1. A liposome having a lipid bilayer encapsulating an aqueous core, where (i) the lipid bilayer contains a lipid having a pKa value of from 5.0 to 7.6； and (ii) the water core includes RNA that encodes an immunogen. 2. The liposome of claim 1, wherein the lipid having pKa ranging from 5.0 to 7.6 contains a tertiary amine. The liposome according to one of the preceding paragraphs, where the pKa in the range from 5.0 to 7.6 is in the range of 5.7-5.9.4. The liposome of claim 1, wherein the lipid having a pKa ranging from 5.0 to 7.6 has the formula given herein for RV01, RV02, RV03, RV04, RV05, RV06, RV07, RV08, RV09, RV011 , RV012, RV016 or RV017.5. The liposome according to claim 1, having a diameter in the range of 20-220 nm. The liposome of claim 1, wherein the RNA molecule encodes (i) an RNA-dependent RNA polymerase that can transcribe RNA from an RNA molecule, and (ii) an immunogen. The liposome of claim 5, wherein the RNA molecule has two open reading frames, where the first encodes an alpha virus replicase and the second encodes an immunogen. The liposome according to claim 1, where the RNA molecule has a length of 9000-12000 nucleotides. The liposome of claim 1, wherein the immunogen can elicit an immune response in vivo against a bacterium, virus, fungus, or parasite. The liposome of claim 1, wherein the immunogen can elicit an in vivo immune response against a respiratory syncytial virus glycoprotein F. A pharmaceutical composition comprising a liposome according to one of the preceding claims. A method for inducing a protective immune response in a vertebrate, comprising the step of administering to the vertebrate an effective amount of liposome according to claims 1-10 or the pharmaceutical composition according to claim 11.13. A method for producing an RNA-containing liposome, comprising the steps of: (a) mixing RNA with a lipid at a pH that is lower than the pKa of the lipid but above 4.5； then1. Липосома, имеющая липидный бислой, инкапсулирующий водное
