Expert reaction to latest weekly summary of Yellow Card reporting following COVID-19 vaccination published by the MHRA*, including updated numbers of cerebral venous sinus thrombosis (CVST) and other thrombosis events with low platelets

Expert reaction to latest weekly summary of Yellow Card reporting following COVID-19 vaccination published by the MHRA*, including updated numbers of cerebral venous sinus thrombosis (CVST) and other thrombosis events with low platelets


Prof Adam Finn, Professor of Paediatrics, University of Bristol, said:

“It’s clear from the level of detail in the many pages of the MHRA weekly analysis print that the UK safety reporting system is extremely thorough. Given the level of interest and concern around rare serious blood clotting events associated with low platelet counts in several European countries over recent days and weeks, the MHRA report highlights information on such cases among the large number of other diverse events being reported to the UK Yellow Card scheme. The report states that these cases are being very carefully investigated to better understand whether or not they may have any causal relationship with vaccination. Nevertheless, the extreme rarity of these events in the context of the many millions of vaccine doses that have been administered means that the risk-benefit decision facing people who are invited to receive COVID19 vaccines is very straight forward: receiving the vaccine is by far the safest choice in terms of minimising individual risk of serious illness or death.”


Prof Sir David Spiegelhalter, Chair, Winton Centre for Risk and Evidence Communication, University of Cambridge, said:

“No medical intervention is ‘safe’, and the balance of benefit to risk is crucial. The MRHA has now identified 30 serious thrombotic events with low platelets out of 18 million AZ vaccinations, so the average risk of less than 1 in 500,000 is low.

“But it is important to know whether this risk is substantially higher in younger age groups, as has been found elsewhere. Vaccinating younger people is as much for the wider community as for their direct benefit, and so it is reasonable to try to avoid them causing harm, even if extremely rarely.

“It is vital that the vaccination roll-out is not delayed. As Sir Patrick Vallance showed last week, even at the current low levels of the virus, a month’s delay in vaccinating 500,000 people between 44-54 would be expected to lead to around 85 severe cases requiring hospitalisation, of which perhaps 5 would die.”


Prof David Werring, Professor of Clinical Neurology, UCL Institute of Neurology, UCL, said:

“Although the number of people diagnosed in the UK with cerebral venous thrombosis after receiving the Astra Zeneca COVID-19 vaccine has increased to 22 cases, this is among 18 million receiving the vaccine. So the absolute risk of CVST after this vaccine remains extremely low (about 1.5 per million) and it’s not clear if this is any higher than the usual expected incidence of CVST (probably around 5 to 15 per million people per year, though the figures vary as it can be difficult to diagnose in some cases).

“However, emerging evidence suggests that some cases of post-vaccination CVST have unusual features, including low blood platelets, male sex (CVST is typically more common in females), a usually rare type of antibody to platelet-factor 4 (PF-4), and a high risk of severe clots. This raises the possibility that the vaccine could be a causal factor in these rare and unusual cases of CVST, though we don’t know this yet, so more research is urgently needed.

“CVST can reduce drainage of blood into the cerebral veins, leading to a rare type of stroke. Common symptoms of CVST include severe, often progressive, headache over hours to days, sometimes with seizures or stroke symptoms (such as weakness of the face, arm or leg, or disturbances of vision or speech). It is usually treated with the anticoagulant drug heparin, but if anti-PF4 antibodies are present, heparin could make things worse so is not recommended. Severe CVST can increase pressure inside the skull (often causing drowsiness), which can sometimes be treated with neurosurgery.

“Like most medical interventions, vaccines can have side effects as well as benefits. The key thing to remember is how rare these brain clots are, and how powerful the proven benefit of vaccination is against COVID. More research is needed, as it remains uncertain whether the vaccine is causing these unusual CVST. However, if within a month of receiving the vaccine, a person develops a headache that is severe and of rapid or progressive onset (hours to days), with symptoms of stroke, seizures, drowsiness or confusion then CVST needs to be considered. Doctors and other health professional need to be aware of the possible link, and know that if the unusual PF4 antibodies are found the usual treatment, heparin, might be hazardous, so should be avoided. It remains crucial that all suspected new cases of CVST after vaccination continue to be reported to the MHRA through the yellow card system.”




Declared interests

Prof Adam Finn: “AF is an investigator in trials and studies of several COVID19 vaccines including Oxford-AZ, Pfizer, Janssen and Valneva and advises the UK government and the WHO on COVID19 and other vaccines. He receives no personal income for this work and is remunerated solely through his employment by the University of Bristol.”

Prof Sir David Spiegelhalter: “No COIs.”

None others received.