Abstract
The first two vaccines administered in the COVID-19 vaccination campaign of India were Covaxin (BBV152) and Covishield (ChAdOx1-nCoV-19). In this study, we evaluate the longevity and sustainability of the humoral immune response after vaccination and various factors influencing it. An observational study was conducted in individuals who received both doses of Covaxin or Covishield vaccine, and their blood samples were analyzed for total-antiRBD-SARS-CoV-2 antibodies. Then, antibody titers were classified based on monthly time-intervals up to 360 days and their trend was analyzed. In addition, the correlation between antibody titers and factors such as previous SARS-CoV-2-infection status, vaccine type and presence of comorbidities was examined. Of the 2069 participants, most (1767;85.4%) had been vaccinated with Covaxin, but the higher antibody titers were induced by Covishield vaccine at all time points. However overall, antibodies persisted for at least 1 year, although a drop in antibody titers occurred in the 3rd and 6th months. In addition, 430 (20.8%) participants had prior SARS-CoV-2 infection (hybrid immunity) with a significantly higher humoral immune response compared with vaccine-induced immunity (naive immunity). No significant differences were observed in antibody titers related to age, sex and presence of comorbidities. We concluded that vaccine-mediated immunity lasts for at least one year. However, antibody titers decrease over time, which may be more pronounced in certain groups such as Covaxin vaccine, vaccine-induced-immunity, presence of comorbidities and > 60 years which should be considered when recommending booster vaccination, as these individuals may have a stronger and longer-lasting immune response to the virus.