The present invention provides a method of nucleic acid, including DNA, immunization of a host, including humans, against disease caused by infection by a strain of Chlamydia, specifically C. pneumoniae, employing a vector containing a nucleotide sequence encoding a 98 kDa outer membrane protein of a strain of Chlamydia pneumoniae and a promoter to effect expression of the 98 kDa outer membrane protein gene in the host. Modifications are possible within the scope of this invention.