Found the reason why AstraZeneca vaccine causes blood clots

Scientists have a new explanation for why adenovirus-based Covid-19 vaccines - like those from AstraZeneca and Johnson & Johnson - carry a risk of serious blood clots.

Rare blood clots linked to some early Covid-19 vaccines (no longer in use), the AstraZeneca and Johnson & Johnson (J&J) vaccines , may have been the result of two out-of-control immune responses occurring at the same time.

One of these immune responses was already known, but the second, published in the journal Blood , was a new discovery.

The discovery helps explain how blood clotting develops and points to better treatments, as well as suggesting ways to create safer vaccines for people vulnerable to side effects.

Covid-19 vaccine produced by AstraZeneca has a risk of rare blood clotting disorder - (Photo: LIVE SCIENCE).

A rare side effect of the vaccine

Vaccine-associated blood clotting disorders, called vaccine-induced immune thrombocytopenia (VITT) , are rare and have been linked to two Covid-19 vaccines, Johnson & Johnson (J&J) and AstraZeneca.

Both shots contain a common cold virus called an adenovirus that has been modified so that it cannot infect cells.

These viruses are modified by carrying part of the DNA of the SARS-CoV-2 virus, the coronavirus that causes Covid-19, into the body.

Unlike the Moderna and Pfizer-BioNTech Covid-19 shots, which contain RNA, the J&J and AstraZeneca vaccines do not require ultra-cold storage, making them more accessible in areas where storage is difficult.

Soon after the J&J and AstraZeneca vaccine campaigns, doctors began reporting cases of blood clots — much like a previously known disorder called heparin-induced thrombocytopenia (HIT).

About 20 to 30 years ago, HIT affected 3.5 percent of patients who had hip or knee replacements, says Dr. Andreas Greinacher, a specialist in blood clotting disorders at the University Hospital Greifswald in Germany. In these patients, heparin—a blood thinner typically used to prevent blood clots—actually caused rapid clotting.

Adenovirus-based Covid vaccines cause a similar condition to HIT, although scientists have given it a new acronym VITT to reflect its different origins.

About 1 in 50,000 people under 50 who received the vaccine developed a blood clot, and about 1 in 100,000 people 50 and older did, the researchers reported.

'Our big goal now is to figure out what element in the vaccine is causing this,' explains Dr Greinacher . 'Once we do, I'm sure there are very clever biotechnologists who can modify the adenovirus vector so that this element is no longer present.'

Clarifying VITT

VITT was first observed in patients vaccinated against Covid-19 in February 2021.

Scientists soon discovered that it involved PF4 , a chemical signal released by platelets, which causes blood cells to form clots.

In rare cases, after receiving an adenovirus-based vaccine, the body will produce antibodies against PF4. These antibodies will attach to PF4 and form clumps, which can then bind to receptors called Fc on other platelets. This will activate the platelets and lead to a rapid clotting response.

New blood research shows that PF4 alone also activates a second set of receptors that cause platelets to accumulate . This may be the second reason why blood clotting goes awry.

The new research suggests that there are actually two different ways PF4 works in VITT, Dr. Nazy said. These two pathways are not exclusive and can work in parallel.

Phillip Nicolson, clinical associate professor of cardiovascular medicine at the University of Birmingham in the UK and lead author of the new study, told Live Science : 'What we've shown is that in addition to that antibody activator, the body itself has PF4 that binds to platelets and activates them, giving it a double effect. That may be why clotting occurs at harmful levels.'

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