The purpose of this study is to learn about the safety and effects of the study vaccine for the possible prevention of influenza. Influenza is a disease that can spread easily from one person to another and cause body aches, fever, cough, and other symptoms. The study vaccine is called Pandemic Influenza modRNA (pdmFlu) Vaccine. This study is seeking for participants who are: * between the ages of 18 to 49 years old or 65 to 84 years old. * willing and able to follow with all scheduled visits, treatment plan, laboratory tests, lifestyle changes, and other study procedures. * healthy as confirmed by medical history, physical examinations, and the study doctor. * capable of signing informed consent. Participants will receive either: * the pdmFlu vaccine, * a licensed influenza vaccine * a placebo. A placebo does not have any medicine in it but looks just like the study medicine. Participants will not know which vaccine they receive. Participants will receive the study vaccines as a single shot in the arm. The study will compare participant experiences to help understand if the pdmFlu vaccine is safe and effective. Participants will take part in this study for up to 13 months. During this time, the participants will receive the study vaccine and take part in follow-up visits.
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Percentage of participants by age group reporting local reactions
Timeframe: For 7 days after vaccination
Percentage of participants by age group reporting systemic events
Timeframe: For 7 days after vaccination
Percentage of participants by age group reporting adverse events
Timeframe: From vaccination to 4 weeks after each vaccination
Percentage of participants by age group reporting serious adverse events
Timeframe: From vaccination to 6 months after last vaccination or through the last visit, whichever is longer
Percentage of participants by age group reporting medically attended adverse events
Timeframe: From vaccination to 6 months after last vaccination or through the last visit, whichever is longer
Percentage of participants by age group with abnormal hematology and chemistry laboratory values
Timeframe: 1 week, 3 weeks, 6 weeks, 8 weeks and 12 weeks after vaccination 1, 1 week and 4 weeks after vaccination 2, 1 week and 4 weeks after booster vaccination
Percentage of participants by age group with grading shifts in hematology and chemistry laboratory assessments
Timeframe: Between baseline and 1 week, 3 weeks, 6 weeks, 8 weeks and 12 weeks after vaccination 1, 1 week and 4 weeks after vaccination 2, 1 week and 4 weeks after booster vaccination
Percentage of participants by age group with new clinically significant ECG abnormalities
Timeframe: 1 week, 3 weeks, 6 weeks, 8 weeks and 12 weeks after vaccination 1, 1 week and 4 weeks after vaccination 2, 1 week and 4 weeks after booster vaccination
Percentage of participants by age group with new troponin I abnormalities
Timeframe: 1 week, 3 weeks, 6 weeks, 8 weeks and 12 weeks after vaccination 1, 1 week and 4 weeks after vaccination 2, 1 week and 4 weeks after booster vaccination