A Phase II Study of a Primary Dose of Investigational RSV Maternal Vaccine, Given Alone or With Boostrix, With a 2nd Dose Investigational RSV Maternal Vaccine

The purpose of this study is to evaluate the safety, ability of GSK Biologicals' investigational RSV maternal vaccine (RSVPreF3) to generate an immune response and the degree to which the vaccine can cause side effects, when administered alone and in combination with Boostrix vaccine in healthy non-pregnant women 18-45 years of age. Two dose levels of RSVPreF3 and 2 Boostrix \[Diphtheria, Tetanus and acellular Pertussis (dTpa) vaccine\] formulations (US and ex-US) will be evaluated. A 2nd dose of RSVPreF3 will be administered in an extension of the study to assess the durability of the immune response after the first dose vaccination, and to assess the safety and immunogenicity following a second dose vaccination of the RSVPreF3 maternal vaccine.