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.