Several countries have suspended use of AstraZeneca's coronavirus vaccine after a number of recipients suffered blood clots in the days and weeks that followed.
But how prevalent have these incidents been - and should Britons who are about to get the jab be concerned?
What has happened?
A review from Norway's medicines agency showed four new cases of "serious blood clotting in adults".
Meanwhile, the European Medicines Agency (EMA) confirmed that one person in Austria was admitted to hospital with pulmonary embolism, where there is a blockage in the arteries of the lungs. They died 10 days after they were vaccinated.
Another death involving a blood clot was reported in Denmark.
A 50-year-old man is also thought to have died in Italy from deep vein thrombosis, and there has been an unconfirmed report of another death in the country.
The EMA has stressed that there is "currently no indication that vaccination caused these conditions" - adding that the jab's benefits continue to outweigh the risks.
A review is being carried out into each reported incident, and the regulator says that - of almost five million people given the vaccine across Europe - there have been just 30 reports of blood clots.
What are British scientists saying?
Overwhelming scientific opinion is that there is no certain link between blood clots and the AstraZeneca jab.
Regardless of vaccination, blood clots are fairly common - and given the sheer number of people who are getting a jab, these incidents could be coincidental.
Professor Stephen Evans of the London School of Hygiene and Tropical Medicine said: "The problem with spontaneous reports of suspected adverse reactions to a vaccine are the enormous difficulty of distinguishing a causal effect from a coincidence.
"This is especially true when we know that COVID-19 disease is very strongly associated with blood clotting and there have been hundreds if not many thousands of deaths caused by blood clotting as a result of COVID-19 disease.
"The first thing to do is to be absolutely certain that the clots did not have some other cause, including COVID-19."
What is the UK's medicines regulator saying?
Earlier in March, the UK's Medicines and Healthcare products Regulatory Agency (MHRA) said that more than 11 million doses of the AstraZeneca vaccine had been administered across the country with no issues.
Dr Phil Bryan, the MHRA's vaccines safety lead, has urged Britons to "still go and get their COVID-19 vaccine when asked to do so".
He added: "We are closely reviewing reports but given the large number of doses administered, and the frequency at which blood clots can occur naturally, the evidence available does not suggest the vaccine is the cause."
The World Health Organisation's spokeswoman, Dr Margaret Harris, has described the AstraZeneca vaccine as "excellent".
Which countries have suspended use of the AstraZeneca jab?
The Netherlands temporarily stopped the rollout of this vaccine, with 43,000 appointments cancelled as a result.
Hours earlier, Ireland also said it would halt the use of the AstraZeneca jab, based on the review released by Norway's medicines agency.
Denmark, Norway and Iceland have all suspended vaccinations while they investigate further.
Confirming that the restrictions would last for 14 days, Denmark's health minister Magnus Heunicke said: "It is currently not possible to conclude whether there is a link. We are acting early, it needs to be thoroughly investigated."
And Geir Bukholm, a senior official at the Norwegian Institute of Public Health, added: "This is a cautionary decision."
Meanwhile, Italy, Austria, Estonia, Latvia, Luxembourg and Lithuania have banned jabs from one particular batch of one million vaccines.
What is AstraZeneca saying?
AstraZeneca says it has completed a review examining how safe its vaccine is - based on data covering the 17 million people who have been vaccinated in the UK and the EU.
In a statement, the company added: "A careful review of all available safety data of more than 17 million people vaccinated in the European Union and UK with COVID-19 Vaccine AstraZeneca has shown no evidence of an increased risk of pulmonary embolism, deep vein thrombosis or thrombocytopenia, in any defined age group, gender, batch or in any particular country."
The company's figures suggest there have been 15 cases of deep vein thrombosis and 22 events of pulmonary embolism so far - with executives stressing this is comparable to the rates seen among other licenced vaccines.
Monthly safety reports are tracking the vaccine's rollout, and are being made available on the EMA website.
Will this affect long-term confidence in the vaccine?
Peter English, who formerly served as a consultant in communicable disease control for the British government, said it was "regrettable" that some countries have stopped vaccinations as a precautionary measure.
He said: "It risks doing real harm to the goal of vaccinating enough people to slow the spread of the virus, and to end the pandemic."