Inflammation and Platelet Activation After COVID-19 Vaccines - Possible Mechanisms Behind Vaccine-Induced Immune Thrombocytopenia and Thrombosis

Abstract

Introduction of vaccines against COVID-19 has provided the most promising chance to control the world-wide COVID-19 pandemic. However, the adenovirus-vector based Oxford/AstraZeneca [ChAdOx1] (AZ) and Johnson & Johnson [Ad26.CoV2.S] COVID-19 vaccines have been linked with serious thromboembolic events combined with thrombocytopenia, denominated Vaccine-induced Immune Thrombocytopenia and Thrombosis (VITT). The pathogenesis of COVID-19 VITT remain incompletely understood; especially the initial events that trigger platelet activation, platelet factor (PF)4 release, complex formation and PF4 antibody production are puzzling. This is a prospective study investigating the impact of different COVID-19 vaccines on inflammation (CRP, TNF-α, IL-1β, IL-6, IL-8, IL-10), vascular endothelial activation (syndecan-1, thrombomodulin, E-selectin, ICAM-1, ICAM-3, VCAM-1), platelet activation (P-selectin, TGF-β, sCD40L) and aggregation (Multiplate^® impedance aggregometry), whole blood coagulation (ROTEM^®), thrombin generation and PF4 antibodies to reveal potential differences between AZ and mRNA vaccines in individuals without VITT. The study included 80 (55 AZ and 55 mRNA) vaccinated individuals and 55 non-vaccinated age- and gender matched healthy controls. The main findings where that both vaccines enhanced inflammation and platelet activation, though AZ vaccination induced a more pronounced increase in several inflammatory and platelet activation markers compared to mRNA vaccination and that post-vaccination thrombin generation was higher following AZ vaccination compared to mRNA vaccination. No difference in neither the PF4 antibody level nor the proportion of individuals with positive PF4 antibodies were observed between the vaccine groups. This is the first study to report enhanced inflammation, platelet activation and thrombin generation following AZ vaccination compared to mRNA vaccination in a head-to-head comparison. We speculate that specific components of the AZ adenovirus vector may serve as initial trigger(s) of (hyper)inflammation, platelet activation and thrombin generation, potentially lowering the threshold for a cascade of events that both trigger complications related to excessive inflammation, platelet and coagulation activation as observed in epidemiological studies and promote development of VITT when combined with high-titer functionally active PF4 antibodies.

Keywords: COVID-19; TTS; VITT; platelet factor 4; thrombocytopenia; thrombosis; vaccines.

Figure 1

Study design. Eighty participants (n=80) included in the Danish national vaccine trial ENFORCE, recently vaccinated with AZ or mRNA vaccines against COVID-19 were invited to provide an early blood sample post-vaccination (median of 11 days (range 8-16) post-vaccination). Pre-vaccination samples were available from all participants via the ENFORCE study. Since standard coagulation tests, platelet aggregation, whole blood coagulation (Thromboelastometry) and thrombin generation were not available at the pre-vaccination sample, non-vaccinated age and gender matched healthy individuals were invited to participate as controls.

Figure 2

Inflammation markers pre- and post-vaccination and delta changes with either AZ (n=55) or mRNA (n=25) vaccines. Levels of C-reactive protein (CRP, µg/ml), Tumor necrosis factor (TNF)-α (pg/ml), interleukin (IL)-1β (pg/ml), IL-6 (pg/ml), IL-8 (pg/ml) and IL-10 (pg/ml) are displayed on log scales. The top figure displays pre- and post-vaccination values, and the bottom figure displays delta changes. The box plots display median, first and third quartile (box mid-line, button and top, respectively) and the whiskers display maximum and minimum values with outliers (circles) and extremes (small asterisks) displayed. AZ vaccinated individuals are displayed by blue bars (top figure: pre=light blue, post=dark blue) and mRNA vaccinated individuals are displayed by gray bars (top figure: pre = light gray, post=dark gray). Differences between AZ and mRNA vaccinated individuals either pre- and post-vaccination and delta changes are investigated by Mann-Whitney U test, with p-values displayed. Changes over time within the AZ or mRNA groups are investigated by Wilcoxon signed-rank test, with **p < 0.0001 and *p < 0.010 (displayed over the post-vaccination bars).

Figure 3

Vascular endothelial activation markers pre- and post-vaccination and delta changes with either AZ (n=55) or mRNA (n=25) vaccines. Levels of Syndecan-1 (pg/ml), thrombomodulin (TM, ng/ml), endothelial (E)-selectin (ng/ml), intercellular adhesion molecule (ICAM)-1 (µg/ml), ICAM-3 (ng/ml) and vascular cell adhesion molecule (VCAM)-1 (µg/ml) are displayed on log scales. The top figure displays pre- and post-vaccination values, and the bottom figure displays delta changes. The box plots display median, first and third quartile (box mid-line, button and top, respectively) and the whiskers display maximum and minimum values with outliers (circles) and extremes (small asterisks) displayed. AZ vaccinated individuals are displayed by blue bars (top figure: pre=light blue, post=dark blue) and mRNA vaccinated individuals are displayed by gray bars (top figure: pre=light gray, post=dark gray). Differences between AZ and mRNA vaccinated individuals either pre- and post-vaccination and delta changes are investigated by Mann-Whitney U test, with p-values displayed. Changes over time within the AZ or mRNA groups are investigated by Wilcoxon signed-rank test, with **p < 0.0001 and *p < 0.010 (displayed over the post-vaccination bars).

Figure 4

Platelet activation markers pre- and post-vaccination and delta changes with either AZ (n=55) or mRNA (n=25) vaccines. Levels of platelet (P)-selectin (ng/ml), transforming growth factor (TGF)-β (pg/ml) and soluble CD40 ligand (CD40L, pg/ml) are displayed on a log scale. The top figure displays pre- and post-vaccination values, and the bottom figure displays delta changes. The box plots display median, first and third quartile (box mid-line, button and top, respectively) and the whiskers display maximum and minimum values with outliers (circles) and extremes (small asterisks) displayed. AZ vaccinated individuals are displayed by blue bars (top figure: pre=light blue, post=dark blue) and mRNA vaccinated individuals are displayed by gray bars (top figure: pre=light gray, post=dark gray). Differences between AZ and mRNA vaccinated individuals either pre- and post-vaccination and delta changes are investigated by Mann-Whitney U test, with p-values displayed. Changes over time within the AZ or mRNA groups are investigated by Wilcoxon signed-rank test, with **p < 0.0001 and *p < 0.010 (displayed over the post-vaccination bars).