Effectiveness of two dose Covaxin, an Inactivated COVID-19 vaccine in protection against SARS-CoV-2 infection: Six-month prospective cohort study

ABSTRACT

Introduction: SARS-CoV-2 affected millions of lives globally and led to devastating impact on public health. India had also witnessed the dreadful effect of SARS-CoV-2 pandemic. Within a short span of time, various SARS-CoV-2 vaccines were developed using different platforms across the world. India has also developed one such indigenous whole-virion inactivated SARSCoV-2 vaccine named as BBV152 (Covaxin). The Covaxin has been found to be immunogenic and second most widely used vaccine in India. Recent studies have also shown significant increase in the humoral and neutralizing antibody response post the administration of booster dose against Omicron variant. Apparently, there is limited data on the long-term persistence of the immune response against the Covaxin in Indian context. Method(s) We evaluated an effectiveness of the Covaxin and comparing its specific immune responses in two categories through prospective cohorts recruited at the vaccination centre, Pune during June 2021 to March 2022. We defined the study population in two groups who were COVID-19 naive individuals (group-1) and COVID-19 recovered individuals (group-2) prior to the immunization with Covaxin. The two cohorts and the study participants were decided considering the baseline antibody titres against SARS-CoV-2, the COVID-19 positivity rate, sample power and loss to follow up. The study population was assessed during three follow-ups at second dose, one and six months post second dose to determine the immune response and effectiveness using S1-RBD IgG ELISA and neutralizing antibody response (NAbs) by plaque reduction neutralization test (PRNT). Result(s) We enrolled participants between age group of 18-80 year (median 32 years). In group-1 and group-2, we recruited 118 and 128 participants respectively. The cohort retention was found to be> 85%,>70% and>40% in 1st, 2nd and 3rd follow up respectively. Loss to the 3rd follow up was coincided with third wave with omicron variant. A rise in geometrical mean titre (GMT) of S1-RBD IgG were observed amongst the participants of both the groups at one-month post immunization (Group 1 S1-RBD 154.4 to 446.3, Group 2 S1- RBD 918 to 1127). However, the GMTs at six months post vaccination found to be slightly raised in Group 1 compared to one-month follow-up. Considering the hybrid immunity in group 2 participants, the GMTs of NAbs were higher than group 1 participants at each follow-up against B.1, Delta, Omicron BA.1 and BA.2. Both the groups had shown significant reduction in the levels of NAbs against Delta, Omicron BA.1 and BA.2 compared to B.1. The lowest GMTs of NAbs was observed against BA.1 variant. The IgG and NAbs persisted till six months in 90% participants in both categories except BA.1 variant. Breakthrough cases were reported at one-month (n = 1) and six-months (n = 2) post vaccination respectively from group 1. While reinfection cases (n = 3) were detected at six months post vaccination from group 2 due to Omicron BA.1 variant. Conclusion(s) A two-dose regimen of the Covaxin vaccine enhanced humoral immune response in adults with/without past COVID-19 infection and protected more than 90% adults against SARSCoV-2 infection. Additionally, IgG and NAb responses persisted for six months postvaccination.