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.
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Presence of any unsolicited systemic Adverse Events (AEs) reported within 30 minutes after vaccination
Timeframe: Within 30 minutes after each vaccination
Presence of solicited injection site and systemic reactions (ie, pre-listed in the participant's diary and in the eCRF (electronic case report form) occurring through 7 days after vaccination
Timeframe: Through 7 days after each vaccination
Presence of unsolicited AEs reported through 28 days after vaccination
Timeframe: Through 28 days after each vaccination
Presence of SAEs (Serious Adverse Events) and AESIs (Adverse Events of Special Interest) throughout the study
Timeframe: Throughout study, approximately 6 months
Presence of out-of-range biological test results (including shift from baseline values) through 7 days after vaccination
Timeframe: Through 7 days after vaccination
Hemagglutinin Inhibition Assay (HAI) antibody (Ab) response to each homologous influenza strain at Day 1 and Day 29
Timeframe: Day 1 and Day 29
RSV A, hMPV A and PIV3 serum neutralizing antibodies (nAb) titers at Day 1 and Day 29
Timeframe: Day 1 and Day 29