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Very rare cases of blood clots linked to the Oxford-AstraZeneca COVID-19 vaccine are well-known and have been publicly recognized since 2021

Posted on:  2024-05-10

Key takeaway

The Oxford-AstraZeneca COVID-19 vaccine was one of the few tools available to reduce the risk of serious illness and death at the initial stages of the COVID-19 pandemic. The vaccine underwent rigorous testing to ensure it was safe and effective. In 2021, a rare but serious condition involving blood clots with low platelet level—known as thrombosis with thrombocytopenia—was associated with the vaccine. However, the chance that this condition will occur after vaccination is very low, and the benefits of vaccination outweigh this small risk.

Reviewed content


AstraZeneca admits for the first time that its COVID-19 vaccine can cause blood clots

Source: The Blaze, The Independent, The Washington Times, Dan Bongino, Liz Wheeler, 2024-04-28

Verdict detail

Misleading: AstraZeneca’s admission that its COVID-19 vaccine can cause very rare cases of blood clots is only “a first” in court. It doesn’t imply that the company previously concealed this side effect.
Lack of context: Public statements from AstraZeneca, public health authorities, and multiple scientific publications demonstrate that blood clots have been acknowledged as a very rare side effect of the Oxford-AstraZeneca COVID-19 vaccine since 2021.

Full Claim

“In a first, AstraZeneca admits its Covid vaccine can cause rare blood clots”; “AstraZeneca vindicates skeptics with admission that its COVID-19 vaccine can cause blood clots”


A claim that the pharmaceutical company AstraZeneca “admitted” for the first time that its COVID-19 vaccine can cause blood clots went viral on social media platforms in late April and early May 2024.

One example is an Instagram reel in which political commentator Liz Wheeler claimed, “people have died as a result of this shot. But we knew it”. In a Facebook post linking to a video with over 30,000 views on Rumble, political commentator Dan Bongino referred to this alleged admission as “SHOCKING”.

Similar headlines appeared in outlets such as the Daily Mail, Blaze Media, The Independent, and The Washington Times.

Many of these publications are based on a 28 April 2024 article in The Telegraph reporting on a U.K. class-action lawsuit against AstraZeneca. According to The Telegraph, the lawsuit involved 51 claims that the Oxford-AstraZeneca COVID-19 vaccine is “defective” and causes “death and serious injury” due to a rare but potentially fatal blood clotting condition called thrombosis with thrombocytopenia syndrome (TTS).

In February 2024, AstraZeneca sent legal documents to the High Court of Justice in London contesting the claims but acknowledging that its COVID-19 vaccine “can, in very rare cases, cause TTS”, The Telegraph reported.

While this might be the first time AstraZeneca acknowledged TTS in court, the posts are misleading because this admission isn’t new. As we will show below, multiple documents and studies demonstrate that TTS has been a publicly recognized side effect of the Oxford-AstraZeneca COVID-19 vaccine since 2021. Moreover, cases of TTS following vaccination are very rare, making this risk extremely small compared to the benefits of vaccination like reducing the risk of serious illness and death.

Rare cases of blood clots have been a recognized side effect of the Oxford-AstraZeneca COVID-19 vaccine since 2021

The Oxford-AstraZeneca COVID-19 vaccine is a viral vector vaccine. Public health authorities, researchers, and AstraZeneca itself have publicly recognized TTS as a very rare side effect of viral vector COVID-19 vaccines, like the Oxford-AstraZeneca, Johnson and Johnson (Janssen), CanSino, and Sputnik V vaccines since 2021.

First approved in the U.K. on 30 December 2020, the Oxford-AstraZeneca COVID-19 vaccine has been marketed worldwide under the brand names Vaxzevria and Covishield. Within the first year of its rollout, the number of vaccine doses administered exceeded 50 million in the U.K. and 2.5 billion globally. An analysis by the UK-based data and analytics company Airfinity estimated that the vaccine saved 6.3 million lives globally during that first year.

However, when such a large number of people receive a vaccine over a short period, rare side effects that are too infrequent to be detected in clinical trials can emerge. This is because the size of clinical trials is never going to be large enough for detecting adverse events that occur at a rate of one per 100,000 or one per million people.

In April 2021, the European Medicines Agency and the World Health Organization (WHO) acknowledged a possible link between TTS cases and the Oxford-AstraZeneca COVID-19 vaccine. Other public health authorities and medical associations in Canada, Australia, the U.K., and the U.S. issued similar statements throughout 2021, in some cases using the term vaccine-induced immune thrombotic thrombocytopenia (VITT)—which refers to TTS cases specifically caused by vaccination.

