PROS1 and infection: To improve the breadth of neutralization and potency of the vaccines against heterologous or multiple variant infections, bivalent vaccines (by mixing two mRNAs encoding S protein of ancestral strain or different variants) or priming-boosting vaccination strategies (by priming with a mRNA targeting ancestral or a variant and boosting with monovalent or bivalent mRNA, or a viral vector, targeting ancestral or one or two variants) are employed [45,47,57,58].