Can spike protein induced cell fusion be triggered by the mRNA vaccine? The mRNA-based vaccines cannot lead to COVID-19 or its symptoms since they only lead to the production of the spike protein in the cell. However, the spike protein itself can lead to cell fusion: Quantitative assays reveal cell fusion at minimal levels of SARS-CoV-2 spike protein and fusion from without00138-3?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS2589004221001383%3Fshowall%3Dtrue) Is this a potential risk of the mRNA vaccine ? I could imagine, that the spike protein needs to be bound to the virus to induce this effect ? I could not find any literature where the effect of the unbound spike protein is tested.

The mRNA vaccines encode a mutant version of the spike protein in which the structural transition needed to to fuse membranes is blocked. This was done to make the immune response focus on the pre-fusion state, which is much better for neutralizing the virus.

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> Fortuitously, Graham and a former postdoc, Jason McLellan, devised a solution to this problem before the pandemic. Through a bit of structural biology and persistent protein engineering, McLellan discovered that adding two prolines—the most rigid of the 20 amino acids—to a key joint of a vaccine’s spike protein could stabilize the structure’s prefusion shape. This 2P mutation worked in preclinical studies of Graham and Moderna’s MERS vaccine, so they applied it to Moderna’s COVID-19 vaccine.

I don't know if cells expressing wild-type spike protein would fuse into syncytia, but it seems very unlikely to happen with the vaccine spike.