PB0352 - Non-pathogenic anti-PF4 antibodies after SARS-CoV-2 vaccines: long term follow-up

Background: A subgroup of anti-platelet factor 4 (PF4) antibodies can activate platelets via Fcgamma RIIA and cause thrombotic and thrombocytopenic diseases such as heparin-induced thrombocytopenia and vaccine-induced immune thrombotic thrombocytopenia. Non-pathogenic anti-PF4 antibodies are detected in 1-7% of healthy blood donors and in 2-8% of SARS-CoV-2 vaccinated individuals.

Aims: In this study, we investigated the long-term course of anti-PF4 antibodies detected after the first SARS-CoV-2 vaccination.

Methods: Five subjects (all female, mean age (range): 40 (29-60) years) who had anti-PF4 antibodies after the first vaccination with ChadOx1 nCov19 (Vaxzevria, AstraZeneca-Oxford) were included. Blood samples collected as part of a longitudinal study (TüSeRe:exact) evaluating the immune response to SARS-CoV-2 vaccines among employees of an University Hospital were used. Anti-PF4/heparin antibodies were measured using a commercially available ELISA assay (Zymutest HIA IgG, Hyphen BioMed, France). Platelet activation was tested with a modified heparin-induced platelet aggregation assay (HIPA).

Results: The median (range) OD for IgG anti-PF4 antibodies was 0.69 (0.60-1.83) after the first vaccination. Blood samples were available up to 16 months after the first vaccination (range: 5-16 months). Anti-PF4 antibody levels decreased in all subjects. However, antibody levels returned to pre-vaccination levels in only one subject. In one subject who had received two doses of ChadOx1 nCov19, anti-PF4 antibodies remained above OD 1.0 at the last follow-up. All samples were negative in the modified HIPA assay.

Conclusion(s): Non-pathologic anti-PF4 antibodies can be detected even several months after the first vaccination with ChadOx1 nCov19. The clinical significance of these antibodies in case of subsequent exposure to an adenoviral vector vaccine or heparin is not yet clear.