This study involves comparing two flu vaccines: **mRNA-1010** and a regular licensed flu shot. The main goals are to check how safe mRNA-1010 is and how well it works against flu-like illnesses caused by flu strains A or B. Adults who are 50 years or older can join this study. They need to give written permission and follow some rules. People who can become pregnant must use birth control and test negative for pregnancy. Some people can't join, like those who are sick with a fever or have certain health conditions.

• The study checks if mRNA-1010 is safe and effective against the flu.
• Participants must be 50 years or older and meet certain health requirements.
• People who are sick or have certain health issues may not be able to join.