This study is testing a new COVID-19 vaccine called BBV152. The study compares the immune responses (the body's defense system) of people in the U.S. to those in India who were in an earlier study. Participants must be healthy adults aged 18 or older. The study checks if the BBV152 vaccine works well in people who have already had other COVID-19 vaccines (like mRNA vaccines or the J&J/Janssen vaccine).

Key Points:

• Participants will receive two shots of either the BBV152 vaccine or a placebo (a harmless shot that looks like the vaccine but isn't) over 28 days.
• Participants must avoid other vaccines during the study. They must also agree to use birth control if they can have children.
• The study monitors for any side effects or health issues. Safety checks by a special board happen at 2 and 6 months.