Before revision:  That both the D614G mutation and the K986P mutation increase the COVID-19 infectivity and why a large number of mutations, including those at interface residues, have not been selected in current SARS-CoV-2 pandemic are interpreted from the presented theory.
After revision:  Possible new SARS-CoV-2 variants from amino acid mutations in 5-9 sites on RBD interface are predicted. The elastic energy variation needed for conformation transition is estimated quantitatively. Taking the elastic-structural change into account more virus variants are possible. Both the D614G mutation and the K986P mutation increase the COVID-19 infectivity and why a large number of mutations, including those at interface residues, have not been selected in current SARS-CoV-2 pandemic are interpreted from the presented theory.