Eleven European countries have suspended use of the vaccination over concerns about side effects.
Vaccine safety experts from the World Health Organization (WHO) are meeting on Tuesday to review the Oxford-AstraZeneca jab, after several European countries halted their rollouts.
There have been a number of cases in Europe of blood clots reported after the vaccine was administered.
But the numbers are below the level you would expect in the general population.
The UK medicines regulator and the WHO say there is no evidence of a link.
The European Medicines Agency (EMA) – the European Union’s medicines regulator – is also meeting on Tuesday.
It is expected to issue its decision on the continued use of the Oxford-AstraZeneca vaccination on Thursday.
About 17 million people in the EU and the UK have received a dose of the vaccine, with fewer than 40 cases of blood clots reported as of last week, AstraZeneca said.
Concerns that there could be a link led to 11 European countries temporarily suspending use of the vaccine, including Germany, France, Italy and Spain.
Other countries, including Austria, have halted the use of certain batches of the drug as a precautionary measure.
However, Belgium, Poland, the Czech Republic and Ukraine said they would continue to administer the AstraZeneca vaccine.
And in Thailand, Prime Minister Prayuth Chan-ocha became the first person in the country to receive the AstraZeneca inoculation.
The key question that has to be asked is whether this is cause or coincidence? Would these clots have happened anyway?
The 37 reported cases are below the level you would expect. What is more, there is no strong biological explanation why the vaccine would cause a blood clot. It is why the WHO and the UK say there is no evidence of a link. And the EMA has suggested the vaccine should continue.
Unsurprisingly, therefore, the decisions by individual nations to pause their rollouts have baffled experts.
And this is not the first time countries in Europe have exercised caution about the AstraZeneca vaccine. Germany, France and others did not initially recommend use of the vaccine for the over-65s. That has now been reversed, but the impact is still being felt, it seems.
Now, with supplies of the AstraZeneca vaccine going to waste, deadly consequences may ensue. France, Germany and the other major European nations all have higher rates of infection than the UK, and face the prospect of things getting worse before they get better.
The WHO says it is investigating the reports of blood clots. On Monday, a spokesman said there was “no evidence” that the incidents were linked to the vaccine.
“As soon as WHO has gained a full understanding of these events, the findings and any unlikely changes to current recommendations will be immediately communicated to the public,” WHO spokesman Christian Lindmeier said.
The EMA – which is also currently carrying out a review into incidents of blood clots – said the vaccine could continue to be administered.
The agency advises that the benefits of having the vaccine outweigh the risks of any side effects.
The UK medicines regulator also said evidence “does not suggest” the jab causes clots, as it urged people in the country to get the vaccine when asked to do so.
The company says there is no evidence of an increased risk of clotting due to the vaccine.
It said that across the EU and the United Kingdom there had been 15 events of deep-vein thrombosis (DVT) – a blood clot in a vein – and 22 events of pulmonary embolism – a blood clot that has entered the lungs – reported among those vaccinated.
These figures were “much lower than would be expected to occur naturally in a general population of this size and is similar across other licensed Covid-19 vaccines”.
Professor Andrew Pollard, director of the Oxford vaccine group which developed the Oxford-AstraZeneca jab, told the BBC on Monday that there was “very reassuring evidence that there is no increase in a blood clot phenomenon here in the UK, where most of the doses in Europe [have] been given so far”.
Finland has also done a “very careful study” and not found an increased risk, he added.
He said it was “absolutely critical that we don’t have a problem of not vaccinating people”.