Investigators from the Statens Serum Institut in Denmark and Imperial College London recently reported results from an early (Phase 1) trial of a candidate vaccine against the sexually transmitted bacterium, Chlamydia trachomatis. The vaccine consists of a derivative of the major outer membrane protein of Chlamydia administered with one of two different adjuvants (compounds added to the vaccine to help trigger an immune response). The study enrolled 35 women (15 randomly assigned to each different vaccine formulation and 5 to placebo) between the ages of 19 and 45. Immunization consisted of 3 injections in the arm at 0, 1, and 4 months followed by two intranasal doses at months 4.5 and 5. The main purpose of the study was to determine the safety of the two versions of the vaccine, but the investigators also measured the immune response to the vaccine. The vaccine was well tolerated, although all women developed pain at the site of the vaccine injection. All women who received the active vaccine developed antibody to the Chlamydia protein in the vaccine. Recipients of one version of the vaccine (called CTH522:CAF01) developed higher antibody levels and more mucosal antibody (antibody that is present at the usual sites of infection) than recipients of the other version of the vaccine. Therefore, that version of the vaccine seems more promising and is likely to undergo further studies. There were no Chlamydia infections detected during the study, so it is not known if the vaccine can protect a person from infection. That is the big question that must be answered in future studies. However, this is an important first step on the road to protecting people from this common infection.