Chlamydia (C.) trachomatis is a highly prevalent sexually transmitted bacterial disease, with 127 million infections reported in 2016. In women, genital tract infections by C. trachomatis can cause severe complications, including infertility, ectopic pregnancy, and chronic pelvic pain. In addition, the impact on susceptibility and transmission of other pathogens, such as HIV, has been reported. There is a consensus that prophylactic vaccination against C. trachomatis would be an important tool in our fight to control the worldwide impact of C. trachomatis. However, vaccine development, already being investigated for more than a century, is hampered by insufficient knowledge of the complex biology of C. trachomatis and its adaptations to evade interactions with the immune system. Current know-how emphasizes the importance of IFNɣ production and neutralizing antibody production as part of a successful immune response. Interestingly, the specific correlates of protection against infection are still to be revealed. This PhD will build on the know-how that is being established by our group using a first-void urine (FVU) sample for monitoring the impact of HPV vaccination programs. We have demonstrated already that FVU sampling can be effectively used to detect the presence of HPV infection and to monitor the humoral immune response. The developed tools to study the pathogen-immune system interaction will be adjusted for the follow-up of C. trachomatis infection and potential vaccine impact.