Statement from the International Society on Thrombosis and Haemostasis on Reports Indicating Blood Clots Associated with the AstraZeneca Vaccine

Earlier this week, the European Medicines Agency (EMA) and UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) completed their assessments of extremely rare blood clotting events associated with low platelet counts that have occurred in the over 34 million people vaccinated with AstraZeneca’s COVID-19 vaccine in the UK and EU. In step with those assessments, both regulatory agencies have concluded that unusual blood clots with low blood platelet counts should be listed as a very rare side effect of the vaccine.

The International Society on Thrombosis and Haemostasis (ISTH) concurs with the recent statements issued by these regulatory bodies based on the careful analysis of reports indicating blood clots in unusual sites and thrombocytopenia, a low platelet count, in association with the AstraZeneca vaccine. Fortunately, these events appear to be exceedingly rare. Although the cause of these findings remains uncertain, an immune response that triggers activation and consumption of platelets and subsequent clotting events has been shown in some patients. The ISTH cautions that more rigorous scientific studies need to be undertaken to clearly determine if and how the vaccine causes these events, how they can best be diagnosed and to recommend the optimal treatments for them.

Since the first cases were reported earlier in the year, members of the ISTH in countries around the world have worked tirelessly and contributed greatly to our scientific understanding about why thrombosis occurs in combination with thrombocytopenia, and in some cases with bleeding. We applaud and support the ongoing efforts of the scientific community as they work to care for patients and understand and determine the underlying causes and optimal diagnostic and treatment protocols.

With new data continuously emerging, the ISTH and its members are in the process of developing consistent recommendations based on the latest science on the definition, diagnosis, and management of these cases to provide better guidance to treating physicians. Additionally, the ISTH recognizes the following statements that have been recently developed and issued by experts in some countries to provide interim guidance based on currently available data.

As a reminder, COVID-19 is associated with a risk of hospitalization and death. The reported combination of blood clots and low blood platelets is exceedingly rare, and the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of thromboembolic side effects.

Information for the General Public

  • Cases of unusual blood clots with low platelet counts have occurred in a small number of people who received the AstraZeneca vaccine.
  • The chance of having this occur is extremely low, but you should still be aware of symptoms so that you can get prompt medical treatment to help recovery and avoid complications.
  • If you have a history of clotting, you should not stop medications or change drug schedules unless it’s advised by your physician.
  • You must seek urgent medical attention if you have any of the following in the first few weeks after your vaccination:
    • New and unusual neurological symptoms, such as severe and persistent headache or blurred vision
    • persistent abdominal (belly) pain
    • shortness of breath
    • chest pain
    • leg pain or swelling
    • tiny blood spots under the skin beyond the site of the injection
  • Speak to your healthcare professional or contact your relevant national health authorities if you have any questions about the roll out of COVID-19 vaccines in your country.

Information for Healthcare Professionals

  • EMA and MHRA have reviewed unusual cases of thrombosis in combination with thrombocytopenia, and in some cases bleeding, in people who received the AstraZeneca vaccine.
  • These very rare types of thrombosis (with thrombocytopenia) included venous thrombosis in unusual sites such as cerebral veins and splanchnic veins as well as arterial thrombosis. Most of the cases reported so far have occurred in women under the age of 60 years and have occurred within days of the person receiving their first dose of the vaccine. There is limited experience with the second dose. Currently, no specific risk factors have been identified.
  • Emerging evidence from studies performed by ISTH members suggests that the vaccine triggers an immune response leading to an atypical heparin-induced thrombocytopenia like disorder.
  • The diagnosis is established by the finding of a low platelet count and antibodies against platelet factor 4 often associated with an elevated D-dimer level.
  • Treatment with intravenous immunoglobulin and non-heparin anticoagulants is recommended. Platelet transfusion should be avoided.
  • Healthcare professionals should be alert to the symptoms and signs of thromboembolism and thrombocytopenia so that they can promptly treat people affected in line with available guidelines.
  • There is currently no evidence that any pre-medications should be given before the vaccine to avoid thrombocytopenia or thrombotic events.
  • Healthcare professionals should tell people receiving the vaccine that they must seek medical attention if they develop:
    • new and unusual neurological symptoms such as severe and persistent headache and blurred vision
    • symptoms or signs of blood clots such as shortness of breath, chest pain, leg pain or swelling, or persistent abdominal pain
    • petechiae
  • The benefits of the vaccine continue to outweigh the risks for people who receive it. The vaccine is effective at preventing COVID-19 and reducing hospitalization and death.
  • Patients with a prior history of blood clots or with a history of thrombophilia should continue to be vaccinated.
  • National authorities may provide additional guidance on the roll out of the vaccine based on the situation in your country.