An ionizable cationic lipid (pKa 6.89) has been used in the formation of LNPs for the delivery of mRNA in vivo administration of LNPs containing lipid AX4 and encapsulating SARS-CoV-2 Delta variant mRNA encoding the SARS-CoV-2 spike glycoprotein RBD induces the production of antibodies that bind to the SARS-CoV-2 RBD in mice