A Commentary on Vaccine-Induced Immune Thrombotic Thrombocytopenia

Authors: Miss Caitlin Dop and Dr Carly Bliss, May 2021.


In response to the SARS-CoV-2 pandemic, a number of COVID-19 vaccines have received emergency licensure. The majority of these vaccines deliver the genetic material encoding the SARS-CoV-2 spike protein. This includes two vaccines that deliver mRNA: Pfizer-BioNTech’s BNT162b2 vaccine and Moderna’s mRNA-1273 vaccine. Adenoviral vectored vaccines delivering spike DNA have been engineered from human adenovirus type 26 (Ad26, species D adenovirus) and Ad5 (species C adenovirus), forming Janssen’s single shot Ad26.COV2-S vaccine and the Russian Sputnik V vaccine using an Ad5-Ad26 combination regimen. Furthermore, the Oxford/Astrazeneca ChAdOx1 nCoV-19 vaccine deploys a species E chimpanzee adenoviral vector in a 2 shot homologous regimen. Since rollout, hundreds of millions of vaccine doses have been administered. A plausible link between vaccination and thrombotic events (blood clots) with thrombocytopenia (low platelet count) has been termed vaccine-induced immune thrombotic thrombocytopenia (VITT) due to the similarities with autoimmune heparin-induced thrombocytopenia. In both cases, cerebral venous thrombosis (CVT, also termed cerebral venous sinus thrombosis) can be a serious concern.


A recent non-peer reviewed preprint from Paul Harrison and colleagues at the University of Oxford quantifies the occurrence of CVT using anonymised electronic health care records from the USA¹. Data from subsets of healthy individuals, COVID-19 patients, and those vaccinated with COVID-19 mRNA vaccines from Pfizer or Moderna were evaluated for the 2-week period following infection/vaccination. An absolute risk of CVT was 42.8 per million in patients with a confirmed COVID-19 diagnosis. The comparator value was approximately 4 per million in those receiving an mRNA vaccine, albeit based on just 2 cases in just under 500,000 vaccinees. The same electronic health record network yields a baseline CVT rate of 0.41 per million people over any 2-week period. However, most reported estimates of CVT rates are generally higher and more variable².


On 23rd April 2021, the CDC reported CVT without thrombocytopenia as 1 per million in those receiving COVID-19 mRNA vaccines from Moderna or Pfizer³, however ruled half of the cases were not related to vaccination. A detailed evaluation of 17 de novo thrombocytopenia cases in mRNA vaccinees highlights that the direct relationship between low platelet count and mRNA vaccination is still uncertain⁴.


Rollout of Janssen’s Ad26.COV2-S vaccine commenced in the USA and was subsequently paused following a joint FDA/CDC statement on 12th April 2021 outlining 6 cases of CVT in combination with thrombocytopenia out of more than 6.8 million doses administered in the USA and over 7.2 million doses globally (<1 case per million doses)⁵. All cases were in women aged between 18 and 48, and prompted a safety review⁶. Then on 23rd April 2021, Ad26.COV2-S vaccine rollout resumed in the USA with the potential risks deemed extremely low⁷.


A recent EMA report states CVT cases following ChAdOx1 nCoV-19 vaccination is 5.0 per million people in EEA and UK vaccinees as of 4th April 2021⁸, with EMA’s safety committee concluding that unusual blood clots with low blood platelets should be listed as very rare side effects of ChAdOx1 nCoV-19 vaccination (on 7th April 2021) and Ad26.COV2-S vaccination (on 20th April 2021)⁹ ¹⁰. Also, on the 7th April 2021, the MHRA outlined 79 reports of thromboembolic (restricted blood flow due to a blood clot) events after the first dose of ChAdOx1 n-Cov-19 in the UK, with 44 of these detailed as CVT with thrombocytopenia. Nineteen of the 79 cases were fatal, with 14 of these 19 deaths involving CVT with thrombocytopenia. At the time, 20.2 million people had been vaccinated with ChAdOx1 nCov-19 in the UK, translating to almost 4 cases of clots per million and almost 2.2 cases of CVT per million ChAdOX1 nCoV-19 vaccinees¹¹. Further data from MHRA on 28th April 2021 outlines 242 major thromboembolic events with concurrent thrombocytopenia, of which 93 suffered CVT and 49 suffered a fatal outcome. This is from a total of 22.6 million first doses of ChAdOx1 in the UK, increasing the estimate of the CVT rate per million ChAdOx1 nCoV-19 vaccinees to 4.1 cases per million¹².


