What is thrombosis, the rare side effect that the AstraZeneca COVID vaccine can cause

Studies revealed in 2021 and 2022 a link between the AstraZeneca vaccine and thrombosis syndrome with thrombocytopenia (REUTERS/Dado Ruvic/Illustration)

Pharmaceutical giant AstraZeneca admitted before British courts that its COVID-19 vaccine can cause, in very rare cases, thrombocytopenia thrombosis syndrome (TTS). The laboratory’s statement occurred within the framework of a class action lawsuit in the United Kingdom, promoted by victims who attribute serious injuries and health damage resulting from the application of the vaccine developed in collaboration with the University of Oxford.

AstraZeneca had previously indicated the possibility of this adverse effect in 2021, through a publication in the journal The Lancet of the study titled “Very rare thrombosis with thrombocytopenia after the second dose of AZD1222: an analysis of the global safety database”, carried out by scientists from the laboratory itself.

There, they noted that “since the launch of the COVID-19 vaccine, very rare cases of TTS, which is known as vaccine-induced immune thrombotic thrombocytopenia (VITT), have been reported.”

Now, the pharmaceutical giant once again assured in the United Kingdom Court that its COVID vaccine shows an acceptable safety profile, backed by global regulators who confirm that the benefits of vaccination outweigh the risks associated with extremely rare side effects.

AstraZeneca noted that product information related to the vaccine was updated in April 2021, with approval from the UK regulator, to include “the possibility that the AstraZeneca-Oxford vaccine may be capable, in very rare cases, of being a TST trigger” (Europa Press)

Thrombocytopenia thrombosis syndrome (TTS) consists of the formation of blood clots accompanied by a low platelet count.

The United States National Institutes of Health (NIH) describes that platelets, also known as thrombocytes, are essential cells for coagulation, since when a person is injured, clots are responsible for sealing the blood vessels. wounds.

A platelet count of less than 150,000 increases the risk of bleeding, which can occur both internally and on the surface of the skin.

The severity of hemorrhages increases as the platelet count decreases and can be life-threatening, especially if they occur in the brain.

Early detection and treatment are key to preventing serious complications associated with this condition.

In the United Kingdom, a group of families argue that the inoculant caused a serious adverse reaction in several people (REUTERS/Yves Herman)

The vaccine developed by the University of Oxford and AstraZeneca is based on the use of an adenovirus (specifically a chimpanzee common cold virus). This modified adenovirus acts as a vehicle to deliver the antigen to the human body without causing disease.

A study published in the American journal Science Advances In December 2021, it had already described that the interaction between the surface of the adenovirus used in the vaccine and platelet factor 4 (PF4) present in human blood can, in rare cases, cause an erroneous immune response.

In some rare cases, the immune system may mistake PF4 for the virus and release antibodies in response. The antibodies would cluster with PF4 and end up leading to blood clots.

The researchers described the process in which small amounts of the vaccine’s ChAdOx1 adenovirus enter the bloodstream through small lesions in the capillaries caused by the injection. This can lead to the formation of a complex between ChAdOx1 and PF4, potentially triggering clot formation.

Importantly, this process requires several intermediate steps before clots form, indicating that the mechanism is not direct or inevitable, but rather the product of an unfortunate and rare chain of events. “

“What we have is the trigger, but there are many steps that must happen next,” clarified Professor Alan Parker, one of the researchers at Cardiff University, in charge of the study published in Science Advances

Following the 2021 investigation, the company noted that although a potential mechanism has been identified, it does not conclusively establish that it is the cause of rare cases of thrombosis. Furthermore, he emphasized that the presence of PF4 antibodies does not automatically lead to the development of clots in most individuals.

And they stressed that the risk associated with the formation of blood clots is significantly higher in cases of COVID-19 infection compared to the administration of any vaccine.

“A new type of very rare adverse event, known as thrombosis syndrome with thrombocytopenia, has been reported after administration of this vaccine. “This syndrome involves rare and severe blood clotting disorders associated with low platelet counts,” the WHO said of the AstraZeneca vaccine (Yuqi Zhou).

Bleeding is responsible for the main symptoms of thrombocytopenia. The signs in the body can appear suddenly or over the course of days. Mild thrombocytopenia usually has no symptoms and is therefore often only detected through a routine blood test.

The United States NIH details that signs of thrombocytopenia may include:

• bleeding that lasts a long time, even from small wounds
• petechiae, which are small, flat red spots under the skin caused by blood loss from blood vessels
• purpura, which is bleeding on the skin that can cause red, purple, or yellow-brown spots
• nose or gum bleeding
• blood in your urine or stool, which may look like red or dark tarry blood
• heavy menstrual bleeding

“TTS can also occur in the absence of the AstraZeneca vaccine (or any vaccine). Causation in any individual case will be a matter of expert evidence,” the laboratory said before the British court (Europa Press)

“TTS is a serious and potentially fatal adverse event,” noted the World Health Organization (WHO) in its “Guideline for the management of clinical cases of TTS after vaccination to prevent COVID-19 disease,” updated in April 2023.

Scientific studies and the WHO Global Advisory Committee on Vaccine Safety corroborated the existence of thrombosis syndrome with thrombocytopenia after the administration of the AstraZeneca vaccine, although they also highlighted that the events “are very rare.” The Committee said: “A new type of very rare adverse event, known as thrombosis syndrome with thrombocytopenia, has been reported following administration of this vaccine. This syndrome involves serious and rare blood coagulation disorders associated with low platelet counts.”

“In countries where SARS-CoV-2 transmission is currently recorded, the benefits of vaccination far outweigh the risks, given the protection conferred by the vaccine against COVID-19,” declared WHO experts.

According to The Telegraph newspaper, lawyer Sarah Moore, a partner in the Leigh Day study, which is part of the lawsuit in the United Kingdom, said that “it has taken AstraZeneca a year to formally admit that its vaccine can cause devastating blood clots, when this fact has been widely accepted by the clinical sector” since the end of 2021.

For its part, in a statement the pharmacist said: “Our solidarity is with anyone who has lost loved ones or reported health problems. “Patient safety is our top priority and regulatory authorities have clear and strict standards to ensure the safe use of all medicines, including vaccines.”