Discussion:
The continuous emergence of new variants of SARS-CoV-2 has become the
most concern in current COVID-19 pandemic, which has caused successive
global waves of infection, and poses great challenges to vaccine
research and development. Up to now, Omicron has become the dominating
variant due to its high transmissibility and immune evasion (9). It is
important to understand whether current immunity induced by vaccine and
pre-infection is effective against different Omicron variants. Previous
studies showed that three or four doses of parental mRNA vaccine did not
elicit robust neutralization against BA.4/5 (10). Recent studies showed
that the SARS-CoV-2 omicron variant has less sensitive to neutralizing
antibody responses induced by vaccination and previous infection than
previous variants (11,12).In the study, we determined IgG antibody
levels and neutralizing antibody levels of the serum from donors
immunized with inactivated COVID-19 vaccine.
After the third dose of the homologous inactivated vaccine in 88 young
population, the neutralizing activity against the Omicron BA.1 and BA.5
variant were decreased significantly compared with the prototype strain
respectively (P <0.05). However, the IgG titers against
the S of the prototype, Omicron BA.1 and BA. 5 variants detected by
ELISA, showed no significant difference between the prototype and
Omicron variants. It may consider that a part of IgG binding to S of
different Omicron variants has no neutralization potency.
Some study demonstrated strong correlation of quantitative test with
neutralization testing. A study showed that Anti-RBD IgG plasma
concentration significantly correlated with the plasma/serum VN activity
against SARS-CoV-2 in vitro (r s=0.888,p <0.0001) (7). The anti-SARS-CoV-2 S1 IgG was also
analyzed in other study, indicating a high correlation with the titers
of neutralizing antibodies (r s=0.819,p <0.0001) (13). In our study, we analyzed the
correlation of anti- SARS-CoV-2 S IgG titers and neutralizing activities
based on pseudovirus. The results showed different levels of correlation
of IgG titers by ELISA and neutralizing antibodies, against SARS-CoV-2
prototype, Omicron variant BA.1 and BA.5, which indicated that
determination of IgG titers against the SARS-CoV-2 Spike protein can not
predict the viral immune evasion against different Omicron variants.
However, there are some limitations in the study. First, the number of
serum samples is limited, and the results need to be conformed in a
large sample size in further study. Second, the neutralization activity
test was on the basis of a pseudotyped virus system. Further studies
based on live viral particles need to be carried out.
In summary, we demonstrated that the Omicron variant BA.1 and BA.5
escapes vaccine-induced immunity by neutralization activity test, and
IgG titers against the SARS-CoV-2 Spike protein can not predict the
viral immune evasion against different Omicron variants.