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 clot together with low blood platelet count. One of the five has since died.
There is no detail as yet on whether they had underlying health conditions.
The Medicines and Healthcare products Regulatory Agency (MHRA) said it was looking at the reports but stressed the events were “extremely rare” and there was a possibility they could have been caused by Covid itself.
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 AstraZeneca vaccine, prompting other countries across Europe to follow.
Officials in Germany received six reports of CVST associated with low platelets – all in younger to middle-age women.
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.”
The MHRA said its overall review of all types of blood clots “does not suggest that blood clots in veins (venous thromboembolism) are caused by Covid-19 Vaccine AstraZeneca.”
It said a detailed review into the five cases of the blood clot in the cerebral veins occurring together with lowered platelets was ongoing.
Lowered blood platelet counts have been found in around 37% of people suffering from Covid-19.
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).
“We have received a very small number of reports of an extremely rare form of blood clot in the cerebral veins…occurring together with lowered platelets soon after vaccination.
“This type of blood clot can occur naturally in people who have not been vaccinated, as well as in those suffering from Covid-19.
“Given the extremely rare rate of occurrence of these events among the 11 million people vaccinated, and as a link to the vaccine is unproven, 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.
“You should therefore continue to get your jab when it is your turn.”
Dr Raine said while investigations were ongoing, as a precautionary measure, anyone with a headache that lasts for more than four days after vaccination, or bruising beyond the site of vaccination after a few days, should seek medical attention.
“However, please remember that mild flu-like symptoms remain one of the most common side effects of any Covid-19 vaccine, including headache, chills and fever,” Dr Raine said.
“These generally appear within a few hours and resolve within a day or two, but not everyone gets them.”
Professor Sir Munir Pirmohamed, chairman of the Commission on Human Medicines, said cases of CVST will be closely monitored “to understand whether there is any potential association”.
He added: “Further work with expert haematologists is under way to further understand the nature of these cases and whether there is a causal association with any of the vaccines.
“Given the extremely rare rate of occurrence of these events, the benefits of the AstraZeneca Covid vaccine, with the latest data suggesting an 80% reduction in hospitalisation and death from Covid disease, far outweigh any possible risks of the vaccine in the risk groups currently targeted in the UK.”