Evidence that Chlamydia pneumoniae, strain TWAR, is not sexually transmitted

Serum specimens from 230 sexually active women were tested for antibodies to Chlamydia trachomatis and Chlamydia pneumoniae. The women were randomly selected as controls in a population-based case-control study for ectopic pregnancy. A total of 123 had antibody to C. pneumoniae and 39 had antibody to C. trachomatis. There was an association between prevalence of antibody to C. trachomatis and an increasing number of lifetime sexual partners, early age at first intercourse, and a history of gonorrheal infection. The prevalence of antibody to C. pneumoniae was not related to any of these factors. These results provide evidence that the two human species of Chlamydia have a different mode of transmission and that C. pneumoniae is not sexually transmitted.