The Medicines and Healthcare Regulator said it was looking at the reports but stressed the events were “extremely rare”.
The UK has received five reports of a specific brain blood clot in people who have had the AstraZeneca coronavirus vaccine, though no causal link has been made with the jab, the medicines regulator has said.
The five people were men aged 19 to 59 who experienced a brain blood clot together with low blood platelet count. One of the five has since died.
It said the cases represented a less than one in a million chance of suffering this type of clot among those who have been vaccinated, while the risk of dying from Covid aged 40 to 49 was one in 1,000.
The MHRA has concluded that any link between the jab and clots is unproven and the benefits of the AstraZeneca vaccine outweigh any risks.
The type of clot – cerebral venous sinus thrombosis (CVST) – prevents blood from draining out of the brain.
It is this type of clot that led Germany to halt its rollout of the Oxford/AstraZeneca vaccine, prompting other countries across Europe to follow.
Dr Phil Bryan, MHRA vaccine safety lead, said the blood clots were “extremely rare” whereas vaccines were “highly effective” in preventing death and hospital admission from Covid-19.
“Where we are now is that no proven causal association with what is still an extremely rare medical event has been proven for the AZ (AstraZeneca) vaccine, but we do know that these are highly effective vaccines,” he said.
“We still have a huge burden of Covid disease in the population. So right now the balance of benefits and known risks of the vaccine are favourable.”
Dr June Raine, MHRA chief executive, said: “We continually monitor safety during use of all vaccines to protect the public, and to ensure the benefits continue to outweigh the risks.
“Our thorough and careful review, alongside the critical assessment of leading, independent scientists, shows that there is no evidence that that blood clots in veins is occurring more than would be expected in the absence of vaccination, for either vaccine (Pfizer or AstraZeneca)."
June added that the 'benefits of the vaccine in preventing Covid-19, with its associated risk of hospitalisation and death, continue to outweigh the risks of potential side effects.'
Meanwhile, The European Union's medicines agency has said today that the Oxford-AstraZeneca coronavirus vaccine is "safe and effective"
The EMA said the benefits outweigh the risks - and the vaccine is not linked to an "overall risk" of blood clots.
However, the agency's safety committee has also said it can't rule out a potential link with a "small number of cases" of a rare clotting disorder occurring after the vaccination.
It has therefore recommended that governments "raise awareness" of the possible effects by including them in product information.
"Drawing attention to these possible rare conditions and providing information to healthcare professionals and vaccinated people will help to spot and mitigate the possible side effects," said EMA executive director Emer Cooke on Thursday.
"We're also launching additional investigations to understand more about these rare cases and we're conducting targeted observational studies."