Both UK and EU regulators confirmed on 7 April that benefits of vaccination with the AstraZeneca COVID-19 vaccine continue to outweigh risks for the vast majority of people.
However, at a Downing Street press conference on 7 April, MHRA chief executive Professor June Raine said evidence was 'firming up' of a link between the vaccine and extremely rare side effects.
With more than 20m doses of the AstraZeneca vaccine administered in the UK, 79 cases of clots have been reported and 19 people have died, the MHRA chief executive said.
Blood clot risk
Of the 79 cases, 51 occurred in women and 28 in men - aged from 18 to 79 years of age. Of the 19 deaths, three were in people aged under 30, with 14 of these cases involving cerebral venous sinus thrombosis (CVST), and five 'other types of thrombosis' in major veins.
Based on the different risk-benefit balance for people in different age groups, the Joint Committee on Vaccination and Immunisation (JCVI) is now recommending that people aged under 30 with no other health conditions that put them at increased risk from COVID-19 are offered alternatives to the AstraZeneca vaccine.
Millions of people have already received the Pfizer/BioNTech vaccine through the UK vaccination programme - and the first UK doses of the Moderna vaccine have now been administered in Wales.
JCVI COVID-19 immunisation chair Professor Wei Shen Lim said that following a review of available evidence, 'in the interest of safety and public benefit' the committee was advising that adults aged 18-29 'who do not have an underlying condition that puts them at higher risk from serious COVID-19 disease should be offered an alternative COVID-19 vaccine in preference to the AstraZeneca vaccine where such an alternative is available'.
'We are not advising a stop to any vaccination for any individual in any age group,' he told the briefing. 'We are advising a preference for one vaccine over another for a particular age group really out of the utmost caution rather than because we have any serious safety concerns.'
The JCVI has also recommended an update to advice for patients to reflect the latest information and has said that people who have received a first dose of AstraZeneca vaccine should continue to receive a second dose in line with the 11- to 12-week schedule.
Commission on Human Medicines chair Professor Sir Munir Pirmohamed told the briefing that anyone who had experienced a cerebral or other major blood clot alongside a low platelet count after a first dose of the AstraZeneca vaccine should not be offered the second dose.
He pointed out that COVID-19 itself was associated with a significant risk of clots and lowered platelets - and said pregnant women should continue to be advised to discuss with healthcare professionals whether the benefits of vaccination outweighed risk.
Professor Raine said the UK vaccination programme had already saved thousands of lives. She said very rare effects were only detected when vaccines were used at scale - and that UK monitoring systems were now detecting a 'potential side effect of the COVID-19 vaccine Astra Zeneca in an extremely small number of people'.
She said: 'The evidence is firming up and our review has concluded that while it is a strong possibility more work is needed to establish beyond all doubt that the vaccine has caused these side effects.'
Deputy chief medical officer Professor Jonathan Van Tam said the UK vaccination programme had been 'the most enormous success'.
He said the update to advice on use of the AstraZeneca vaccine was a 'course correction' for the vaccination programme but that expected supply of Pfizer and Moderna vaccine over the coming weeks would likely mean the impact on rollout of vaccination to UK adults would be 'zero or negligible'.
He added: 'If you sail a massive liner across the Atlantic, it is not reasonable that you will not have to make at least one course correction during that voyage. Vaccines continue to be the way out for the UK and the way we can get our lives back to normal.'
Professor Lim added that there was a 'slight gradation in the risk of this serious adverse event occurring in younger people compared with older people' - but made clear that the key factor behind the advice on preference for other vaccines in younger people was 'the benefit-risk balance'.
Meanwhile a review by the European Medicines Agency did not conclude based on available evidence that age and gender were risk factors for blood clots in patients receiving the AstraZeneca vaccine.