Lipid nanoparticle (LNP) encapsulating self-amplifying mRNA, compositions, and methods of using the novel nucleic acid constructs and compositions are disclosed. LNP constructs include novel ionizable lipid. Novel sa-mRNA constructs encode a modified SARS-CoV-2 spike protein, wherein the polynucleotide has been truncated to not include nucleotides encoding a SARS-CoV-2 transmembrane domain and short cytosolic domain amino acids and immunomodulators. Sa-mRNAs are useful in for use as a therapeutic, diagnostic and/or prophylactic agent to mammalian cells or organs.