The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) bears an abundance of the characteristic spike protein, which mediates its entry into and fusion with the host cell, as well as syncytium formation by the host cells themselves.
Among the devastating complications of severe or critical coronavirus disease 2019 (COVID-19), clot formation ranks high in the adverse impact it creates. An important question is whether SARS-CoV-2-spike-based vaccines also cause such coagulopathies. A new study, released as a preprint on the medRxiv* server, examines this risk for the first time.
Clotting in SARS-CoV-2 infection
The virus is known to cause platelet activation and hypercoagulability, which is associated with a higher risk of strokes, heart attacks and thromboembolic events. This is mediated by the immunodominant spike protein, a known thrombosis promoter and platelet activator.
Platelets can be stimulated by the spike protein alone, apart from the virus, to release procoagulant factors, which lead to the formation of primary white cell-platelet masses or aggregates.
Some recent studies have shown that platelet reactivity is increased in a dysregulated manner even in mild COVID-19. Corresponding to this, platelet activation markers were seen to be expressed at higher levels, as well as the regulatory transmembrane proteins GPIIb/IIIa, GPIX and GPIa. The mechanism of coagulopathy is unclear.
However, despite the fact that the same antigen is also highly expressed in the score or so of vaccines that are currently being rolled out, not much work has been done on platelet activity following vaccination. Much reliance is placed on the fact that all vaccines in use today have passed phase III trials.
Clotting events after vaccination
The association of some rare clotting events after vaccination with BNT162b2 as well as other vaccines has led to widespread doubt about the safety of these vaccines. Such concerns are raised not only by ordinary citizens but even governmental bodies.
Secondly, a vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) has been reported in some Oxford (ChAdOx1) vaccine recipients. This resembles autoimmune heparin-induced thrombocytopenia, in that it is triggered by anti-platelet factor 4 antibodies.
The Pfizer-BioNTech (or BNT162b2) vaccine, is based on spike expression via a messenger RNA (ribonucleic acid) platform, encoding the spike protein for synthesis within the host cell. The continuing synthesis of the spike in its correct conformation ensures a higher specificity for the elicited antibodies and cytotoxic cells.
The study examined transmembrane receptors on platelet membranes, adhesion proteins, and markers of platelet activation, in 12 healthy individuals, at five points in time within the first four weeks following vaccination with the Pfizer-BioNTech vaccine. The researchers used mass cytometry by time of flight (CyTOF).
The time points included day 1 before vaccination, and post-vaccination days 1-2, 3-4, 14, 22 (one day after the second dose), and 28. These were examined for non-stimulated platelets, as well as those stimulated by adding thrombin receptor activating peptide (TRAP).
Platelet activation at baseline was estimated by assaying the expression of five markers, such as P-Selectin, LAMP-1, and LAMP-3, before and after vaccination.
What were the findings?
Of the donors, most of whom were female, without any medical conditions, all platelet activation markers remained unchanged after vaccination. Two of them, namely, CD40L and LAMP-1, were undetectable at both time points.
Transmembrane receptors and adhesion proteins such as CD41, CD61, CD42a, and CD42b were also found to be unchanged.
After TRAP stimulation, all these receptors and activation markers were upregulated, but without any significant difference following vaccination. Computational analysis failed to show any variations related to vaccination.
When compared to platelet protein expression in symptomatic COVID-19 patients in hospital, but no other medical condition, the healthy donors showed lower expression levels, as in another independent cohort of healthy virus-negative donors who had been tested earlier for platelet activation. The platelet markers in the two healthy donor groups were similar in expression.
What are the implications?
The main finding of this study is that the exposition to spike protein upregulation through BNT162b2 does not alter the expression of the most important platelet adhesion proteins and receptors.”
The advantage of the CyTOF method is the ability to quantify the expression of multiple markers on single cells simultaneously. Here, its use allowed platelet phenotypes to be clarified with regard to activation marker, a transmembrane protein, and adhesion protein expression.
Earlier reports suggested the dose-dependent activation of platelet aggregation and dense granule secretion, with ATP release, in the presence of isolated spike protein. In addition, transmembrane proteins and activation markers were upregulated by the spike protein even without other agonists.
The current study did not confirm earlier findings, showing no change in activation phenotype after BNT162b2 vaccination. This was validated by comparison with a similar, but independent, control cohort tested by the same approach, as well as with a group of symptomatic hospitalized COVID-19 patients, where the first group showed unchanged low expression of these markers. Still, the second had higher levels of these molecules.
TRAP stimulation did not change these results, with equivalent responses in vaccinated and non-vaccinated individuals. Overall, therefore, “our study shows that the vaccination does not generally lead to platelet activation or dysfunctional reactivity.”
Further studies will be necessary, using other vaccines that use different vectors, but these findings remain highly significant and promising.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.