The European Union's medicines agency has said the Oxford-AstraZeneca coronavirus vaccine is "safe and effective" to use following an investigation into reports of blood clots in a small number of recipients.
The decision by the European Medicines Agency (EMA) comes after more than a dozen European countries - including Germany, Italy, Spain, the Netherlands, France and Sweden - halted the vaccine's rollout over clotting fears.
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.
"We're also launching additional investigations to understand more about these rare cases and we're conducting targeted observational studies."
Earlier on Thursday, the UK's medicines regulator said people who experience a persistent headache for more than four days after the jab should seek medical attention, as should anyone with bruising in places other than the site of injection.
The Medicines and Healthcare products Regulatory Agency (MHRA) said this new advice was a "precautionary measure" after a "very small" number of reports of sinus vein thrombosis (CVST) occurring with thrombocytopenia after the vaccine.
CVST is an extremely rare form of blood clot, while thrombocytopenia describes a low number of platelets - the cells that help clot blood.
In the UK, there have been five cases of these co-occurring conditions reported to the MHRA after vaccination.
That's a rate of one in one million vaccinated people. The MHRA said this could also naturally occur whether people have had the jab or not, or if they have COVID.
Concerns about the vaccine were initially raised after a report from the Norwegian Medicines Agency revealed blood clotting events in four adults who had the COVID jab.
AstraZeneca also said after a thorough review of its COVID-19 immunisation data, that it found no evidence of any increased risk of blood clots in any age group or gender in any country.
The World Health Organisation (WHO) added its weight behind the debate ahead of the EMA's announcement, urging countries to continue using the jab.
"As of now, we do not know whether some or all of the conditions have been caused by the vaccine or by other coincidental factors," said Hans Kluge, the WHO's European director.
"At this point in time, however, the benefits of the AstraZeneca vaccine far outweigh its risks - and its use should continue, to save lives."
Prime Minister Boris Johnson has said there is "no reason at all" to stop the vaccine's rollout.
"It will certainly be Oxford-AstraZeneca that I will be having," he said on Wednesday, after revealing he has been called up to have his COVID jab.
Everyone aged 50 and over in England is now being invited to book a coronavirus vaccination on the NHS website.
However, next month could see a slowdown of the vaccination programme in the UK due to international supply issues.
Analysis: Trust in AstraZeneca's jab may still be hard to overcome in some EU nations
By Michelle Clifford, Europe correspondent
The EMA's conclusion that the AstraZeneca vaccine is safe was widely expected. The European regulator had already said this week that the benefits outweighed any risk.
But experts still had to look at the reported cases of blood clots in those who had the jab in Europe, case by case.
It was, the EMA said, not just an issue of safety but of public confidence - and the issue of trust may yet prove hard to overcome in many nations.
The decision early on by Germany and France not to give the jab to the elderly led to questions and nervousness amongst many in the wider population. It wasn't hard in EU vaccine centres to find people asking for an alternative.
And the blood clots development - although now scientifically dismissed (except in a tiny number of very rare cases which are still being examined) - may leave a sense of worry among those waiting their turn for a vaccine.
The EMA said European nations who had halted the vaccine rollout could now take an informed decision about whether to restart.
But there's still the issue of supply which has led to rancour between the EU and UK. European Commission President Ursula Von Der Leyen threatening to block exports across the channel because of a need at home. She accused AstraZenenca of failing its commitments to meet EU vaccine orders.
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