 For female participants: subjects will be excluded if pregnant. There are no known risks to pregnant women from the PPSV23 vaccine also indicated in the Vaccine Information Statement VIS attachment provided to participants, and there is no additional risk associated with becoming pregnant during the study. However, as pregnancy affects immune system responses, this may affect the molecular readout in this study and introduce confounding factors in the analysis by the investigators. If a participant is already enrolled and becomes pregnant during this study, the investigators will temporarily withdraw them from the study from the day the participants become pregnant. If the participants would like to stay in the study, the investigators may continue their participation after their delivery. 