Several European countries suspended use of the AstraZeneca-Oxford Covid-19 vaccine.
Temporary bans were put in place after a small number of people who received the drug suffered blood clots.
We look at the key questions surrounding the situation.
Should people be worried about taking AstraZeneca’s vaccine?
No. Many scientists criticised European countries for acting too hastily in suspending the vaccine while the pandemic is ongoing.
Health experts said the number of blood clots reported in vaccinated people is no higher than in the general population, and that there is no evidence to link the vaccine directly to blood clotting.
“I think it is very clear that the benefits of being vaccinated at the moment … outweigh the possible concern over this rather rare type of blood clot,” Prof Peter Openshaw from Imperial College London told the BBC.
“It really is a completely one-sided argument statistically that we need to be vaccinating.”
Dr Simon Clarke, a microbiologist at the University of Reading, said it was reckless to stop using the vaccine.
“I keep hearing the phrase 'abundance of caution' being used in reference to countries pausing rollout of the Oxford vaccine, but is it really caution?” he said.
Prof Stephen Evans from the London School of Hygiene and Tropical Medicine said some European countries were taking a “super-cautious approach based on some isolated reports”.
"The problem with spontaneous reports of suspected adverse reactions to a vaccine is the enormous difficulty of distinguishing a causal effect from a coincidence," he said.
Why did European countries suspend AstraZeneca’s vaccine?
A small number of blood-clotting incidents were reported in people who had received the vaccine made by AstraZeneca-Oxford.
Austria suspended a batch of the drug on March 9 while authorities investigated a death from a pulmonary embolism, an acute lung disease caused by a dislodged blood clot.
Denmark was next after a 60-year-old woman died from a blood clot after she was given a dose of AstraZeneca's Covid vaccine from the same batch used in Austria.
Deaths from blood clots were also reported in Norway and Italy.
The medicines regulator said there were 30 cases of "thromboembolic events" among five million people in Europe who had received at least one dose of the vaccine.
French immunologist Alain Fischer, who heads a vaccination advisory board, said the cases caused alarm.
"There were a few very unusual and troubling cases which justify this pause and the analysis," Prof Fischer told France Inter radio.
"It's not lost time."
Which European countries suspended AstraZeneca’s vaccine?
Germany, France, Italy and Spain on Monday suspended AstraZeneca's Covid vaccinations, joining Ireland, Denmark, Norway, Bulgaria and Iceland.
Sweden on Tuesday became the latest country to halt distribution.
Austria suspended vaccinations from one batch of the drug, as did Estonia, Latvia, Luxembourg, Lithuania and Romania.
How did the European Medical Association react?
The EU's medicines regulator said on Tuesday it was "firmly convinced" that the benefits of AstraZeneca's Covid shot outweighed potential risks, insisting there was no evidence linking the vaccine to blood clots.
"We are still firmly convinced that the benefits of the AstraZeneca vaccine in preventing Covid-19 with its associated risk of hospitalisation and death outweigh the risk of these side effects," EMA executive director Emer Cooke said.
"At present there is no indication that vaccination caused these conditions".
Ms Cooke said that the regulator was "looking at adverse events associated with all vaccines".
How did British authorities react?
Authorities in Britain see no evidence of more frequent blood clots among those inoculated with the vaccine.
The country has administered more than 11 million AstraZeneca-Oxford doses.
The UK’s Healthcare Products Regulatory Agency said people should continue to receive their Covid-19 vaccines.
“We are closely reviewing reports but the evidence available does not suggest the vaccine is the cause,” MHRA safety lead Dr Phil Bryan said.
“Blood clots can occur naturally and are not uncommon … the number of blood clots reported after having the vaccine is not greater than the number that would have occurred naturally in the vaccinated population.”
Prime Minister Boris Johnson hailed the effectiveness of the British-made vaccine and said it demonstrated the country's worldwide influence.
"That vaccine is safe and works extremely well, and now, only six months later, it is being made in multiple places from India to the US, as well as Britain, and it is being used around the world," he wrote in The Times.
What is the next step?
The EMA’s safety committee reviewed blood clot data on Tuesday and an extraordinary meeting was called for Thursday to consider whether any action should be taken.
If a strong association is found with vaccination, the regulator could put procedures in place to avoid similar problems occurring.
Several countries said they would lift the ban on AstraZeneca vaccines if the committee decided on Thursday decided against further action.
What did AstraZeneca say?
AstraZeneca said 15 events of deep-vein thrombosis – a blood clot in a vein – and 22 events of pulmonary embolism – when a blood clot entersthe lungs – were reported among those vaccinated in the UK and EU.
The company said 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".
Has the EU had problems with AstraZeneca before?
The EU was embroiled in a major dispute with AstraZeneca this year after the pharmaceutical company said it would reduce deliveries to the bloc owing to a shortage of doses.
On March 5, Italy used the bloc’s transparency mechanism to block a shipment of vaccines made by AstraZeneca bound for Australia.
How will the ban affect Europe’s struggling vaccination drive?
The temporary bans came as Europe’s inoculation campaign was beginning to gather pace after a difficult start.
As well as supply shortages, some European nations experienced hesitancy, with people reluctant to take the AstraZeneca shot.
The drug was labelled “quasi-ineffective” by French President Emmanuel Macron.
German and French regulators added to the problem after recommending the vaccine for people aged under 65, citing a lack of safety data in vaccinated groups.
Those decisions were reversed but the damage was already done, with vaccine centres stocked with doses from AstraZeneca pictured empty.
Prof Openshaw said the latest news was likely to further damage the drug’s reputation in Europe.
“I think it is a disaster for the vaccination uptake in Europe, which is already on slightly unsteady ground in some countries,” he said.
