Introduction:
Serological assays are important tools in the monitor of infectious diseases and the detection of immunity induced by vaccine. In current COVID-19 pandemic, to better understand whether current immunity induced vaccine and pre-infection is effective against different Omicron variants, developing countries have faced crucial challenges to develop validated, standardized serological assays to assess the antibody response against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
The neutralizing antibody assays based on live viral particles which detected and quantified in a neutralization assay is considered as the reference gold standard, such as microneutralization assay (1) or plaque-reduction neutralization test (2). Neutralizing assays based on pseudo-neutralization assay is also widely used in the evaluation of immune efficacy in development of vaccines and antibody drugs (3,4), or the screening of recovered patients for plasma therapy (5).
However, these serological assays, which assessing the antibodies that inhibit infection of cultured cells, is labor-intensive or restricted to biosafety level 3 laboratories. To overcome these limitations, standardized enzyme-linked immunosorbent assay (ELISA) based on recombinant SARS-CoV-2 antigens have developed, including spike (S) protein, S1 domain and receptor-binding domain (RBD) (6,7). Evaluating the IgG titers targeting the S protein of SARS-CoV-2 by ELISA assays have been used in the development of several anti-SARS-CoV-2 vaccines in China. It is unclear whether the IgG levels against S protein tested by ELISA can monitor immune evasion against different Omicron variants.
In current study, we have first determined IgG antibody levels of the serum from donors immunized with inactivated COVID-19 vaccine, against the S protein of the prototype, Omicron BA.1 and BA.5 variants. Meanwhile, we have assessed the neutralizing antibody levels with the same serum based on pseudovirus system. We have also investigated the level of correlation between standardized ELISA for the detection of anti-SARS-CoV-2 S IgG and neutralization activity by pseudo-neutralization assay.