1. Antibody titers against Spike protein after interchangeability of the study vaccines at baseline; 28 days of first and second dose; 6 months and 1 year. <br/ >2. Incidence of mild; moderate and serious adverse events rate of the Covid-19 vaccines after interchangeability <br/ >3. Breakthrough infection rate of the Covid-19 vaccines (Covishield/Covaxin/Sputnik V) after interchangeability.Timepoint: At at baseline; 28 days of first and second dose; 6 months and 1 year.

1. Antibody titers against Spike protein after interchangeability of the study vaccines at baseline; 28 days of first and second dose; 6 months and 1 year. <br/ >2. Incidence of mild; moderate and serious adverse events rate of the Covid-19 vaccines after interchangeability <br/ >3. Breakthrough infection rate of the Covid-19 vaccines (Covishield/Covaxin/Sputnik V) after interchangeability.Timepoint: At at baseline; 28 days of first and second dose; 6 months and 1 year.