Serum levels of anti-PF4 IgG after AZD1222 (ChAdOx1 nCoV-19) vaccination

Rare cases of thrombosis with thrombocytopenia syndrome (TTS) have been reported after AZD1222. Anti-platelet factor-4 (PF4) antibodies were observed in patients following presentation of TTS, however it is unclear if AZD1222 was responsible for inducing production of anti-PF4. Paired samples (baseline and day-15) from a phase 3 trial of AZD1222 vs placebo were analyzed for anti-PF4 levels; 19/1727 (1.1%, AZD1222) vs 7/857 (0.8%, placebo) participants were anti-PF4-IgG-negative at baseline but had moderate Day-15 levels ( P = 0.676) and 0/35 and 1/20 (5.0%) had moderate levels at baseline but high Day-15 levels. These data indicate that AZD1222 does not induce a clinically relevant general increase in anti-PF4 IgG.