A preservative in AstraZeneca’s Covid vaccine may trigger a rare overreaction from the immune system that causes blood clots, a scientist has claimed.
Tiny numbers of people given the jab or the one made by Johnson & Johnson have developed a clotting condition that spooked some countries into turning their backs on it.
The reasons for this have been unclear but one German expert has suggested it may begin with a chemical used to make the vaccine.
EDTA, or ethylenediaminetetraacetic acid, might trigger an immune overreaction by forming clumps using platelets in the bloodstream.
EDTA — commonly used in medicines and cosmetics — may make more ingredients of the intra-muscular jab leak into blood vessels.
Dr Andreas Greinacher said they could make the body over-produce antibodies and trigger a second reaction from the immune system which then starts clotting blood.
Medical regulators in the UK have spotted 262 cases of the clots in 23.36m people given AstraZeneca’s jab – a rate of around one in 100,000.
The clots remain rare and far less of a risk than Covid but people under the age of 40 have been advised to get a different vaccine if they can, to be on the safe side.
The AstraZeneca Covid vaccine remains one of the most widely used in the world because it prevents severe Covid and is widely available, but some countries have restricted its use only for older people who have a higher risk from coronavirus that outweighs danger of side effects (Pictured: A nurse in Dublin fills a syringe with vaccine)
MHRA boss Dr June Raine said last week the benefits of AstraZeneca’s continue to outweigh the risks for the ‘vast majority of people’.
But she said: ‘The balance of benefits and risks is very favourable for older people but is more finely balanced for younger people.
‘We advise that this evolving evidence should be taken into account when considering the use of the vaccine.’
The Government opted to make it policy to give adults aged 40 or younger a choice of vaccines when they go to their appointment because the rate of the clots with low platelets appeared more common among them, at around one in 60,000 people.
The NHS rollout expanded to 38 and 39-year-olds today for the first time.
One of the main theories for why the clots were happening was that the body was over-producing antibodies that triggered a reaction that led to blood clotting.
Now Dr Andreas Greinacher, a blood expert and immunologist at the University of Greifswald, may be able to explain why, Pharmaceutical Technology reports.
Dr Greinacher and colleagues did studies on mice that suggest that EDTA causes proteins in the vaccine liquid to leak into the bloodstream and activate platelets by bumping into them.
Platelets are tiny components of clots and are always circulating in the blood in case they are needed to heal an injury.
But once they are sparked into action they start a chain reaction of immune responses, releasing another protein, known as PL4, which sticks to more proteins from the vaccine and starts to form more lumps.
These lumps then alarm the immune system and antibodies – not Covid ones – are produced to destroy them.
Large amounts of these antibodies then trigger a higher level reaction from the immune system which can include swelling in the blood vessels and blood clots, potentially leading to the clotting being seen in vaccine patients.
The most high profile of the cases were in a vein leading out of the brain, a condition known as CVST, but the majority of cases in the UK (163 out of 262) were elsewhere in the body.
A total of 51 people are known to have died in the UK after developing a blood clot following vaccination.
Some countries, including Denmark and Norway, have stopped using the AstraZeneca vaccine completely while others have restricted it to older people.
Last week health officials in the UK said people under 40 should be offered an alternative to the jab because the risks and benefits were finely balanced.
For older people who are at a genuine risk of dying if they catch Covid, the benefits of protection from the virus clearly outweighed any negative side effects.
Experts said the infection rate in the UK is now so low that the risk of the rare clots outweigh that of Covid in younger adults, who often only suffer mild illness.
They will be offered the Pfizer or Moderna vaccines instead, so long as there is enough supply and it won’t delay the rollout.
Anyone, no matter what age, who has been given their first dose of the AstraZeneca jab and didn’t suffer the complication is being urged to come forward for their second.
England’s deputy chief medical officer, Professor Jonathan Van-Tam, claimed the change would not affect the Government’s target to vaccinate all adults by July 31.
‘Our vaccine supply schedule will support the change without limiting the speed and scale of the vaccine roll-out,’ he told a televised Downing Street press conference.
‘I do expect that we are still on target to offer a first dose to all adults by the end of July.’
It was previously recommended on April 7 that healthy under-30s should be offered an alternative to AstraZeneca.