This study is checking how safe and effective a new vaccine is against a type of flu called H5N1. The vaccine uses a special method called *messenger RNA (mRNA)* to teach our bodies to fight the flu. The study has two parts: **Phase 1** and **Phase 2**. In Phase 1, they will try different amounts of the vaccine on healthy adults aged 18 to 85, and compare it with a placebo (a fake treatment) to see which dose works best. Phase 2 will use the best dose from Phase 1 and compare it with a regular flu vaccine. People in the study will get two shots of the vaccine, 21 days apart, and they will be checked for reactions and side effects up to 29 days after the first dose.

To join the study, you must be a healthy adult between 18 and 85 years old, and you need to be willing to follow the study rules. You can't join if you have certain health problems, are pregnant, or have been in another flu study before.

• The study involves multiple visits and injections.
• Participants will be closely monitored for health and safety.
• Compensation may be provided for participation.