On 7 April 2021, the European Medicines Agency (EMA) announced the results of an investigation that suggest a possible link between the AstraZeneca Covid-19 vaccine and very rare cases of blood clots.
By 16 March 2021, around 20 million people in the UK and EEA had received the vaccine, and by 18 March 2021, the EMA had reviewed 25 cases of blood clots.
Investigations at the time found that the overall the number of blood clots reported after vaccination, both in studies before the vaccine was licensed and in reports after rollout of vaccination campaigns, was lower than that expected in the general population. The EMA did not rule out a link between the blood clots and the vaccine, but the numbers did not suggest a significant link was likely.
More recently, an EMA committee carried out an in-depth review of 86 cases of blood clots, of which 18 were fatal. The cases were reported in the UK and EEA where around 25 million people had then received the vaccine. This is an incidence rate of 3.4 per million and a death rate of 0.7 per million. The blood clots occurred in the brain, abdomen and arteries and were mostly found in women under 60.
The recent review found that those suffering from these rare blood clots also have low blood platelets. According to the committee, one plausible explanation for the combination of blood clots and low blood platelets is an immune response that leads to a condition similar to one seen sometimes in patients treated with heparin, a blood thinner. However, the investigation was unable to identify any specific risk factors.
The EMA reminds healthcare professionals and people receiving the vaccine to remain aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within 2 weeks of vaccination. It says that patients should seek medical assistance immediately if they have the following symptoms:
- shortness of breath
- chest pain
- swelling in your leg
- persistent abdominal (belly) pain
- neurological symptoms, including severe and persistent headaches or blurred vision
- tiny blood spots under the skin beyond the site of injection
The EMA also stresses that Covid-19 is associated with a risk of hospitalisation and death. The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of the vaccine in preventing Covid-19 outweigh the risks of side effects, wrote the EMA.
EMA press release (in English)