Pre-existing antibody levels negatively correlate with antibody titers after a single dose of BBV152 vaccination.

Many adults in India have received at least one dose of COVID-19 vaccine with or without a prior history SARS-CoV-2 infection. However, there is limited information on the effect of prior immunity on antibody response upon vaccination in India. As immunization of individuals continues, we aimed to assess whether pre-existing antibodies are further boosted by a single dose of BBV152, an inactivated SARS-CoV-2 vaccine, and, if these antibodies can neutralize SARS-CoV-2 Delta and Omicron variants. Here we show that natural infection during the second wave in 2021 led to generation of neutralizing antibodies against other lineages of SARS-CoV-2 including the Omicron variant, albeit at a significantly lower level for the latter. A single dose of BBV152 boosted antibody titers against the Delta and the Omicron variants but the antibody levels remained low against the Omicron variant. Boosting of antibodies showed negative correlation with baseline neutralizing antibody titers.