Pandemic Influenza Modified RNA Vaccine (Pfizer)

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.