the primary sequence is important to consider, there must be a long enough stretch of sequence similarity to facilitate antibody binding.

the homologous sequence must also be exposed to the exterior solvent system, not folded away inside the 2' and 3' structure. thus folding is important.

perhaps an ELISA comparing syncytin-1 with Cov-19 S protien for binding homology of either set of antibodies, would be revealing either way.

for your conveinience :

https://en.wikipedia.org/wiki/Antibody

https://en.wikipedia.org/wiki/Immune_complex