COVID-19 vaccine thrombosis update


Barts-MS rose-tinted-odometer: ★★

Last week I heard from an Italian colleague that in Italy when many people arrive for the COVID-19 vaccine slot and find out that the vaccine on offer is the Oxford-AstraZenca (AZ) vaccine they say no thank you and leave. The main reason they give for turning down the AZ vaccine is the thrombosis risk. I wonder if these people are aware of the new data that emerged last week (see below). 

In this big data study from the US, the incidence of cerebral venous thrombosis (CVT) after COVID-19 diagnosis was 39.0 cases per million people who get COVID-19. This was higher than the CVT incidence after influenza (0.0 per million people or adjusted relative risk = 6.7) or after receiving the Pfizer-BionTech or Moderna RNA vaccines (4.1 per million people, adjusted relative risk = 6.4 higher).

The bottom line is COVID-19 is way riskier than the COVID-vaccines in causing thrombus and contrary to a common belief the risk is not only restricted to the AZ or J&J vaccines, which use adenoviral vectors but with the COVID-19 mRNA vaccines well. This suggests it may be the immune response to the SARS-CoV-2 spike protein that induces cross-reactivity and sets up a very rare thrombotic state.  

It is never easy to explain risks and relative risks, but the following infographic doing the rounds on social media may help. What do you think? 

Please #StayCalm and #GetVaccinatedASAP

Taquet et al. Cerebral venous thrombosis: a retrospective cohort study of 513,284 confirmed COVID-19 cases and a comparison with 489,871 people receiving a COVID-19 mRNA vaccine. OSF 15-April-2021.

Using an electronic health records network we estimated the absolute incidence of cerebral venous thrombosis (CVT) in the two weeks following COVID-19 diagnosis (N=513,284), or influenza (N=172,742), or receipt of the BNT162b2 or mRNA-1273 COVID-19 vaccines (N=489,871). The incidence of portal vein thrombosis (PVT) was also assessed in these groups, as well as the baseline CVT incidence over a two-week period. The incidence of CVT after COVID-19 diagnosis was 39.0 per million people (95% CI, 25.2–60.2). This was higher than the CVT incidence after influenza (0.0 per million people, 95% CI 0.0–22.2, adjusted RR=6.73, P=.003) or after receiving BNT162b2 or mRNA1273 vaccine (4.1 per million people, 95% CI 1.1–14.9, adjusted RR=6.36, P<.001). The relative risks were similar if a broader definition of CVT was used. For PVT, the incidence was 436.4 per million people (382.9-497.4) after COVID-19, 98.4 (61.4-157.6) after influenza, and 44.9 (29.7-68.0) after BNT162b2 or mRNA-1273. The incidence of CVT following COVID-19 was higher than the incidence observed across the entire health records network (0.41 per million people over any 2-week period). Laboratory test results, available in a subset of the COVID-19 patients, provide preliminary evidence suggestive of raised D-dimer, lowered fibrinogen, and an increased rate of thrombocytopenia in the CVT and PVT groups. Mortality was 20% and 18.8% respectively. These data show that the incidence of CVT is significantly increased after COVID-19, and greater than that observed with BNT162b2 and mRNA-1273 COVID-19 vaccines. The risk of CVT following COVID-19 is also higher than the latest estimate from the European Medicines Agency for the incidence associated with ChAdOx1 nCoV-19 vaccine (5.0 per million people, 95% CI 4.3–5.8). Although requiring replication and corroboration, the present data highlight the risk of serious thrombotic events in COVID-19, and can help contextualize the risks and benefits of vaccination in this regard

Conflicts of Interest

Preventive Neurology




General Disclaimer: Please note that the opinions expressed here are those of Professor Giovannoni and do not necessarily reflect the positions of the Barts and The London School of Medicine and Dentistry nor Barts Health NHS Trust.

About the author

Prof G

Professor of Neurology, Barts & The London. MS & Preventive Neurology thinker, blogger, runner, vegetable gardener, husband, father, cook and wine & food lover.


  • I don’t think showing how rare the side effects are a good way to encourage vaccinations especially to the readers of the blog who ironically have a 1 in a 100,000 disease

    • Knowlege is power, Steve and you’re wrong about the incidence of MS

      MS estimated prevalence is 190 cases per 100,000 population, with 105,800 individuals in England.
      MS is more than twice as common in females than males, 272 versus 106 per 100,000 population

      • Apologies for the misinformation but the thought still holds value. It’s like a reverse Monte Carlo fallacy.

  • This study has several BIG problems:
    1. Conflict of interest! (Oxford AstraZeneca)
    2. The matched group “COVID-19” does not take into account that….
    a) not the entire population is infected (but will be vaccinated)
    b) not all infected people develop COVID (some are asymptomatic)
    c) not all sick people are recognized, so the “ICD” selects symptomatic COVID patients (they are the ill ones)
    3. Previous illnesses of the 23 cases of SVT under COVID19 were not (!) taken into account (however, there were relevant previous illnesses in these patients)
    > Table 1: 34.8% art. Hypertension, 21.7% CHD, 17.4% “venous disease”, 26.1% stenosis of arteries supplying the brain, 26.1% COPD, 13% malignancy, 9% liver disease, 26.1% diabetes mellitus
    How does that compare to healthy young women, who died from SVT/AZ?
    4) The study calculates an overall SVT risk of 5: 1,000,000 for A.-Z. (EMA) and does not make a calculation according to different age groups (cf. e.g. Norway: SVT approx. 1: 26,500 = 8x higher risk than in this study!!!)
    5) Pat. with SVT-AstraZeneca often had concomitant thrombocytopenia and PF4-AK; this is a different disease with a different prognosis than a “usual” thrombosis! You cannot compare!
    6) A prospective comparison of the ABSOLUTE risks related to the different age groups (!!!) and matching the patient with regard to previous illnesses would be scientifically meaningful: what is the ABSOLUTE risk fpr a 20 year old woman to die from COVID (no matter hwat cause – SVT, resp. insuff., …) vs. the ABSOLUTE risk to die from SVT/AZ-Vaccine.

  • Although the benefits of the vaccine generally outweigh the disadvantages of potential side effects, the decision to vaccinate should be based on the current context in the country, including risk of infection and disease, population immunity, epidemic control, disease burden and availability of other vaccines. The published data are important, but decisions must be made on a broader basis and include other data and considerations.
    The example from Italy may actually represent well informed decisions.

    • Not sure I agree. This virus is going nowhere and will become endemic, therefore, at some point in time, you will be infected with the virus if you are not vaccinated.

      You are either prepared to get COVID-19 and take your chances (1 in 1,000 mortality if you are 40-49 and a 1 in 10 risk of developing long-COVID) or you have the vaccine with its risk. Choose your poison; it really is playing Russian roulette, but clearly with different odds.

By Prof G



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