The Gamaleya Center, who spearheaded the development of Sputnik V, has released a statement outlining no cases of CVT during clinical trials or mass vaccination campaigns using the Sputnik V vaccination regimen. They state that differences in the adenoviral vector, antigen signal sequence, spike antigen and cell line used for vaccine propagation are key differences between Sputnik V and the ChAdOx1 nCoV-19 and Ad26.COV2-S vectored vaccines. As such they highlight that safety data from other vaccines should not be extrapolated to Sputnik V¹³. This should be a key finding, considering the use of 2 adenoviral vectors in this vaccine regimen, however inconsistencies in various published data surrounding Sputnik V currently cast some doubt over the accuracy of vaccine evaluation/reporting of the Sputnik V regimen¹⁴.


Clearly, the risk of CVT following SARS-CoV-2 infection is substantially higher than baseline CVT rates (~100-fold higher) and markedly higher than following COVID-19 vaccination (~8-10 fold)¹. Despite this, restrictions have been placed on the rollout of ChAdOx1 nCoV-19: alternative vaccines are being offered to the under 40s in the UK following an age stratified risk-benefit analysis¹⁵ ¹⁶; and across Europe, Germany and Finland have placed age restrictions on the use of ChAdOx1-nCov-19¹⁷ ¹⁸, Denmark and Norway have removed ChAdOx1 from their vaccination programmes, with Norway also postponing the rollout of Janssen’s Ad26.COV2-S¹⁹ ²⁰.


The most detailed characterisation of this phenomenon published to date was performed in 11 patients in Germany and Austria in whom thrombosis or thrombocytopenia developed after vaccination with ChAdOx1 nCov-19²¹. They used an ELISA to measure platelet factor 4 (PF4) antibodies and PF4-heparin antibodies, and deployed a modified (PF4-enhanced) platelet-activation test to detect platelet-activating antibodies. All patients showed strong reactivity of serum samples for both PF4 and PF4–heparin antibody by ELISA. In fact, these results extended to a panel of 28 patients with clinically suspected vaccine-induced immune thrombotic thrombocytopenia. Serum samples showed platelet activation with the addition of PF4 in almost all patients tested, but not with control volunteer serum. The authors concluded ChAdOx1 nCoV-19 vaccination can result in the rare development of immune thrombotic thrombocytopenia mediated by platelet-activating antibodies against PF4, which clinically mimics autoimmune heparin-induced thrombocytopenia. Further non-peer reviewed analyses from similar authors have reported an acute inflammatory post-vaccination environment driven by the adenovirus plus the cell culture-derived proteins and EDTA found in the ChAdOx1 nCoV-19 vaccine preparation, and this leads to platelet activation. They outline that activated platelets release PF4, which complexes with the vaccine and results in anti-PF4 antibody production and the downstream formation of neutrophil extracellular traps and overall activation of the coagulation system²². Separate studies corroborate the presence of PF4 antibodies in VITT patients following ChAdOx1 nCoV-19 vaccination²³ ²⁴. Published individual case reports also outline similar PF4 antibody reactivity following Janssen’s Ad26.COV2-S vaccine⁵ ²⁵. There are currently no studies of PF4 antibodies following mRNA COVID-19 vaccine administration.


It is also possible the spike antigen component contributes to this very rare occurrence of VITT. An RGD motif in the spike can bind integrins on endothelial cells, which has potential to lead to endothelium activation and initiation of blood clotting²⁶. This could explain the similar rate of CVT (4 per million) in those receiving mRNA vaccines and adenoviral vectored vaccines, as outlined by Harrison and colleagues and through EMA/MHRA reporting.


Despite news reports in some countries outlining a higher number of blood clotting cases among females, neither the MHRA or EMA have confirmed a risk factor related to biological gender ⁹ ²⁷. One possible explanation for this gender observation is that healthcare workers were among the first to be vaccinated globally, and with a greater proportion of these roles fulfilled by women, a higher number of females were vaccinated early on during vaccine rollout. Identifying whether biological gender plays a role in VITT is clearly important for vaccine licensure. Equally, non-evidence-based reports in this regard could be damaging, as they are likely to increase the gender inequalities in healthcare access that are already present in many countries.


Multifaceted research is ongoing into the causes, outcomes and treatments of VITT. Meanwhile the detailed statistical analyses demonstrating the positive effects of vaccine rollout upon case rates, hospitalisation and mortality from COVID-19 are eagerly awaited. Prior to vaccine licensure, the best preventative measure against case rate surges was complete national lockdown. Therefore, these analyses are expected to outline the overwhelming positive impact of such vaccination programmes. At the time of writing, 127,640 people across the UK have died within 28 days of a positive SARS-CoV-2 test, and estimates of over 3.3 million COVID-19 deaths globally are likely a vast under-representation²⁸. Devastatingly, this is orders of magnitude higher than the suspected fatalities from VITT, giving important perspective in these highly unprecedented times.

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