No changes of parameters nor coagulation activation in healthy subjects vaccinated for SARS-Cov-2.

Background: Recent reports of thrombotic events after SARS-Cov-2 vaccination raised concern. However, modifications of the most common coagulation parameters after vaccination are unknown.

Aims: We measured parameters of coagulation including (i) basic coagulation tests, (ii) procedures aimed to assess the ex-vivo potential capacity to generate thrombin and (iii) in vivo thrombin activity. We also assessed anti-platelet factor 4 (aPF4) with two methods.

Design: Laboratory measurements were performed for a cohort of subjects (n = 30) before (baseline) and after (7 and 21days after first dose, and 14days after second dose) SARS-Cov-2 vaccination.

Results: All subjects received the Pfizer-BioNTech vaccine, and none developed symptomatic thrombotic events during the study period. None of the parameters showed clinically relevant variations at different time-points before and after vaccination. Only platelet count showed a slight increase, and F1.2 and the thrombin generation parameters ETP and ETP-TM ratio, showed a small decline, at the last time-point after vaccination when compared to baseline. aPF4 was negative in all the subjects, except two, who were positive (one with the chemiluminescent and the other with the ELISA assay).

Conclusions: The study shows no modifications of the coagulation parameters nor the presence of biochemical signs of coagulation activation following the administration of the Pfizer-BioNTech vaccine.