This study tests a new vaccine called OCU500 designed to protect against COVID-19. The vaccine uses a special delivery system called a ChAd36 Vector, which is a harmless virus modified to help the body recognize the COVID-19 virus spike protein. Participants will receive the vaccine either through the nose or by inhaling it. This study is for healthy adults ages 18-64 who have already had their first COVID-19 vaccines and boosters. The study is divided into four groups, each with 20 people, and looks at how safe the vaccine is and how well it works in producing an immune response.

• The study requires participants to be available for all study visits.
• Participants will be monitored for safety and immune responses.
• The study lasts up to 6 months, with some restrictions on travel and vaccines.