This study is testing a COVID-19 vaccine called **BBV152**. It will compare the immune responses of people in the US to those in India who were in Phase 3 trials. Participants will receive two doses of the vaccine or a placebo (a shot with no medicine) over 28 days. The study checks if the immune system reacts the same way in different groups and if BBV152 works well in people who had other COVID-19 vaccines like mRNA (a kind of vaccine that uses a small piece of the virus's genetic code) or J&J. **Key Points:** - Participants will get two shots over one month and have check-ups for safety. - It's for adults aged 18 and older who are healthy and have had certain COVID-19 vaccines. - Safety checks with medical teams are included, and participants must avoid other vaccines during the study. Safety is closely monitored by experts. Participants must be in stable health and follow all study rules, including not taking other vaccines during the study. Pregnant or breastfeeding women, or those with serious health conditions, cannot join.