In a separate update, the European Medicines Agency (EMA) safety committee concluded that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the AstraZeneca vaccine.
However, the Medicines and Healthcare products Regulatory Agency (MHRA) said the benefits still outweigh the risks overall – but while it has not concluded that the vaccine causes rare brain clots, it said the link is getting firmer.
Some European countries have restricted the vaccine’s use in younger people, while Boris Johnson has sought to reassure the public the vaccine is safe.
Regulators have recommended that people aged 18 to 29 should be offered Pfizer, Moderna or other vaccines that come on stream as the programme continues to rollout across the UK.
The prime minister sought to boost public confidence in the Oxford-developed shot, telling reporters: “These vaccines are safe, they’ve saved many thousands of lives and people should come forward to get their jabs and we’ll make sure that they get the right jabs.”
Health secretary Matt Hancock also took to the airwaves on Thursday morning in an attempt to reassure the public over Wednesday's findings.
He said everyone should take a vaccine when their time comes, and the risk of experiencing a brain clot was the same as “taking a long-haul flight”.
Watch: What we know about the Moderna Covid-19 vaccine
He urged the under-30s, who will be offered an alternative vaccine to AstraZeneca, to take a jab to protect loved ones and avoid the risk of long COVID, adding there were plentiful supplies of Moderna and Pfizer for this age range.
Speaking directly to younger people who may be thinking they do not need a vaccine, Hancock told BBC Breakfast: “The vaccines are safe, and if you want to have the Pfizer vaccine or Moderna vaccine instead then that is fine.
“COVID is a horrible disease and long COVID affects people in their 20s just as much it seems as any other age group and can have debilitating side effects that essentially ruin your life.”
He added: “The safety system that we have around this vaccine is so sensitive that it can pick up events that are four in a million (the chance of developing a rare brain blood clot) – I’m told this is about the equivalent risk of taking a long-haul flight.”
MHRA’s chief executive Dr June Raine told a press conference that there was a “reasonably plausible” link between the AstraZeneca jab and rare blood clots, but stressed these were “extremely rare”.
She said: “Based on the current evidence, the benefits of the COVID-19 vaccine AstraZeneca against COVID-19 and its associated risks – hospitalisation and death – continues to outweigh the risks for the vast majority of people.
“Our review has reinforced that the risk of this rare suspected side effect remains extremely small.”
The MHRA said that up to 31 March, across the UK it had received 79 reports of blood clots accompanied by low blood platelet count, all in people who had their first dose of the vaccine, out of around 20 million doses given.
Of these 79, a total of 19 people have died, the regulator said, although it has not been established what the cause was in every case. Of the 19 who died, three were under the age of 30.
The figures suggest the risk of rare blood clot is the equivalent to four people out of every million who receive the vaccine.
The MHRA said that those who have had their first dose of the AstraZeneca vaccine should still get their second dose.
Watch: Should I be worried about the AstraZeneca COVID vaccine?