The purpose of the RSV F-021 study that will be conducted in an observer-blind manner during Epoch 1 and single blinded during Epoch 2, is to rank the 3 different doses of the non-adjuvanted investigational RSV PreF-vaccine (30, 60 and 120 μg) based on safety/reactogenicity and immunogenicity data up to 1 month post-vaccination (Day 30). Non-pregnant women aged 18-45 years will be randomized in a 1:1:1:1 ratio to receive one of three dose levels (30, 60, 120 μg) of the RSV PreF vaccine or placebo. The doses eliciting strong immune responses while maintaining an acceptable safety profile will be considered for further evaluation, including in studies vaccinating pregnant women. This will allow evaluation of a wider antigen dose range to determine if there is a dose response relationship in terms of antibody response at the higher dose range that was not present at the lower range.