Primary objectives: 1. To evaluate the safety and tolerability of INO-4800 at different dose levels in healthy subjects (>= 18 and < 60 years old as well as >= 60 and <= 85 years old); 2. To evaluate the humoral immune response of INO-4800 at different doses in healthy subjects (>= 18 and < 60 years old as well as >= 60 and <= 85 years old).

Secondary objective: 3. To evaluate the duration of humoral response of INO-4800 at different doses in healthy subjects at different ages ((>= 18 and < 60 years old as well as >= 60 and <= 85 years old).

Exploratory objective: 4. To evaluate the cellular immune response of INO-4800 at different doses in healthy subjects ((>= 18 and < 60 years old as well as >= 60 and <= 85 years old).