The Coronavirus Disease 2019 (COVID-19) pandemic has claimed over 5 million livesglobally. Fortunately, a substantial and growing number of SARS-CoV-2 vaccines with veryhigh efficacy have been developed, manufactured, and rapidly approved. Novel mRNAvaccines such as the BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) have reported astunning >94% efficacy against COVID-19. However, global access has not been equitable,with many low- and middle-income countries having no vaccine access or access underemergency use mainly to traditional inactivated SARS-CoV2-2 vaccines such as BBIBP-CorV(Sinopharm Beijing), CoronaVac (Sinovac) and BBV152 (Bharat Biotech). Emerging studieshave shown that lower concentrations of neutralizing antibodies (Nab) are attained afterCoronaVac than after a mRNA-based vaccine in healthy individuals. This difference seemsto be more pronounced in immunocompromised patients who are at higher risk of severeCOVID-19 and death from COVID-19. As such several countries including United States,Israel and Chile have recommended a third vaccine dose for this vulnerable population.In this observational study we will explore the humoral response to the BNT162b2 vaccinein patients who received two previous doses of the inactivated vaccine Coronavac or twodoses of BNT162b2.