A Study About Modified RNA Vaccines Against Influenza in Healthy Adults

The purpose of this study is to learn if modified RNA (modRNA) vaccines for the prevention of influenza are: * safe; and * how these vaccines produce an immune response in generally healthy adults. Immune response is the way the body protects itself against things it sees as harmful or foreign. RNA (also called ribonucleic acid) is one of two types of nucleic acid made by cells. RNA contains information that has been copied from DNA (the other type of nucleic acid). Cells make several different forms of RNA, and each form has a specific job in the cell. Many forms of RNA have functions related to making proteins. RNA is also the genetic material of some viruses instead of DNA. RNA can be made in the laboratory and used in research studies. Also called ribonucleic acid. Influenza is term used for flu illness. It is an infection caused by a virus that affects your mouth, nose, and throat. The study is seeking for participants who: * are at least 18 years of age * have not received an influenza vaccine within the last 6 months * are generally healthy This study will be divided into three sub-studies: Substudy A (SSA), Substudy B (SSB), and Substudy C (SSC). All participants, regardless of sub-study, will receive 1 dose of either of the following vaccines as an injection into their arm: * 1 of the modRNA influenza vaccines that is being studied; or * an approved influenza vaccine approved for use in their respective age group. Participants will be involved in this study for about 6 months. During this time, participants will have at least 3 clinic visits.