Vaccinating previously infected induces more potent antibodies less susceptible to escape from SARS-CoV-2 variants

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the causal agent of the coronavirus disease 2019 (COVID-19) pandemic, which has claimed more than 4.3 million lives worldwide. Even after implementing various measures, such as vaccination and social restrictions, SARS-CoV-2 and its variants of concern (VoC) leave the world's public health systems and economies in turmoil.

In most developed countries, scientists report that vaccination programs have commenced. The programs result in a decrease in the rate of hospitalization and mortality. However, vaccination has failed to control the spread of infection by the SARS-CoV-2 VoCs.

Previous studies have correlated COVID-19 protection with the production of neutralizing antibodies against the virus's spike (S) protein. The entry of the SARS-CoV-2 virus into the host cell is mediated by the S protein, which is a metastable and trimeric class 1 fusion glycoprotein. It contains two domains, namely, S1 and S2. The main function of the S1 subunit is to bind to the angiotensin-converting enzyme 2 (ACE2) receptor of the host, while the S2 subunit is involved with the fusion of membranes.

Neutralizing Antibodies and SARS-CoV-2 Variants of Concern

Effective neutralizing antibodies can recognize the S1 subunit of each monomer, for example, the receptor-binding domain (RBD) and N-terminal domain (NTD) immunodominant sites. Most neutralizing antibodies can bind to the receptor-binding motif (RBM) within the RBD, while a few target the NTD. Several neutralizing antibodies against S2 subunits have been reported to possess low potency. Previous studies reported that post-infection neutralizing antibodies are mostly derived from germline IGHV3-53 gene and are closely related to IGHV3-66 with very few somatic mutations.

Since the summer of 2020, the emergence of several SARS-CoV-2 variants was reported that could escape neutralizing antibodies. Some of these variants, such as the B.1.1.7 (alpha), B.1.351 (beta), B.1.1.248 (gamma), and B.1.617.2 (delta), have been classified as VoCs by the World Health Organization. Due to these variants being more infectious than the original strain, and could evade neutralizing antibodies. At present, the Delta variant is circulating globally and causing difficulty for the vaccinated population.

Identification of cross-neutralizing SARS-CoV-2 S protein-specific nAbs. (A) The graph shows supernatants tested for binding to the Wuhan SARS-CoV-2 S protein antigen stabilized in its prefusion conformation. Threshold of positivity has been set as two times the value of the blank (dotted line). Dark blue and red dots represent mAbs that bind to the S protein for seronegative and seropositive vaccinees respectively. Light blue and red dots represent mAbs that do not bind the S protein for seronegative and seropositive vaccinees. (B) The bar graph shows the percentage of not-neutralizing (gray), neutralizing mAbs from seronegatives (dark blue), and neutralizing mAbs for seropositives (dark red). The total number (n) of antibodies tested per individual is shown on top of each bar (C) Graphs show the fold change percentage of nAbs in seronegatives and seropositives against the alpha, beta and gamma VoCs compared to the original Wuhan SARS-CoV-2 virus. The heatmaps show the overall percentage of Wuhan SARS-CoV-2 nAbs able to neutralize tested VoCs.

Hybrid Immunity in Controlling SARS-CoV-2 and its Variants of Concern

Previous studies have indicated that vaccination of convalescent individuals can produce neutralizing antibodies a thousand-fold higher than those induced by infection or vaccination. These results implied that one of the measures to contain the pandemic could be initiating a hybrid immunity-like response via a third booster dose.

A new study published on the bioRxiv* preprint server focused on comparing the nature of the neutralizing antibody response against the original SARS-CoV-2 strain and the VoCs in naïve and convalescent participants vaccinated with the BNT162b2 mRNA vaccine at a single-cell level.

This study consisted of ten donors who were vaccinated with the BNT162b2 mRNA vaccine. Among them, five were naïve subjects or seronegative before vaccination, and five were convalescent donors who were seropositive before vaccination. The current study has shown that immunization of individuals, who are already seropositive to the virus, increases the frequency, potency, and breadth of neutralizing antibodies. Therefore, this strategy could be effective against emerging variants.

Previous studies had reported that COVID-19 vaccination of convalescent people stimulates the production of hybrid immunity. They further reported that the amount of neutralizing antibodies in these individuals was much higher than those elicited post-vaccination of seronegative naïve people. These results were in line with the reports of the current research. The present study also highlighted that vaccinating seropositive candidates produced neutralizing antibodies, which are much more effective against SARS-CoV-2 variants.

The current research revealed that most antibodies could neutralize the original Wuhan SARS-CoV-2 variant but fail to do so for beta and gamma variants. The percentage of antibodies neutralizing the alpha and delta variants is even less. The reason is that beta and gamma variants can escape natural immunity. Concerning alpha and gamma, they are far more infectious and became the dominant variant in a community.

Potency and breadth of neutralization of nAbs against SARS-CoV-2 and VoCs. (A – E) Scatter dot charts show the neutralization potency, reported as IC100 (ng/mL), of nAbs tested against the original Wuhan SARS-CoV-2 virus (A) and the B.1.1.7 (B), B.1.351 (C), B.1.1.248 (D) and B.1.617.2 (E) VoCs. The number and percentage of nAbs from seronegatives vs seropositives, fold-change and statistical significance are denoted on each graph. A nonparametric Mann–Whitney t test was used to evaluate statistical significances between groups. Significances are shown as *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. (F) The table shows the IC100 geometric mean (GM) of all nAbs pulled together from each group against all SARS-CoV-2 viruses tested.

Main Significance of the Study

The uniqueness of this study lies in the molecular analysis, which revealed that seropositive candidates respond to vaccination with more B cells producing antibodies. These antibodies possess greater neutralization potency and are effective against the variants that are reported to evade immune responses. The authors of this study explained that only seropositive candidates, post-vaccination, could produce antibodies derived from the IGHV2-5; IGHJ4-1 germline. This study indicated the absence of the above-stated germline in seronegative vaccinees, a finding which is consistent with earlier studies. This study proposes that germline-targeting vaccination, which has shown promising results for HIV, may be an effective strategy to combat the ongoing COVID-19 pandemic.

A Limitation of the Study

One of the limitations of this study is that it did not include candidates who received a third booster dose of vaccine. The reason being that, during the study period, administering a third booster dose was not implemented. This study indicated that a booster vaccine could positively enhance the frequency of memory B cells producing effective neutralizing antibodies that are more effective against variants.

*Important Notice

bioRxiv 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.

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.