Nucleic acid, including DNA, for immunization to generate a protective immune response in a host, including humans, to a major outer membrane protein of a strain of Chlamydia, preferably contains a nucleotide sequence encoding a fragment that generates antibodies that specifically react with MOMP and a promoter sequence operatively coupled to the first nucleotide sequence for expression of the MOMP fragment in the host. The non-replicating vector may be formulated with a pharmaceutically-acceptable carrier for in vivo administration to the host.