What do we know about the AstraZeneca vaccine and blood clotting?

This article was published on
March 17, 2021

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April 8, 2021: The European Medicines Agency’s (EMA) Pharmacovigilance Risk Assessment Committee (PRAC) EMA’s safety committee (PRAC) concluded that unusual blood clots with low blood platelets should be listed as very rare side effects of Vaxzevria the AstraZeneca COVID-19 vaccine. 

To reach this conclusion, the committee took into consideration all currently available evidence, which included reviewing 86 cases of blood clot reported in the EU drug safety database (EudraVigilance) as of 22 March 2021, 18 of which were fatal.So far, most of the cases reported have occurred in women under 60 years of age within 2 weeks of vaccination, though specific risk factors have not been confirmed. 

The reported combination of blood clots and low blood platelets is very rare, and the committee reported that the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects.

UK regulators responded more cautiously, and advised that people under the age of 30 be given an alternative to the AstraZeneca vaccine, given that the Joint Committee on Vaccines and Immunisation -- an independent expert advisory committee that advises United Kingdom health departments on immunization -- said on that the risk-to-benefit tradeoff was "more finely balanced" in younger people given their relatively lower risk of dying from COVID-19.

March 28, 2021: Most European countries who temporarily suspended their use of AstraZeneca's COVID-19 vaccine have resumed its usage. The European Medicines Agency will meet on March 29 to further investigate the impacts of the vaccine on blood clots. Scientists around the world are currently investigating the vaccine's side effects and conducting research to determine whether or not the shots may lead to unique side effects relating to blood clots.

March 18, 2021: The European Medicines Agency Pharmacovigilance Risk Assessment Committee stated that "...[t]he vaccine is not associated with an increase in the overall risk of blood clots (thromboembolic events) in those who receive it; there is no evidence of a problem related to specific batches of the vaccine or to particular manufacturing sites; however, the vaccine may be associated with very rare cases of blood clots associated with thrombocytopenia..."

So far, roughly 20 million people in the UK and the European Economic Area have received the vaccine as of March 16, 2021, and the group has reviewed seven cases of blood clots in multiple blood vessels and 18 cases of cerebral venous sinus thrombosis. The group noted "A causal link with the vaccine is not proven, but is possible and deserves further analysis."

March 17, 2021: The European Medicines Agency said its investigation on AstraZeneca's COVID-19 vaccine has found no causal link between the injections and recent reports of blood clots, bleeding, and other similar events in people who have been vaccinated with the drug. Thirteen European Union member states have suspended use of the vaccine until the EMA meets on March 18 to discuss their official findings.

A recent string of blood clotting events in people who had received the AstraZeneca vaccine caused several European nations to halt their use of the shots while they investigated. The European Medicines Agency said that the benefit of the vaccine outweighs the risks and though there have been no direct links between these blood clotting events and the vaccine itself, they will continue to research.

A recent string of blood clotting events in people who had received the AstraZeneca vaccine caused several European nations to halt their use of the shots while they investigated. The European Medicines Agency said that the benefit of the vaccine outweighs the risks and though there have been no direct links between these blood clotting events and the vaccine itself, they will continue to research.


What our experts say

In March of 2021, several nations including the Netherlands, Norway, Denmark, Ireland, and Iceland suspended use of the AstraZeneca COVID-19 vaccine in their countries. The pause is a precautionary measure that was taken in response to potential side effects from the vaccine that were not expected, in comparison to what researchers saw in early clinical trial data. The primary concern listed by these nations was blood clotting (also called 'thrombosis') appearing in adults who had recently received the vaccine in Norway and Denmark. Additionally, reports surfaced that vaccine recipients also experienced lower levels of blood platelets in people under 50 years of age, bleeding under the skin, and a brain hemorrhage in one patient that led to death.