The fact the TTS has only been associated with the viral vector but not with the mRNA COVID-19 vaccines suggests that this side effect might be related to the vaccine platform rather than to the COVID-19 vaccines in general. The precise mechanism leading to TTS isn’t well understood.  However, there is evidence that the adenovirus binding to a protein released from platelets called platelet factor 4 (PF4) might trigger an abnormal immune response to vaccination in some individuals[1,2].

TTS has been listed in the Oxford-AstraZeneca COVID-19 vaccine inserts since mid-2021. Examples of these inserts are publicly available on the WHO and the U.K. government websites. For example, the information for U.K. recipients stated:

“Extremely rare cases of blood clots with low levels of blood platelets (thrombosis with thrombocytopenia syndrome) have been observed following vaccination with COVID-19 Vaccine AstraZeneca […] Some cases were life-threatening or had a fatal outcome. It is important to remember the benefits of vaccination to give protection against COVID-19 still outweigh any potential risks.”

At least two documents published by AstraZeneca in 2021 show that the company also recognized TTS as a potential side effect of vaccination. One example is a September 2021 press release regarding a settlement agreement with the European Commission over vaccine supply, in which AstraZeneca stated:

“Incidents of thrombosis with thrombocytopenia (TTS) have been reported in a small number of people. Early diagnosis allows appropriate treatment of these events, and there is no elevation of the risk of TTS at the second dose, compared to the rates expected in the general population.“

AstraZeneca also referred to TTS in a real-world evidence summary from October 2021:

“Incidents of an extremely rare blood disorder, thrombosis with thrombocytopenia (TTS) have been reported in a small number of people who have received the vaccine (2.3 events per million after second dose compared with 8.1 events per million after the first dose). NICE [U.K. National Institute for Health and Care Excellence] and UN recommendations state that TTS in these patients are treatable. The likelihood of developing very rare TTS events following the second dose are comparable to those seen in an unvaccinated population and are several- fold lower than after the first dose.”

Multiple scientific studies published since 2021 further show that cases of TTS associated with the Oxford-AstraZeneca vaccine weren’t concealed. On the contrary, this association has been extensively investigated since this potential risk was first identified[3-7].

The likelihood of developing blood clots following COVID-19 vaccination is very small

The fact that a particular vaccine side effect is acknowledged in a court doesn’t mean that the side effect wasn’t previously recognized, nor does it make it more dangerous.

All vaccines can cause side effects, and COVID-19 vaccines aren’t an exception. However, it’s important to weigh these side effects against the benefits of vaccination, such as the reduced risk of serious illness and death.

When TTS was initially flagged as a potential side effect of the Oxford-AstraZeneca COVID-19 vaccine, several countries paused the administration of this vaccine as a precautionary measure. However, many resumed using this vaccine after data showed that such cases were infrequent.

The WHO’s 2023 guidance for managing vaccine-associated TTS explains that most cases occur within 3 to 30 days following vaccination with a viral vector COVID-19 vaccine, generally after the first dose. The rate of TTS ranges from 0.5 to 6.8 cases per 100,000 people vaccinated, depending on the population. It’s higher in younger adults compared to older adults and in women compared to men.

In other words, the occurrence of TTS following COVID-19 vaccination is extremely rare. A 2021 BBC explainer showed that the risk of serious harm due to vaccine side effects is comparable to that of dying in a road accident and at least ten times lower than that of dying due to an accident or injury.

It is also important to highlight that not any TTS that happens after vaccination with a viral vector is necessarily caused by the vaccine. As reported by The Telegraph, AstraZeneca’s court documents stated, “TTS can also occur in the absence of the AZ vaccine (or any vaccine). Causation in any individual case will be a matter for expert evidence”.

Public health regulators in the European Union and countries like the U.K. and Australia agree that the benefits of COVID-19 vaccination outweigh the small risk of blood clots, particularly when the circulation of the virus is high. But to further minimize this risk, many countries, including Spain, Belgium, Italy, and the U.K., restricted the Oxford-AstraZeneca COVID-19 vaccines to older adults and offered young adults alternative vaccines.

On 8 May 2024, AstraZeneca announced it would withdraw its COVID-19 vaccine worldwide. The company cited a “surplus of available updated vaccines”, which led to a decline in demand, as the primary reason for the withdrawal. An article published in The Conversation analyzed how this decline in demand might have been at least partially influenced by misinformation about the risks and benefits of the Oxford-AstraZeneca COVID-19 vaccine throughout the pandemic.


The Oxford-AstraZeneca COVID-19 vaccine was an invaluable tool for reducing the risk of serious illness and death during the initial stages of the COVID-19 pandemic. But like any other vaccine, it also came with the risk of side effects.

Contrary to viral claims, a document submitted to court by AstraZeneca is by no means the first time its vaccine was acknowledged to cause rare but serious cases of blood clotting. Such cases are well documented and recognized by public health authorities, researchers, and the company itself since 2021.


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