SUMMARY

COVID-19 vaccines carry the spike protein (S or "Spike") of the SARS-CoV-2 virus as an alleged antigen to trigger the immune response, which shares high genetic and protein similarity with two human proteins, Sincitin-1 and Sincitin-2.

Human syncytins are the product of the expression of the genes of the envelope (Env) of human endogenous retroviruses (HERV): they are proteins that mediate fusion between cells and have immunosuppressive properties.

Syncytins are physiologically expressed during pregnancy: they intervene in the development of the placenta, trophoblast differentiation, the implantation of the embryo in the mother's uterus and the immunosuppression of the mother's immune system to prevent allogeneic rejection of the embryo.

Because of the similarity between syncytins and the spike protein of SARS-CoV-2, COVID-19 vaccine-induced antibody responses could trigger a cross-reaction against syncytins, causing allergic, cytotoxic and/or autoimmune side effects affecting human health and reproduction.