A review by the drugs regulator the Medicines and Healthcare products Regulatory Agency (MHRA) found by the end of March 79 people in the UK suffered rare blood clots after vaccination - 19 of whom died.
The regulator said this was not proof the jab had caused the clots but said the link was getting firmer.
Dr June Raine, chief executive of the MHRA said the benefits of the AstraZeneca vaccine outweigh the risks for the “vast majority” of people after cases of blood clots were detected in an “extremely small” number of people.
However, due to a small number of blood clots in younger people, those under the age of 30 will be offered Pfizer or Moderna instead as the MHRA has concluded the weigh up is more finely balanced for younger people because the risk from Covid is much lower.
Dr Raine added: “Our review has reinforced that the risk of this rare suspected side effect remains extremely small.
“The evidence is firming up and our review has concluded that, while it’s a strong possibility, more work is needed to establish beyond all doubt that the vaccine has caused these side effects.”
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.
England's deputy chief medical officer Professor Jonathan Van-Tam said the changes being proposed to the vaccination rollout were a "course correction".
He told a press conference: "The UK vaccine programme has been the most enormous success indeed.
"If you had said to me that by March 2021 we would not have needed a course correction, that also would have amazed me."
People in the under 29 age group who have already had a first Oxford/AstraZeneca jab will still be offered a second.
Professor Van-Tam acknowledged the change in recommended use of the AstraZeneca vaccine might result in delays and longer journeys to receive the jab. However, he said the impact on the government’s vaccine roll out would be “negligible” as long as vaccine supplies were delivered as anticipated.
He told the conference: “The NHS has a message that we will get the right vaccine to you in the right time according to the new JCVI advice.
“There might be a small delay sometimes, there might be a slightly greater distance that some people might be asked to travel.
“But the NHS is all over this and understands the challenge of making the advice from JCVI truly operational in a smooth way.”
The professor, known as JVT, used a nautical analogy to describe the “course correction” in the vaccination programme.
“This is a massive beast that we are driving along at enormous pace with enormous success, this vaccine programme,” he said.
“If you sail a massive liner across the Atlantic then it’s not really reasonable that you aren’t going to have to make at least one course correction during that voyage.”
He said that it would be “pretty absurd” for the AstraZeneca vaccine to be withheld form 40-49 year-olds because the benefits of the jab far outweighed the risks in that age group.
Professor Wei Shen, chairman of the Joint Committee on Vaccination and Immunisation said the recommendation to prefer other vaccines to AstraZeneca for the under-30s was “out of the utmost caution” rather than because of “any serious safety concerns”.
The Professor said giving the Oxford AstraZeneca vaccine to under 30s was not being ruled out in the future but more research was needed.
He added: “We are not advising a stop to any vaccination for any individual in any age group.
“We are advising a preference for one vaccine over another vaccine for a particular age group, really out of the utmost caution rather than because we have any serious safety concerns.”
Separately, a review by the European Medicines Agency’s safety committee has concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the Oxford/AstraZeneca vaccine.
But the regulator said the benefits of the vaccine “overall outweighs the risks of side effects”.
The EMA said most of the cases of blood clots reported have occurred in women under 60 within two weeks of vaccination with the AstraZeneca jab, but that no specific risk factors had been identified based on current evidence.
Following a review of all available evidence, its Pharmacovigilance Risk Assessment Committee said the blood clots reported had been found in veins in the brain, the abdomen and arteries, combined with low levels of blood platelets and sometimes bleeding.
The EMA said symptoms associated with the blood clots include shortness of breath, chest pain, swelling in the leg, persistent abdominal pain, severe headaches, blurred vision and tiny blood spots under the skin beyond the area where the injection wad administered, and anyone who displayed them should seek medical attention.
The committee carried out an in-depth review of 62 cases of clots in the brain and 24 cases of clots in the abdomen as of March 22, with 18 of the combines cases proving fatal.
They came from reporting systems in the European Economic Area and the UK, from around 25 million people who had received the vaccine.
Dr Sabine Straus, the European Medicines Agency’s chairwoman, said side effects developing were “not unexpected”.
She said: “We know we are rolling out vaccinations in very large scale, (in) very many healthy people, and the difficulty is you will see events occurring, some of them just by chance, and some like this one might be an adverse effect from the vaccine.”
Emer Cooke, the regulator’s executive director, added: “The situation evolves as we have more information available, as more vaccines are available, and as different risk groups are identified.
“We can expect there will be new information and recommendations as time goes on.”
Boris Johnson thanked the experts for their work on the AstraZeneca vaccine and for explaining “the potential for extremely rare side effects”.
The Prime Minister said: “As the regulators have said, this vaccine is safe, effective and has already saved thousands of lives – and the vast majority of people should continue to take it when offered.
“We will follow today’s updated advice, which should allow people of all ages to continue to have full confidence in vaccines, helping us save lives and cautiously return towards normality.”
Health Secretary Matt Hancock tweeted: “The @MHRAgovuk & @EMA_News have confirmed that the Oxford/AstraZeneca vaccine is safe, effective & that the benefits far outweigh the risks for the vast majority of adults. We’re on track to offer a first jab to all adults by the end of July. When you get the call, get the jab.”
He added: “Many thanks to the excellent regulators & my clinical advisors for their work, their clarity & their total transparency. This is the excellence of UK institutions in action: clear, transparent, communication of advice based on the best quality science.”