The European Union is said to be set for a showdown with the United Kingdom over a stockpile of millions of AstraZeneca vaccines that are coveted by both sides.
Much has been made of a potential ban on vaccine exports in the bloc of 27 member states, whose Covid-19 vaccine rollout has been marred by delays and supply issues.
With the EU experiencing tensions with first vaccine manufacturer AstraZeneca, and now the UK, we explain what is going on.
What is happening?
The EU refined its rules on the export of Covid-19 vaccines on Wednesday, giving it a clearer right to block shipments of vaccines made in the EU to countries such as the UK with higher inoculation rates and to those not exporting their own vaccine doses.
EU officials said export restrictions could also kick in if vaccine manufacturers do respect quarterly contracts, but instead backload supplies at the end of the period.
The new rules also widen the net of an existing mechanism that requires the EU to authorise vaccine exports, to include 17 countries that were previously exempt including Israel, Norway and Switzerland.
So are vaccine export blocks now in place?
Not quite — the tightened rules are proposals from the European Commission and are set to be a topic of discussion on Thursday at an online summit of EU leaders.
"In practice, all this is, is a piece of paper that says please take this stuff into consideration when you're looking at approving export authorisations," one EU official said.
Why is the EU threatening to block exports?
The EU wants to make sure the 27-nation bloc gets its fair share of jabs, the EU Commission said on Tuesday.
The regulation is aimed at making vaccine trade reciprocal and proportional, so that other vaccine-making countries sell to Europe and the EU does not export much more than it imports.
It also seeks to encourage vaccine manufacturers to live up to their contractual obligations.
The EU is the largest manufacturer of Covid-19 vaccines and the largest global supplier, having exported more than 40 million doses to more than 35 countries, its Health Commissioner said on Tuesday.
So this latest row is also related to AstraZeneca's delivery shortfalls?
Yes. European Commission President Ursula von der Leyen has said AstraZeneca could face a ban on exports of its doses from the EU, if it does not meet its delivery obligations to the bloc.
On Tuesday, the European Commission's chief vaccine negotiator Sandra Gallina said the EU will use all available means to secure vaccines produced by AstraZeneca, though she did not elaborate on which tools would be used.
However, she did note that only one of the five vaccine-production plants listed in the EU contract with AstraZeneca was delivering vaccines to the EU.
AstraZeneca said earlier in March it would aim to deliver to the European Union 100 million vaccines by the end of June — three times fewer than it had committed to in the contract with the EU.
So why is the spotlight also on the UK?
Britain has demanded that EU authorities allow the delivery of vaccines it has ordered and has warned Brussels against "vaccine nationalism".
However, recent comments from British prime minister Boris Johnson have further potential to inflame the row already stoked by post-Brexit tensions, after he said that capitalism and greed had given Britain its success in vaccinating its population.
The Sun newspaper quoted Mr Johnson as making the comments to Conservative lawmakers on a Zoom meeting on Tuesday evening, before he tried to row back and said: "Actually I regret saying it".
The UK has so far mounted the fastest Covid-19 vaccine programme of any big country.
What has the reaction to the potential export blocks been from EU members?
Countries such as France have come out in favour of the export curbs.
The French presidential adviser said on Tuesday that the EU should not be the "useful idiot" of the Covid-19 pandemic by exporting vaccines while other countries keep supplies for themselves.
With some 29 million doses of AstraZeneca vaccines found at a plant in Italy over the weekend, an official at the French presidency has said that the question of blocking the shipment should be on the table if they are confirmed to be export-bound.
What has the Irish Government said?
Taoiseach Micheál Martin has warned that an EU export ban on vaccines would be counterproductive and a retrograde step.
Earlier in the week, he told RTÉ radio’s News at One that it was vital that supply chains for ingredients for vaccines remained open.
Hasn't the EU already blocked vaccine shipments?
The EU this month used an export control mechanism, set up at the end of January, to block an AstraZeneca vaccine shipment to Australia.
Under the scheme, companies must get an authorisation before exporting shots and may have export requests denied if they do not respect their supply commitments with the EU.
The European Union has agreed to export a total of 43 million doses of Covid-19 vaccines since the scheme was set up, with Brussels granting 380 export requests to 33 different destinations and blocking one — the shipment to Australia.
So how are these latest rules any different?
The EU's original mechanism has been amended under the new rules proposed on Wednesday, so that it will now be able to block exports to cover companies that respect their quarterly contracts but backload supplies to the end of the period.
Beforehand, exports could only be blocked in the case of companies not meeting contracted quarterly delivery targets.
The new rules could see manufacturers such as Johnson & Johnson affected, as it has committed to delivering 55 million doses to the EU between April and June, but plans to start deliveries in the second half of April.
So, will the UK ultimately face a vaccine shortage under the new rules?
With no numerical targets, the change is unlikely to trigger mass export bans of EU-made vaccines, one EU official with insight into the announcement said on Wednesday.
"I just really, really don't see that happening because we have our international obligations and we want to keep supply chains going and the global system moving and flowing," the official said.