VPB0108 - Incidence of anti-platelet factor4/polyanionic antibodies, thrombocytopenia, and thrombosis after COVID-19 vaccination with ChAdOx1 nCoV-19 in Thais

Background: Prevalence of antiplatelet-factor 4 (PF4)/polyanionic antibodies occurring after vaccination with ChAdOx1 nCoV-19 was low. Most of these antibodies are weak and not associated with vaccine-induced thrombotic thrombocytopenia. It remains unknown whether these antibodies are preexisting or occur after the vaccination.

Aims: In this study, we demonstrated the incidence of anti-PF4/polyanionic antibodies, thrombocytopenia, and thrombosis after vaccination with ChAdOx1 nCoV-19 in Thais.

Methods: We conducted a prospective study in health care workers and the general population who received COVID-19 vaccination with ChAdOx1 nCoV-19. Blood collection for complete blood count, D-dimer, and anti-PF4/polyanionic antibodies was performed before vaccination (day 0), day 10, and day 28 after vaccination. Anti-PF4/polyanionic antibodies were detected using enzyme-link immunosorbent assay (ELISA). Functional assay with platelet aggregation was performed for all positive anti-PF4/polyanionic antibody ELISA tests.

Results: A total of 720 participants receiving the first, second, or third booster dose of ChAdOx1 nCoV-19 were included in the study. Baseline characteristics are presented in Table 1. Three participants developed seroconversion. Therefore, the incidence of anti-PF4/polyanionic antibodies was 0.42% (95% confidence interval 0.08, 1.23). However, these antibodies were low titer. Fourteen (1.9%) participants had preexisting anti-PF4/polyanionic antibodies before the vaccination but the optical density of anti-PF4/polyanionic antibodies did not significantly increase over time (Figure 1). None of the anti-PF4/polyanionic positive sera induced platelet aggregation. Abnormal D-dimer levels following the vaccination were not different among the positive and negative anti-PF4/polyanionic groups (11.8% vs 13.2%, p=0.86). Thrombocytopenia occurred in one person with negative anti-PF4/polyanionic antibodies. No clinical thrombosis occurred.

Conclusion(s): We found a low incidence of seroconversion of anti-PF4/polyanionic antibodies after vaccination with ChAdOx1 nCoV-19 in Thais. Most of the anti-PF4/polyanionic antibodies are preexisting and did not significantly increase after vaccination with ChAdOx1 nCoV-19. Some participants with anti-PF4/polyanionic antibodies had elevated D-dimer levels. However, no thrombocytopenia and thrombosis were observed.