Disclosed soluble PD-1 (sPD-1) proteins and nucleic acids, and therapeutic compositions comprising sPD-1 proteins and nucleic acids, for enhancing immunity of a subject against coronavirus infection. Disclosed are soluble PD-1 fusion proteins that include a soluble PD-1 protein fragment and an antigenic protein fragment, preferably where the antigenic protein fragment comprises a coronavirus protein fragment. In some forms, the coronavirus protein fragment is derived from a coronavirus receptor binding domain (RBD) or a coronavirus nucleoprotein (N). In some forms, the sPD-1 proteins, nucleic acids, and compositions are formulated as a vaccine composition. Also disclosed are methods for treating a subject at risk of or suffering a coronavirus infection.