Since in RSV F-021 the 120ug dosage of the PreF-based investigational RSV vaccine will be tested for the first time in humans, appropriate safety monitoring is planned for this study, including pausing enrolment when the first 25% of subjects from each study group have been vaccinated until review of day 7 post-vaccination safety data by an unblinded GSK internal Safety Review Committee (iSRC) has been completed. The enrolment/vaccination of the remaining subjects can only start following the favourable outcome of this iSRC safety review.