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.