ireland"

The other paragraph on Northern Ireland says, in my understanding: 'But if Britain were to try to move vaccines through Northern Ireland to get around our export controls, then we could block that' https://twitter.com/danieldschade/status/1355185013139111939?s=20

By citing Article 16 in a bid to square this circle, the Commission has provoked uproar in the North.
"An incredible act of hostility" says DUP's Arlene Foster.
UK govt spox says Michael Gove has spoken to Maros Sefcovic to express concern... https://twitter.com/DUPleader/status/1355213059183271937?s=20