CoviVac vaccination induces production of neutralizing antibodies against Delta and Omicron variants of SARS-CoV-2

Vaccines are proven to be an effective tool in prophylaxis of severe COVID-19, but emerging mutated SARS-CoV-2 variants constantly challenge vaccines’ protectivity. We have evaluated the ability of the sera from individuals vaccinated with two variants of inactivated vaccine CoviVac and COVID-19 convalescents (May-December 2020) to neutralize SARS-CoV-2 variants Delta and Omicron. Four groups of serum samples (CoviVac vaccinees; COVID-19 convalescents; mice immunized with CoviVac preparations based on prototype B.1.1 strain and Delta variant) were evaluated in virus neutralization test against SARS-CoV-2 heterologous B.1.1 virus, Delta and Omicron variants. CoviVac preparations based on B.1.1 and Delta induced neutralizing antibodies against SARS-CoV-2 B.1.1 and two variants of concern. We observed a decrease in neutralization capacity in the sera from CoviVac (based on B.1.1 strain) vaccinees: 57.1% samples had detectable neutralizing antibodies against Delta and 61.9% against Omicron variants. Sera samples of all (100%) mice immunized with a candidate vaccine based on the SARS-CoV-2 Delta variant strain had neutralizing antibodies against all tested strains.