'Trigger' for rare blood clots linked to AstraZeneca vaccine identified by scientists

Researchers believe a chain reaction sparked in the immune system can cause blood clots

Experts have called for further studies on the cause of blood clots linked to the Oxford-AstraZeneca shot. Reuters

Scientists believe they may have found the “trigger” behind the rare blood clots stemming from the Oxford/AstraZeneca Covid-19 vaccine.

An international team of researchers from the UK and US found the interaction between the vaccine and a protein known as platelet factor 4 could be behind the cases of thrombosis with thrombocytopenia syndrome.

Researchers believe this may spark a chain reaction in the immune system that can culminate in blood clots – a condition known as vaccine-induced immune thrombotic thrombocytopenia (VITT).

“VITT only happens in extremely rare cases because a chain of complex events needs to take place to trigger this ultra-rare side effect," said Prof Alan Parker, from the School of Medicine at Cardiff University.

“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.

“We hope our findings can be used to better understand the rare side effects of these new vaccines – and potentially to design new and improved vaccines to turn the tide on this global pandemic."

Prof Parker told the BBC the adenovirus had an “extremely negative surface” that fit well with platelet factor 4’s “extremely positive” components.

He emphasised the rarity of the side effects in comparison to the overall number of vaccine doses administered to people.

"You could never have predicted it would have happened and the chances are vanishingly small, so we need to remember the bigger picture of the number of lives this vaccine has saved," Prof Parker said.

Dr Will Lester, a consultant haematologist at University Hospitals Birmingham NHS Trust, commended researchers but told the BBC that “many questions still remain unanswered”.

He said further studies were needed to determine “whether some people may be more susceptible than others" and why "the thrombosis [clotting] is most commonly in the veins of the brain and liver”.

The side effects prompted several countries to suspend the use of the vaccine in their inoculation programmes.

Scientists from AstraZeneca also took part in the research, which was published in the Science Advances journal.

“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," a spokeswoman for the company told the BBC.

Previously, a German researcher suggested an ingredient in the vaccine may cause blood vessels to leak, sparking a chain reaction in the immune system.

The hypothesis by Dr Andreas Greinacher of the University of Greifswald generated headlines around the world after vaccination campaigns were suspended because some people who took the vaccine suffered blood clots, some of which were fatal.

Finding the cause could offer several benefits, experts said. It could allow researchers to amend the vaccine to prevent the clotting risk and help doctors to treat patients who suffer the condition.

Meanwhile, the University of Oxford said it could update its vaccine quickly “if it should be necessary” amid growing concerns about the Omicron variant.

Academics at the university, who pioneered the vaccine developed by AstraZeneca, said the “necessary tools and processes” were in place if the shot needed to be tweaked.

But the university emphasised the vaccine continued to provide high levels of protection against Covid-19 despite the appearance of coronavirus variants such as Delta.

“Due to the very recent discovery of the new B.1.1.529 [Omicron] strain of coronavirus, there are limited data available at this time," a university representative said.

“As with any new variant, we will carefully evaluate the implications of the emergence of B.1.1.529 for vaccine immunity.

“Despite the appearance of new variants over the past year, vaccines have continued to provide very high levels of protection against severe disease and there is no evidence so far that Omicron is any different.

“However, we have the necessary tools and processes in place for rapid development of an updated Covid-19 vaccine if it should be necessary."