Safety and Immunogenicity Study of Self-Amplifying RNA Pandemic Influenza Vaccine in Adults

The goal of this clinical trial is to evaluate the safety and immune responses of three different dose levels a self-amplifying RNA pandemic influenza vaccine (ARCT-2304) in adults. The key objectives of the study are: * To evaluate safety and reactogenicity of different dose levels of the ARCT-2304 vaccine * To describe the Immune responses of different dose levels of the ARCT-2304 vaccine as measured by hemagglutination inhibition (HAI) and neuraminidase enzyme-linked lectin (ELLA) antibody responses Researchers will compare the results with licensed influenza vaccines to select the most optimal dose level and schedule for vaccine administration in terms of safety and immunogenicity for further development of the vaccine. Participants will receive 2 doses of the ARCT-2304 vaccine or 1 dose of licensed influenza vaccine and 1 dose of placebo. They will be asked: * to complete a daily diary for 7 days after each vaccination, answering questions how they have been feeling on that day. * to provide blood samples at each visit in the clinic * to comply with all study visits and procedures (e.g., be available for planned telephone contacts and unscheduled clinic visits, if required)