The Oxford-AstraZeneca COVID-19 vaccine ChAdOx1 nCoV-19 is associated with a risk for vaccineinduced immune thrombosis with thrombocytopenia syndrome in the range of one to two cases per 100 000 vaccinations, with younger women showing the highest risk. 1, 2 Additional cases have been reported for the Johnson & Johnson adenoviral vector-based Ad26. CoV2. S COVID-19 vaccine. 3 Vaccine-induced antibodies against platelet factor 4 have been implicated in the pathogenesis. 1, 2 These antibodies might be amplified by booster vaccination with an adenoviral vector, which prompted recommendations to boost with an mRNA-based vaccine instead, although data on safety and efficacy of heterologous prime–boost regimens are sparse. 4 We quantified the vaccine-induced antibody response in vaccinees in Germany who received a heterologous COVID-19 vaccination scheme using ChAdOx1 nCoV-19 as prime and BNT162b2 mRNA (BioNTech-Pfizer) as boost vaccination. The results were compared with those of cohorts of health-care workers or volunteers who received homologous BNT162b2 or ChAdOx1 nCoV-19 vaccination