In a recent study posted to the medRxiv* pre-print server, researchers evaluated whether neutralizing immunity elicited against Omicron sub-variant BA.1 in unvaccinated and vaccinated individuals adequately protected against infection by Omicron sub-variants, BA.4 and BA.5.
The BA.1 sub-lineage was the first to emerge from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron (B.1.1.529) variant of concern (VOC). It had a remarkable potential to escape neutralizing immunity elicited by the previous infection from other SARS-CoV-2 variants, vaccines, or both.
Later, Omicron’s two new sub-variants, BA.4 and BA.5, emerged in South Africa, which differed from the BA.1 sub-lineage due to the F486V and L452R mutations in the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) protein.
About the study
In the present study, researchers recruited Omicron/BA.1-infected individuals between November and December 2021. Of these, 24 individuals were unvaccinated against coronavirus disease 2019 (COVID-19), and 15 were vaccinated. More specifically, eight and seven participants had received the BNT162b2 and Ad26.CoV.2S COVID-19 vaccine, respectively.
The team collected the blood samples from all the study participants at a median of 23 days after symptoms onset, when neutralizing immunity developed to Omicron/BA.1 had plateaued. Whole-genome sequencing confirmed that Omicron/BA.1 sub-variant infected all the study participants.
All the experiments with the live BA.4 and BA.5 viruses were performed in biosafety level 3 containment, adhering to protocols approved by the Africa Health Research Institute Biosafety Committee. The researchers used the H1299-E3 cell line for viral expansion and infecting the Vero E6 cells.
The team used viral supernatant from the co-culture of angiotensin-converting enzyme 2 (ACE2)-expressing H1299-E3 and Vero E6 cells for all the study experiments. They used an in vitro, live-virus focus reduction neutralization test (FRNT). Notably, FRNT50 is the reciprocal dilution of serum that neutralizes 50% of the inoculated virus or the infective dose (ID50).
The authors diluted virus stocks with plasma and attained the most concentrated plasma dilution of 1:25, which were extrapolated to FRNT50 values <25. Further, they log-transformed FRNT50 or fold-change in FRNT50 per participant to calculate confidence intervals (CIs). The team exponentiated CIs to obtain the upper and lower 95% CIs on the geometric mean FRNT50.
Study findings and conclusion
In the unvaccinated BA.1 infected participants, FRNT50 declined from 275 to 36 and 37 for BA.1, BA.4, and BA.5, respectively. Conversely, in the vaccinated group infected with BA.1, FRNT50 declined from 507 to 158 and 198 for BA.1, BA.4, and BA.5, respectively.
The aggregate BA.4- and BA.5-neutralization levels were about five-fold higher in the vaccinated group compared to the unvaccinated group. Thus, they appeared to be more protected, although waning immunity might decrease the conferred protection again among the vaccinated population.
The neutralizing immunity elicited in response to Omicron/BA.1 infection in unvaccinated participants was low. Hence the BA.4 and BA.5 fold-drops led to low residual neutralization levels. Presumably, the low immunity to BA.1 reflected this variant’s lower immunogenicity.
Since the sequence differences between BA.4 and BA.5 sub-variants nested outside SARS-CoV-2 S, they both showed similar immune evading capabilities. However, the observed immunity escape was strikingly higher than the non-substantial escape detected for BA.2.
Vaccination increased neutralization capacity against emerging Omicron sub-variants and would likely offer good protection against severe COVID-19. However, it is possible that a BA.4/BA.5 infection wave could hit the unvaccinated population since the findings showed low residual virus neutralization and higher immune escape.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.