Importantly, there has been no conclusive data linking the vaccine to the deaths of these patients. On March 14, 2020 AstraZeneca stated of the 17 million people who had received the vaccine in the European Union (EU) and the United Kingdom, there has been "...[N]o evidence of an increased risk of pulmonary embolism, deep vein thrombosis (DVT) or thrombocytopenia, in any defined age group, gender, batch or in any particular country.” The company noted that these issues occur frequently in the general population and no evidence has linked these adverse effects with the vaccine itself.

AstraZeneca further elaborated on the number of reports it has seen and the potential links to the injections: "So far across the EU and UK, there have been 15 events of DVT and 22 events of pulmonary embolism reported among those given the vaccine, based on the number of cases the Company has received as of March 8, 2020. This is much lower than would be expected to occur naturally in a general population of this size, and is similar across other licensed COVID-19 vaccines. Furthermore, in clinical trials, the number of clotting events was lower in the vaccinated group of test subjects. There has also been no evidence of increased bleeding in over 60,000 participants enrolled."

The World Health Organization is now reviewing reports of the blood clots in a group of people who received the vaccine. The group's chief scientist, Dr. Soumya Swaminathan, said about the matter "it remains unclear if the vaccine is causing the blood clots." The EU's drug regulation agency has not reported any issues with the vaccine and its side effects nor did any clinical trial data but is also investigating these claims. In fact, most of the countries postponing usage of the AstraZeneca vaccine have not reported any severe blood clotting issues among its vaccine recipients but are halting its use out of an abundance of caution.

From what the data shows now, it is very unlikely that the AstraZeneca vaccine has caused an abundance of blood clots in people receiving the vaccine. But until additional studies on this issues are concluded, we cannot definitely rule out the possibility.

Context and background

When large populations are given vaccines across different ages, gender identities, ethnic backgrounds, and health conditions, it is not surprising to see side effects arise. However, most clinical trials are able to document which side effects occur most commonly in patients due to the large size of the participants in the trial.

Throughout the world it is estimated that one in every four deaths are caused by blood clots, so the potential for these issues to occur in adults is relatively like whether or not they received a vaccine. Until more evidence has been conducted about the cause of these blood clots in people who received the AstraZeneca vaccine, we cannot say whether or not these medical issues were caused by the injections.

Globally, the pause of AstraZeneca's vaccine is a major issue given that it has received emergency approval for use in over 70 countries on six continents. It has also received an emergency use listed by the World Health Organization which may increase the number of countries using the vaccines to 142 countries through the COVAX facility and distribution. If there is a link between the vaccine and blood clots, it could impact billions of people around the world so researchers are approaching it with the highest level of safety and rigorous evidence in mind.


  1. Ireland suspends AstraZeneca vaccine amid blood clot reports (AP)
  2. Netherlands halts use of AstraZeneca COVID-19 vaccine (Reuters)
  3. Update on the safety of COVID-19 Vaccine AstraZeneca (AZ)
  4. Covid: What is the Oxford-AstraZeneca vaccine? (BBC)
  5. Ireland halts use of AstraZeneca vaccine following blood clot reports in Norway (CNN)
  6. Countries Piling In To Suspend AstraZeneca Vaccine Over Blood-Clotting Concerns (Yahoo!)
  7. Global Burden of Thrombosis (AHA)
  8. Explainer: How worried should we be about reports of blood clots and AstraZeneca's vaccine? (Reuters)
  9. Should You Be Concerned About Blood Clots, Bleeding and the AZ-Vaccine? (NYT)
  10. COVID-19 Vaccine AstraZeneca: benefits still outweigh the risks despite possible link to rare blood clots with low blood platelets (European Medicines Society)
  11. A rare clotting disorder may cloud the world's hopes for AstraZeneca's COVID-19 vaccine (Science)
  12. AstraZeneca’s COVID-19 vaccine: EMA finds possible link to very rare cases of unusual blood clots with low blood platelets Share (European Medicines Agency)
  13. In rare instances, AstraZeneca’s Covid-19 vaccine linked to blood clots, regulators say (STAT)

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