Prevalence of thrombocytopenia, anti-platelet factor 4 antibodies and D-dimer elevation in Thai people After ChAdOx1 nCoV-19 vaccination

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare distinctive syndrome characterized by unusual site thrombosis accompanied by thrombocytopenia after ChAdOx1 nCoV-19 vaccination. Platelet-activating anti-platelet factor 4-dependent antibodies (anti-PF4 Abs) were detected in most cases of VITT. To date, data from Asian countries are lacking.

Objectives: To determine the prevalence of thrombocytopenia, anti-PF4 Abs, and D-dimer elevation in Thai people administered the ChAdOx1 vaccine.

Patients/methods: A total of 521 vaccinated and 146 nonvaccinated subjects were enrolled. Blood samples were collected to determine platelet counts, anti-PF4 Abs using ELISA and D-dimer levels 5 to 30 days after the first vaccination.

Results: None of the participants developed thrombocytopenia or had significantly decreased platelet counts from baseline after ChAdOx1 vaccination. The frequencies of anti-PF4 Abs between vaccinated (16/521; 3.1%; 95% confidence interval [CI], 1.8-4.9) and nonvaccinated Thai people (6/146; 4.1%; 95% CI, 1.5-8.7) were similar. None of the detectable anti-PF4 Abs activated platelets in vitro. The average D-dimer levels between vaccinated and control groups were similar (282.2 ± 286.3 vs 267.8 ± 219.3 ng/mL; P = 0.58). Four vaccinated and one nonvaccinated participants had markedly elevated D-dimer levels >2000 ng/mL without detectable anti-PF4 Abs. Imaging studies of these asymptomatic subjects revealed incidental pulmonary embolism in a vaccinated elderly woman.

Conclusions: This study demonstrated a low prevalence of thrombocytopenia and pathogenic anti-PF4 Abs after ChAdOx1 vaccination. D-dimer testing revealed no significant coagulation activation. Routine tests for platelet counts, anti-PF4 Abs, and D-dimer levels are not recommended for VITT screening without clinical suspicion.