As a precaution, national health authorities may sometimes temporarily suspend the use of a vaccine batch or a vaccination campaign in the course of the investigation of a severe adverse event or a cluster of adverse events following immunization. In most cases, the investigation will lead to a different explanation and a causal relation between the event and the vaccine will not be found.
Concretely, the detection of a series of thromboembolic cases coincidental in time with vaccination in Austria, in Denmark, or elsewhere in the first quarter of 2021 led to the opening of an investigation and a precautionary suspension of the use in a few countries of the vaccine.
The Pharmacovigilance Risk Assessment Committee of the European Medicines Agency (EMA) taking into account all available evidence and advice concluded that a causal relationship between vaccination with the Oxford-AstraZeneca (Vaxzevria) vaccine and very rare cases of thrombosis together with thrombocytopenia, sometimes accompanied by bleeding, is plausible.
The reported thromboses with thrombocytopenia include venous thrombosis, also in unusual sites such as cerebral venous sinus thrombosis (where blood clots in the brain's venous sinuses prevent blood from draining out of the brain) and splanchnic vein thrombosis (which involves one or more veins in the abdomen), as well as arterial thrombosis. Although such side effects are very rare, the reported case numbers exceeded what is seen in the general population. The majority of these cases occurred within 14 days after vaccination and mostly in women under 60 years of age; some cases had a fatal outcome.
Based on the available data, no specific risk factors were identified. The product information for Vaxzevria has been updated accordingly, and specifies thrombosis in combination with thrombocytopenia as a new very rare side effect (occurring in less than 1 in 10,000 persons).
One plausible explanation for the combination of blood clots and low blood platelets is an immune response, leading to a condition similar to one seen sometimes in patients treated with heparin (heparin induced thrombocytopenia).
Both EMA and the Global Advisory Committee on Vaccine Safety) GACVS clearly state that the benefits of Vaxzevria in preventing COVID-19 continue to outweigh the risks; and there are no recommended changes regarding the use of this vaccine.