A Study on the Safety and Immunogenicity of Influenza, Respiratory Syncytial Virus, Human Metapneumovirus and Parainfluenza Vaccines in Adults 18 to 49 Years of Age.

The objective of this study is to evaluate the safety and immunogenicity of different vaccines of hemagglutinin formulations of trivalent influenza vaccine or of a combined respiratory syncytial virus / human metapneumovirus / parainfluenza virus type 3 vaccine in healthy participants 18 to 49 years of age. A lipid nanoparticle will be used in this study. Overall, the study is designed to: * Assess the safety profile of the candidate formulations * Describe the immunogenicity profile of the candidate formulations * Eligible participants will be randomized to receive a single intramuscular injection of either one of the vaccine formulations. Participants will be provided with a diary to solicit reporting of injection site reactions and systemic reactions, unsolicited Adverse Events (AEs), Serious Adverse Events (SAEs) and Adverse Events of Special Interest AESIs). Participants will also be required to record their daily temperature on the diary.

The duration of study participation will be approximately 6 months for each participant.