Nucleic acid immunization is achieved by delivering a nucleic acid (NA), e.g., a mRNA or a DNA, encapsulated within a lipid-NA nanoparticle. The NA encodes an immunogenic compound of interest. The lipid-NA nanoparticle is effective for in vivo delivery of NA to a vertebrate cell, including upon administration to a subject. The lipid-NA nanoparticle are incorporated in pharmaceutical compositions for immunizing subjects against various diseases.