The UK medicines regulator has said benefits of the Oxford/AstraZeneca [AZ] vaccine continue to outweigh any risks but added that under 30s will be offered an alternative jab.
Meanwhile, in a separate update on its investigations on Wednesday, the European medicines watchdog has ruled that unusual blood clots were "very rare side effects" of the jab.
The government is keen to emphasise people should continue to keep getting their jabs with more than 20 million doses of the AZ vaccine now given across the UK, saving an estimated 6,000 lives.
Here are the key questions and answers as under 30s will be offered Pfizer or Moderna jabs instead.
What have the regulators said?
A review by the European Medicines Agency (EMA) safety committee concluded on Wednesday that "unusual blood clots with low blood platelets should be listed as very rare side effects" of the AstraZeneca vaccine.
Emer Cooke, executive director of the EMA, said: "These are very rare side effects. The risk of mortality from Covid is much greater than risk of mortality from these side effects."
The MHRA said there were still huge benefits of the vaccine in preventing Covid-19 and serious disease but added that due to a very small number of blood clots in younger people, those under the age of 30 will be offered Pfizer or Moderna jabs instead.
MHRA chief executive Dr June Raine gave a list of symptoms that should prompt the public to come forward for swift medical advice.
She told the briefing: "Anyone who has symptoms four days after vaccination or more should seek prompt medical advice, a new onset of a severe or persistent headache or blurred vision, shortness of breath, chest pain, leg swelling, persistent abdominal pain or indeed unusual skin bruising or pin-point spots beyond the injection site."
How many people have been affected?
Up to March 31, the MHRA has received 79 reports of blood clots accompanied by low blood platelet count, all in people who had their first dose of the vaccine.
Of these 79, a total of 19 people have died, although it has not been established what the cause was in every case. The 79 cases occurred in 51 women and 28 men, aged from 18 to 79.
Of the 19 who died, three were under the age of 30, the MHRA said. Some 14 of those cases were CVST while the other five were thrombosis.
There have also been reports of CVST cases in Germany and cases of blood clot clusters in Norway, the Netherlands and Denmark but experts say these incidents are rare.
How have other countries reacted?
Germany, France, the Netherlands, Sweden and Canada have restricted the use of the AZ vaccine in younger people while Denmark and Norway have paused administering it.
But several European countries - such as Greece, Italy and Portugal - are using the vaccine without such restrictions.
What has AstraZeneca said?
Last month, AstraZeneca said it was analysing its database to understand "whether these very rare cases of blood clots associated with thrombocytopenia (low blood platelet count) occur any more commonly than would be expected naturally in a population of millions of people".
Meanwhile, on Tuesday, a trial of the AZ vaccine on children was paused, but the scientists involved said there were no safety concerns with the trial itself and they were waiting for further information from the MHRA.
The University of Oxford said in a statement: "Whilst there are no safety concerns in the paediatric clinical trial, we await additional information from the MHRA on its review of rare cases of thrombosis/thrombocytopaenia that have been reported in adults, before giving any further vaccinations in the trial."
What are the experts saying?
Scientists in the UK have been largely in agreement with the regulators, saying the benefits of the vaccine outweigh potential risks.
Professor Adam Finn, from the University of Bristol and a member of the Joint Committee on Vaccination and Immunisation (JCVI), said it was vital to keep vaccines going as society opens up, in order to help stave off rising infection rates.
He urged people being offered the vaccine at the moment to take it, saying the "risk-benefit is very strongly in favour of receiving the vaccine".
Professor Calum Semple, a member of the Scientific Advisory Group for Emergencies (Sage), which advises ministers, said he was "not worried one little bit" about headlines around the AstraZeneca vaccine.
Speaking in a personal capacity, he told LBC radio: "I'll take (it) myself, I'm 53, my risk of death from Covid is about one in 13,000, for me it's a no-brainer, I need to have the vaccine."
Meanwhile, former MHRA chief Professor Sir Kent Woods also told LBC radio: "Covid itself - the infection itself - is known to be associated with a substantial increased risk of blood clots of various kinds.
"At a time when the population has got lots of Covid going around, it's very difficult to know what the actual background rate of these clotting events is without the vaccine.
"We can say I think that if there is a connection, it's a very, very rare one."
What has the government said?
A spokesperson for the government has said the Oxford AZ vaccine is effective, safe and has already saved thousands of lives.
They reiterated the MHRA and JCVI's confirmation that the benefits of the vaccine outweigh the risks for the vast majority of adults.
The spokesperson said: "Everybody who has already had a first dose of the AZ vaccine should receive a second dose of the same brand, irrespective of age, except for the very small number of people who experienced blood clots with low platelet counts from their first vaccination.
“The government will follow today’s updated advice, which sets out that, as a precaution, it is preferable for people under the age of 30 with no underlying health conditions to be offered an alternative vaccine where possible once they are eligible.
“When people are called forward, they should get their jab. Vaccines are the best way out of this pandemic and provide strong protection against Covid-19.
“We are very grateful for the work of our world-leading regulator and our expert advisors as they continue to address this issue."
The government has said more than 37 million jabs have been administered already, and they believe they are on track to offer jabs to all over 50s by April 15, as well as all adults by the end of July.
What other vaccines are available?
The UK is currently using two vaccines, Pfizer/BioNTech and Oxford/AstraZeneca, while a third coronavirus vaccine, the Moderna jab, began its rollout in Wales on Wednesday.
Meanwhile, preliminary results announced on Tuesday from the trials of the Valneva Covid-19 vaccine, which is set to be manufactured in the UK, has shown it produces a "strong immune response", paving the way for a phase three clinical trial.
The UK has an in-principle agreement for 60 million doses of the Valneva jab, with an option to acquire a further 130 million doses from 2022-2025.
The country has also ordered 30 million doses of a coronavirus vaccine developed by Johnson & Johnson, which has been shown to be 66pc effective in preventing coronavirus infection.
Both the Valneva and Johnson & Johnson jabs will need regulatory approval for use in the UK, once data from later-stage trials become available.
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