mRNA-1777

**mRNA-1777 (V171)** is an investigational mRNA-based prophylactic vaccine developed by Moderna for preventing respiratory syncytial virus (RSV) infection. It encodes the full-length, membrane-associated RSV fusion (F) glycoprotein stabilized in its prefusion conformation (preF) using mDS-Cav1 modifications to elicit potent neutralizing antibodies targeting this key antigenic form, which is preferentially recognized by human antibodies during natural infection. Delivered via lipid nanoparticles (LNP) containing cholesterol, DSPC, MC3, and PEG2000-DMG, it was tested in a phase 1 trial (mRNA-1777-P101) in healthy younger (18-49 years) and older (60-79 years) adults, administered as single intramuscular doses of 25-300 µg, showing good tolerability with no vaccine-related serious adverse events and inducing dose-dependent humoral responses including RSV A/B neutralizing titers, preF-binding antibodies, D25-competing antibodies, and CD4+ T-cell responses.[2][3][4]