This study looks at how safe and effective a new flu shot is for people aged 18 and older. The shot uses something called mRNA to help teach the body to fight the flu. **mRNA** stands for *messenger RNA*, which is like a set of instructions for our cells to make proteins that help fight germs. The study checks if the shot causes any side effects and how well it helps the body defend against the flu.

**Eligibility**:

• Participants must be at least 18 years old and healthy or have stable medical conditions.
• Women who can have children need to use birth control and take a pregnancy test before joining.
• People with allergies to flu shots or mRNA shots, or certain health issues, cannot join.

**Key Points**:

• Participants will visit the study site several times.
• They must keep a diary of any side effects and follow study instructions.
• People who are pregnant or breastfeeding cannot take part.

Participants will need to sign a consent form to join. The study is checking if the vaccine is both safe and effective for different age groups.