Vaccinated Brits are being urged to seek medical attention if they suffer from a headache that lasts for more than four days amid concerns over the AstraZeneca jab.
In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) has concluded that any link between the jab and clots is unproven, and the benefits of the vaccine outweigh any risks.
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Five men in the UK have suffered an "extremely rare" blood clot problem in the brain after having the AstraZeneca vaccine, with one of them subsequently dying, though no causal link with the jab has been established.
The 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.
Dr June Raine, MHRA chief executive, told a Downing Street press conference that, as a precautionary measure, the MHRA advised "anyone with a headache that lasts more than four days after vaccination or bruising beyond the site of vaccination after a few days to seek medical attention".
She said: “While we continue to investigate these cases [of blood clots], as a precautionary measure we would advise anyone with a headache that lasts for more than four days after vaccination, or bruising beyond the site of vaccination after a few days, to 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. These generally appear within a few hours and resolve within a day or two, but not everyone gets them.
“We will continue to robustly monitor all the data we have on this extremely rare possible side effect.”
England's chief medical officer Professor Chris Whitty said Covid-19 was still a "common disease" that was "dangerous" for many people.
He said there were "anecdotal reports" of small numbers of people not turning up for vaccine appointments following the controversy, but he expected many of those would decide to get the jab after "a pause for thought".
