Notes: Lipid AX4 is an ionizable cationic lipid (pKa = 6.89) that has been used in the formation of lipid nanoparticles (LNPs) for the delivery of mRNA in vivo.{72032} Administration of LNPs containing lipid AX4 and encapsulating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant mRNA encoding the SARS-CoV-2 spike glycoprotein, also known as surface glycoprotein, receptor-binding domain (RBD) induces the production of antibodies that bind to the SARS-CoV-2 spike RBD in mice.