Under 30s in the UK should not be offered the Oxford/AstraZeneca vaccine due to blood clot risks, regulators have said.
Adults in that age range will be offered Pfizer or Moderna jabs instead.
The Medicines and Healthcare products Regulatory Agency (MHRA) said there were still huge benefits of the vaccine in preventing Covid-19 and serious disease.
It concluded the balance of risk for the vaccine is “very favourable for older people” but more finely balanced for younger groups.
Dr June Raine, chief executive of the MHRA, said: “The balance of benefits and risks is very favourable for older people but it is more finely balanced for the younger people and we at the MHRA are advising that this evolving evidence should be taken into account when considering how the vaccine is used
“This is extremely rare and with the proven effectiveness against the disease that is still a huge risk to our population, the balance of benefits and known risks of the vaccine is still very favourable for the vast majority of people.”
Raine told a news conference the safety monitoring systems were detecting “a potential side effect of the Covid-19 vaccine AstraZeneca in an extremely small number of people”.
She added: “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,” she said.
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 cases of the 19 were cerebral venous sinus thrombosis (CVST), a specific type of clot that prevents blood from draining from the brain.
The other five cases were thrombosis.
England’s deputy chief medical officer Professor Jonathan Van-Tam said the changes being proposed to the vaccination rollout were a “course correction”.
He said “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.”
And Professor Wei Shen, chairman of the Joint Committee on Vaccination and Immunisation, said the recommendation to prefer other vaccines to Oxford/AstraZeneca for the under-30s was “out of the utmost caution” rather than because of “any serious safety concerns”.
More than 20 million doses of the AstraZeneca vaccine have now been given in the UK, saving an estimated 6,000 lives.
In the UK, the MHRA said that those who have had their first dose of the AstraZeneca vaccine should still get their second dose.
Only those who suffered a rare blood clot after the first dose should not get vaccinated.
Anyone with blood disorders that leave them at risk of clotting should discuss the benefits and risks of vaccination with their doctor before going for a jab.
A separate review by the European Medicines Agency’s safety committee concluded on Wednesday that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the Oxford/AstraZeneca vaccine.
Executive director Emer Cooke told a Brussels press briefing: “It’s important that both vaccinated people, and healthcare professionals, are aware of the signs and symptoms of these unusual blood clotting disorders, so that they can be spotted quickly to minimise any possible risks.
“We will continue to monitor the scientific evidence available on both effectiveness and safety of all the authorised Covid-19 vaccines, and we will issue further recommendations if necessary on the grounds of science and robust evidence.
“This case clearly demonstrates one of the challenges posed by large-scale vaccination campaigns.
“When millions of people receive these vaccines, very rare events can occur that were not identified during the clinical trials.
“The role of pharmacovigilance, the monitoring of the side effects, is to help us to rapidly detect and analyse these risks, and their impact on the benefit-risk profile of the vaccine.”
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