Coronaviruses are sharing several protein regions notable the spike protein (S) on their enveloped membrane surface, with the S1 subunit recognizing and binding to the cellular receptor, while the S2 subunit mediates viral and cellular membrane fusion. This similarity opens the question whether infection with one coronavirus will confer resistance to other coronaviruses? Investigating patient serum samples after SARS-CoV-2 infection in cross-reactivity studies of immunogenic peptides from Middle East respiratory syndrome coronavirus (MERS-CoV), we were able to detect the production of antibodies also recognizing MERS virus antigens. The cross-reactive peptide comes from the heptad repeat 2 (HR2) domain of the MERS virus spike protein. Indeed, the peptide of the HR2 domain of the MERS spike protein, previously proven to induce antibodies against MERS-CoV is sharing 74% homology with the corresponding sequence of SARS-CoV-19 virus. Sera samples of 47 convalescent SARS-CoV-2 patients, validated by RT-PCR-negative testes 30 days post-infection, and samples of 40 sera of control patients (not infected with SARS-CoV-2 previously) were used to establish eventual cross-bind reactivity with the MERS peptide antigen. Significantly stronger binding (p<0.0001) was observed for IgG antibodies in convalescent SARS-CoV-2 patients compared to the control group. If used as an antigen, the peptide of the HR2 domain of the MERS spike protein allows discrimination between post-Covid populations from non-infected ones by the presence of antibodies in blood samples. This suggests that polyclonal antibodies established during SARS-CoV-2 infection has the ability to recognize and probably decrease infectiveness of MERS-CoV infections as well as other coronaviruses. The high homology of the spike protein domain suggests in addition that the opposite effect can also be true: coronaviral infections producing cross-reactive antibodies affective against SARS-CoV-19. The collected data prove in addition that despite the core HR2 region being hidden in the native viral conformation, its exposure during cell entry makes it highly immunogenic. Since inhibitory peptides to this region were previously described, this opens new possibilities in fighting coronaviral infections.