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expert reaction to study looking at risk of thrombocytopenia and thromboembolism after COVID-19 vaccination and SARS-CoV-2 infection

A study published in the British Medical Journal (BMJ) looks at risk of thrombocytopenia and thromboembolism after the first dose of COVID-19 vaccine and after a positive SARS-CoV-2 test.

This Roundup accompanied an SMC Briefing


Dr Richard Francis, Head of Research at the Stroke Association said:

“We have known since early in the pandemic that being infected by the COVID-19 virus has led to strokes in some people and it became apparent during the vaccination rollout that the AstraZeneca vaccine slightly increased the risk of an incredibly rare type of stroke. Our charity has supported stroke survivors throughout the pandemic, no matter what their cause of stroke. 

“This new study tells us about the risk of cardiovascular events that could affect the brain in people after the vaccine, or with the virus, in comparison to those that have had neither. It shows that your risk of having a stroke due to catching COVID-19 before vaccination is much greater than your risk of stroke from having the vaccine. 

“The risk of all types of stroke for all groups of people is lower after taking the vaccine; this means that the benefits of the vaccine for stroke risk alone, is greater than the risk. Knowing that the vaccine could save you from the deadly or severely disabling effects of stroke, as well as protect you from getting a nasty bout of COVID-19 should be enough to persuade anyone to get vaccinated.” 


Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice, Immediate past Chair of the BMA Public Health Medicine Committee, said:

“This is a very important paper.

“The authors have rigorously sought evidence for the occurrence and frequency of various adverse events that have been associated with Covid-19 vaccines both in people who have received these vaccines, AND in people who have had Covid-19 infections.

“The fact that such adverse events were identified in vaccine recipients, despite being very uncommon, is testimony to the high quality of our vaccine adverse event surveillance systems.

“Clarifying whether such events are actually caused by vaccination is complex – you have to be clear that the rate of events is higher in people who have been vaccinated than you would expect, given background rates; and the background rates are often not well known.2-4

“So there is the question of whether the adverse event may be more likely to occur in people who are vaccinated. The other important question is – how likely is the event to occur in people who have the disease the vaccine is intended to prevent. It is by no means unusual for vaccines to cause some of the same complications or sequelae as arise following the disease they are intended to prevent – but usually much less frequently. Idiopathic Thrombocytopenic Purpura (ITP), a generally mild and self-limiting clotting disorder, arises extremely rarely following (and as a result of) vaccination against measles, mumps and rubella (MMR vaccination); but it is a relatively common consequence of rubella infection (aka German measles). Given that, without a vaccine programme, most people would catch rubella, the risks of ITP are much greater if you don’t get vaccinated than if you do.

“Similarly, Guillain Barré Syndrome is a rare and usually-self-limiting but sometimes severe autoimmune neurological condition. When it occurs, it often appears to have been triggered by a viral infection. It sometimes also occurs after vaccination; but much less frequently than after the disease that the vaccine aims to prevent.3

“We have known for a long time that common sequelae of Covid-19 and some other infections (like influenza) include clotting disorders and their consequences (e.g heart attacks and strokes).

“So one of the risk assessments that regulators – and people considering vaccination – have to make is not just “are you more likely to have this adverse event after the vaccine”, but “are you more likely to have the disease the vaccine is intended to prevent and have this adverse reaction as a result of the disease than if you get vaccinated”.

“At present we have a very high rate of new cases in the UK (currently 339 per 100,000 population per week and rising5), and the delta variant is extremely infectious, so if you are not vaccinated or already immune, you are very likely to get Covid-19. This paper shows clearly that you are far more likely to suffer these adverse events (which are very rare following vaccination) if you get Covid-19.

“There is one final remaining calculation to be made, and that relates to whether the risks differ between vaccines. The rarity of these adverse events makes it difficult to quantify precisely their frequency after specific vaccines. As we accumulate more data, we will become more confident in our comparisons; and it may be that this will enable us to identify which vaccines (if any) are to be preferred in different categories (age, sex, etc.) of recipient with increased confidence.”

  1. Hippisley-Cox J, Patone M, Me XW, Saatci D, Dixon S, Khunti K, et al. Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study. BMJ 2021;374:n1931. (
  2. English PMB. Adverse events associated (correctly or otherwise) with vaccination, – some historical context; and a Scandinavian decision. Peter English’s random musings [blog]. Selected Covid-19 vaccine Q&As 2021; Updated 19 Jan 2021; Accessed: 2021 (23 Mar): (
  3. English PMB. How should we react to reports of adverse events following vaccination? Peter English’s random musings [blog]. Selected Covid-19 vaccine Q&As 2021; Updated 19 Jan 2021; Accessed: 2021 (23 Mar): (
  4. World Health Organisation Global Advisory Committee on Vaccine Safety. Causality assessment of adverse events following immunization. Weekly epidemiological record 2001(12):85-89. ( or ).
  5. Cases in United Kingdom Coronavirus (COVID-19) in the UK (“Covid Dashboard”) 2021; Updated 25 Aug 2021; Accessed: 2021 (26 Aug): (



Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study’ by Julia Hippisley-Cox et al. was published in the BMJ at 00:01 UK time Friday 27 August.

DOI: 10.1136/bmj.n1931



All our previous output on this subject can be seen at this weblink:



Declared interests

Dr Peter English: “Dr English is on the editorial board of Vaccines Today: an unpaid, voluntary, position. While he is also a member of the BMA’s Public Health Medicine Committee, this comment is made in a personal capacity. Dr English sometimes receives honoraria for acting as a consultant to various vaccine manufacturers, most recently to Seqirus.”

None others received.


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