Scientists believe they have solved the mystery behind the extremely rare blood clots caused by the Oxford-AstraZeneca vaccine.
A team of international experts, involving researchers from AstraZeneca, say that in a very small number of cases — about one in 100,000 in the UK — the vaccine can set off a chain reaction which leads to the body confusing its own blood platelets for fragments of virus.
The British-made vaccine is thought to have saved about a million lives from Covid and was the backbone of the UK’s initial rollout earlier in the year, helping it to become the most vaccinated country in the West.
But concerns about clots saw its restriction in under-40s in the UK in spring and led Pfizer and Moderna’s vaccines being favoured for young adults and as boosters. It was outright banned in many European countries and the US decided not to purchase a single dose.
The UK Government gave an emergency grant to a team of scientists led by Cardiff University to investigate the clotting phenomenon.
They found that the shell of the vector vaccine — the weakened cold virus used to teach cells how to neutralise Covid — sometimes acts like a magnet and attracts platelets, a protein found in the blood.
For reasons the scientists are still probing, the body then mistakes these platelets as a threat and produces antibodies to fight them. The combination of the platelets and the antibodies clumping together leads to the formation of dangerous blood clots.
But they stress this is extremely rare, with only 426 cases in the UK recorded the cases so far out of about 50million doses of the vaccine, equivalent to less than one in 100,000. The side effect has been linked to 73 deaths in the UK.
Researchers are now doing further work to learn more about the process that causes these clots and if the vaccine can be tweaked to reduce this risk.
The Oxford University-AstraZeneca jab is a adenovirus vaccine, meaning it contains a genetically altered virus, in this case chimpanzee cold virus, modified to be incapable of infecting the human body.
The vaccine works by using the chimpanzee cold virus to deliver a portion of the Covid virus’s genetic code which the body then learns to recognise and prepare itself for a real infection from the virus.
Adenovirus technology is also used in the single dose Johnson and Johnson Covid vaccine which as also been linked to a small number of life threatening blood clot cases. The UK has donated the 20million doses of the Johnson and Johnson it ordered to the COVAX scheme — the UN’s vaccine sharing programme.
While the AstraZeneca jab has been proven to have saved thousands of lives, the rollout was marred after it was revealed there was a rare chance of developing a life threatening blood-clot after it was administered and deaths were reported.
The fallout out led to the jab being restricted to the over 40s in the UK and banned entirely in some countries, although it was later revealed the risk of developing a similar life threatening blood clot from Covid itself was higher.
Now the scientists involved in the new study, who published their findings in the Science Advances, say they may have uncovered the trigger causing theses rare vaccine triggered blood clots.
Essentially, after being delivered into the body adenovirus binds with a specific protein in the blood, known as platelet factor 4 (PF4), which is normally used by the body to promote coagulation in case of injury.
Using incredibly detailed images of the adenovirus in the vaccine the scientists demonstrated the adenovirus in the Oxford-AstraZeneca is negatively charged, and could attract positively charged proteins like a magnet.
The researchers believe that in a case of ‘mistaken identity’ the body’s immune system considers this platelet cluster as threat and releases antibodies to attack it, clumping together to it and triggering potentially life threatening blood clots.
This condition is called vaccine-induced immune thrombotic thrombocytopenia (VITT).
Professor Alan Parker, an expert in using adenoviruses in medicine from Cardiff University, and who was involved in the study, said: ‘VITT only happens in extremely rare cases because a chain of complex events needs to take place to trigger this ultra-rare side effect.
‘Our data confirms PF4 can bind to adenoviruses, an important step in unravelling the mechanism underlying VITT. Establishing a mechanism could help to prevent and treat this disorder.’
Professor Parker said the team hopes their findings can be used to both better understand the rare side affects of the new Covid vaccines and design better jabs in the future.
Researchers from Arizona State University were also involved in the study and used electro-microscope equipment to take incredible detailed images of the adenovirus used in the Oxford-AstraZeneca vaccine.
A spokeswoman for the company told the BBC: ‘Although the research is not definitive, it offers interesting insights and AstraZeneca is exploring ways to leverage these findings as part of our efforts to remove this extremely rare side effect.’
Other Covid vaccine that use the adenovirus technology include the Johnson and Johnson single dose jab.
The Medicines and Healthcare products Regulatory Agency, which body that examines safety of vaccines in the UK has identified 425 cases of major blood clots in Britons who have had the Oxford-AstraZeneca jab. A total of 73 of these were fatal.
Of the 425 cases, 215 in women and 206 in men, 154 cases related to blood clots in the head and 271 in other parts of the body.
Such blood clots are extremely rare however, with nearly 25 million people having received one dose and 24 million having received their second jab.
Studies have showed the benefits of giving AstraZeneca’s vaccine to 40-49 year-olds outweighed the potential risks, with the jab preventing 1.7 ICU admissions per 100,000 people, compared to the risk of 1.2 blood clots per 100,000 people.
However this risk/benefit calculation swung the other way when it came to younger age groups.
The AstraZeneca jab was pivotal to the UK’s initial vaccine rollout in the closing weeks of 2020, helping it become the most vaccinated nation in the West at the time.
Last week AstraZeneca boss Pascal Soriot suggested the UK’s widespread adoption of the jab, compared to EU nations, could explain why the continent is starting to record higher intensive care rates despite having similar case numbers to Britain.
Mr Soriot told the BBC Radio 4’s Today programme: ‘When you look at the UK there was a big peak of infections but not so many hospitalisations relative to Europe. In the UK this vaccine was used to vaccinate older people whereas in Europe initially people thought the vaccine doesn’t work in older people.
‘T-cells do matter…it matters to the durability of the response especially in older people, and this vaccine has been shown to stimulate T-cells to a higher degree in older people.
‘We haven’t seen many hospitalisations in the UK, a lot of infections for sure…but what matters is are you severely ill or not.’