> More than 90% of total human plasma immunoglobulin G (IgG) is found in a fucosylated form, but specific IgGs with low core fucosylation (afucosylated IgGs) are found in response to infections with enveloped viruses and to alloantigens on blood cells. Afucosylated IgGs mediate immunopathology in severe COVID-19 and dengue fever in humans. In COVID-19, the early formation of non-neutralizing afucosylated IgG against the spike protein predicts and directly mediates disease progression to severe form. IgG lacking core fucosylation causes dramatically increased antibody-dependent cellular toxicity mediated by intense FcγR-mediated stimulation of macrophages, monocytes, natural killer cells, and platelets. The mechanism and the context within which afucosylated IgG formation occurs in response to enveloped virus antigens have remained elusive thus far in COVID-19, dengue fever, and other infections. This study demonstrates that administration of human bone marrow megakaryocytes infected by SARS-CoV-2 into the circulation of K18-hACE2 transgenic mice drives the formation of pathogenic afucosylated anti-spike IgG antibodies, and is sufficient to reproduce severe COVID-19 manifestations of pulmonary vascular thrombosis, acute lung injury, and death in mice.