WHO Statement: Update on AstraZeneca COVID-19 Vaccine

The following statement was published on April 16, 2021 by the WHO Global Advisory Committee on Vaccine Safety in response to the latest review of rare adverse blood coagulation events with AstraZeneca COVID-19 Vaccine (Vaxzevria and Covishield).


A very rare new type of adverse event called Thrombosis with Thrombocytopenia Syndrome (TTS), involving unusual and severe blood clotting events associated with low platelet counts, has been reported after vaccination with COVID-19 Vaccines Vaxzevria and Covishield. A specific case definition for TTS is being developed by the Brighton Collaboration1. This will assist in identifying and evaluating reported TTS events and aid in supporting causality assessments.

The biological mechanism for this syndrome of TTS is still being investigated. At this stage, a ‘platform specific’ mechanism related to the adenovirus-vectored vaccines is not certain but cannot be excluded. Ongoing review of TTS cases and related research should include all vaccines using adenoviral vector platforms. The GACVS noted that an investigation has been initiated into the occurrence of TTS following the Johnson & Johnson vaccine administered in the United States. The TTS syndrome has not been linked to mRNA-based vaccines (such as Comirnaty or the Moderna mRNA-1273 vaccine)...

Read Full Statement Here

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