The disclosure provides, in various embodiments, polypeptides (e.g., antibodies and antigen binding fragments thereof) that specifically bind to S2 domains of betacoronavirus Spike glycoproteins, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Spike glycoproteins. The disclosure also provides, in various embodiments, fusion proteins comprising one or more of polypeptides, polynucleotides encoding polypeptides, vectors and host cells suitable for expressing polypeptides, and methods for treating viral infections (e.g., COVID-19).