SARS-CoV-2 Variant Biology and Immune Response Dynamics

The ongoing evolution of SARS-CoV-2 has produced successive waves of variants characterised by alterations in spike protein structure, transmissibility and immune evasion. These changes often arise through amino acid substitutions, insertions, deletions or, in some cases, recombination events that enhance receptor binding or modify entry pathways. Variants such as Delta and Omicron have demonstrated shifts in cell‐entry strategies, with some relying more heavily on endosomal uptake rather than TMPRSS2-mediated plasma membrane fusion. Such shifts can influence tissue tropism and pathogenicity. Concurrently, the human immune response—comprising innate defences, neutralising antibodies, memory B cells and T-cell responses—adapts to these viral changes. While neutralising antibodies target the receptor-binding domain and N-terminal domain of spike, T-cell epitopes remain more conserved, providing a measure of protection against severe disease. Breakthrough infections and waning antibody titres have emphasised the importance of booster vaccinations and updated immunogens. Global surveillance and functional studies of emerging lineages continue to inform vaccine design, antiviral deployment and public health strategies.

Research from Nature Portfolio

Recent studies have revealed that recombination among Omicron sublineages can yield variants with enhanced fitness. One investigation demonstrated that the XBB lineage arose through recombination of two BA.2 progenitors, leading to a spike protein that combines immune-evasive regions with increased cell-fusion capacity. Structural analyses elucidated how the recombinant receptor-binding domain interacts with ACE2, while animal models confirmed comparable or attenuated pathogenicity relative to parental strains. Another series of experiments examined how the Omicron BA.1 spike dramatically alters its usage of host proteases. Unlike Delta, BA.1 shows reduced TMPRSS2 dependence and a preference for endosomal entry, correlating with lower fusogenicity in lung and gut cells and reduced syncytium formation. These phenotypes map to distinct regions of the S1/S2 cleavage site and the S2 domain. A complementary study has highlighted the pronounced evasion of vaccine-elicited and therapeutic monoclonal antibodies by Omicron BA.1 and BA.2, and the shift away from plasma membrane fusion. Booster immunisation restores some neutralisation breadth, underscoring the need for updated vaccines aligned with variant antigenicity.

SARS-CoV-2 Variant Biology and Immune Response Dynamics publication trend

The graph below shows the total number of articles in sars-cov-2 variant biology and immune response dynamics across all publications each year (not limited to Nature Index journals).

Technical terms

Spike protein: The surface glycoprotein of SARS-CoV-2 that mediates attachment to the ACE2 receptor and fusion with host cells.

Receptor-binding domain (RBD): A subregion of the spike protein responsible for direct interaction with the ACE2 receptor on host cells.

Fusogenicity: The capacity of viral spike proteins to induce membrane fusion and syncytium formation between infected and neighbouring cells.

TMPRSS2: A host cell serine protease that cleaves the spike protein, facilitating plasma membrane entry of the virus.

Endosomal entry: A pathway by which the virus enters cells via endosomes, often following spike activation by cathepsins rather than TMPRSS2.

Immune evasion: The ability of viral variants to resist neutralisation by pre-existing antibodies, either from infection or vaccination.

Syncytium: A multinucleated cell formed by the fusion of infected cells, commonly used as a proxy for viral fusion activity.

References

1. Virological characteristics of the SARS-CoV-2 XBB variant derived from recombination of two Omicron subvariants. Nature Communications (2023).
2. Altered TMPRSS2 usage by SARS-CoV-2 Omicron impacts infectivity and fusogenicity. Nature (2022).
3. SARS-CoV-2 Omicron is an immune escape variant with an altered cell entry pathway. Nature Microbiology (2022).
4. Virological characteristics of the SARS-CoV-2 BA.2.86 variant. Cell Host & Microbe (2024).
5. Virological characteristics of the SARS-CoV-2 Omicron BA.2 subvariants, including BA.4 and BA.5. Cell (2022).