This study looks at how safe and effective a new combination vaccine is for adults aged 50 and older. It combines a COVID-19 vaccine with an influenza (flu) vaccine. The COVID-19 part is called *rC19* and the flu part is called *IIV-HD*. A vaccine helps your body recognize and fight viruses. The study will compare different ways of giving the rC19 and IIV-HD vaccines together or separately.

Each participant will get two shots in their upper arms at the first visit. The study involves checking in several times over a year: on Day 1 (D01), Day 9 (D09 - phone call), Day 30, Day 182 (D182 - phone call), and Day 366 (D366 - phone call). The study will happen across different places in the USA.

Key Points:

• The study lasts for about 12 months.
• Participants must be able to receive injections in both arms.
• Participants must have already received COVID-19 vaccinations and a booster.

This study aims to see how well the combination vaccine works and if it is safe for people to use. It is important to make sure you meet the eligibility criteria before joining.