Several countries have suspended the use of the Oxford/AstraZeneca Covid-19 vaccine.
No link has been established between the two and UK scientists have said they are not worried about the jab as the incidents could be entirely coincidental.
Here, we take a look at the key questions surrounding the situation.
Is the AstraZeneca vaccine safe and what has happened so far?
There have been a small number of reports of people experiencing blood clots in the days and weeks after their Oxford/AstraZeneca vaccination.
Last week, the European Medicines Agency (EMA) reported one person in Austria was diagnosed with blood clots and died 10 days after vaccination, but it stressed there is "currently no indication that vaccination has caused these conditions".
Another person was admitted to hospital in Austria with pulmonary embolism (blockage in arteries in the lungs) after being vaccinated, while one death involving a blood clot was reported in Denmark.
Denmark's National Board of Health said: "There is good evidence that the vaccine is both safe and effective."
But added: "We and the Danish Medicines Agency have to react to reports of possible serious side effects."
A 50-year-old man is also thought to have died in Italy from deep vein thrombosis (DVT), while there has been an unconfirmed report of another death in Italy.
Which countries are stopping the AstraZeneca vaccinations?
Denmark, Norway and Iceland said last Thursday, March 11, they were temporarily halting all AstraZeneca vaccinations to investigate the reports.
Thailand followed suit a day later, saying the decision was based on the decision made by Denmark, Austria and Iceland, as a precaution.
These are all the countries that have suspended rollouts of the Oxford/AstraZeneca jab to date:
Austria, Estonia, Latvia, Luxembourg and Lithuania have banned jabs from one particular batch of one million AstraZeneca vaccines, which was sent to 17 countries, after reports of a death.
Very few details have been given about the individuals, including whether they had any underlying conditions that already raised the risk of blood clots.
What do the European and UK medicines regulators say?
The EMA has also backed the jab’s safety, saying that as of March 10, there are just 30 reports of blood clots among almost five million people given the vaccine in Europe.
It has been continuing its investigation on all the data related to blood clots over the weekend. Its safety committee will further review the information on Tuesday and has called an extraordinary meeting on Thursday to conclude on its investigation.
It said on Monday: "Experts are looking in great detail at all the available data and clinical circumstances surrounding specific cases to determine whether the vaccine might have contributed or if the event is likely to have been due to other causes."
The EMA statement continued: "While its investigation is ongoing, EMA currently remains of the view that the benefits of the AstraZeneca vaccine in preventing Covid-19, with its associated risk of hospitalisation and death, outweigh the risks of side effects."
The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has said there is no evidence to suggest the vaccine causes blood clot problems, and people should still get their Covid-19 jab when asked to do so.
Dr Phil Bryan, MHRA vaccines safety lead in the UK, said more than 11 million doses of the Covid-19 AstraZeneca vaccine have now been administered across the UK with no issues.
"Reports of blood clots received so far are not greater than the number that would have occurred naturally in the vaccinated population," he added.
Professor Anthony Harnden, deputy chairman of the UK's Joint Committee on Vaccination and Immunisation, said there was “no demonstrable difference” in the number of blood clots seen between the general population and the 11 million who have so far received the jab.
He said: "I spent all yesterday in our practice vaccinating with the Oxford/AstraZeneca vaccine – I would not be immunising my own patients unless I felt it was safe."
He added: "One ought to also remember that Covid causes blood clots. So, the risks of not having the Covid vaccination far outweigh the risks from the vaccinations."
The World Health Organisation (WHO) have echoed this stance.
What has AstraZeneca said?
AstraZeneca says it has not found any increased risk of blood clots.
It said: "An analysis of our safety data of more than 10 million records has shown no evidence of an increased risk of pulmonary embolism or deep vein thrombosis in any defined age group, gender, batch or in any particular country.
"In fact, the observed number of these types of events are significantly lower in those vaccinated than what would be expected among the general population."
The head of the team who developed the Oxford/AstraZeneca jab, Professor Andrew Pollard, told the BBC Radio 4’s Today programme on Monday it was “important to understand that a lot of stuff happens to people all the time in normal times and, in the case of blood clots here in the UK, we see about 3,000 cases of blood clots happening every month.
“So, when you then put a vaccination campaign on top of that, clearly those blood clots still happen and you’ve got to then try and separate out whether, when they occur, they are at all related to the vaccine or not.”
Finland has also done a “very careful study” and have not found an increased risk, he added.
Professor Pollard said there were “huge risks” from Covid and “if we have no vaccination and we come out of lockdown in this country, we will expect tens of thousands of more deaths to occur during this year”.
He added 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 been given so far”.
Are UK scientists worried?
No. The overwhelming scientific opinion is that there is no certain link between blood clots and the vaccine, and the reported cases could easily be coincidental.
They argue the risks from Covid-19 far outweigh any potential side-effects from the jab, with many saying blood clots are fairly common, regardless of vaccination.
Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & 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."