SARS‐CoV‐2 Variants and Immune Evasion: Mapping the Future of Vaccine Design

ABSTRACT

The evolutionary trajectory of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2) has progressed through several distinct phases since its zoonotic emergence, transitioning from initial human adaptation to an era of rapid antigenic drift and complex immune evasion. As of early 2026, the global landscape is dominated by highly evolved sublineages of the Omicron (B.1.1.529) variant, including the JN.1‐descendent subvariants NB.1.8.1 and XFG. This review provides a comprehensive overview of the molecular mechanisms driving viral fitness, with a primary focus on the structural transformations within the spike (S) protein's receptor‐binding domain (RBD), N‐terminal domain (NTD), and S2 subunit. We examine the biophysical impacts of pivotal mutations, such as E484 K, K417 N, and F486P, alongside the phenomenon of convergent evolution and epistatic compensation. Furthermore, we provide an integrated analysis of current knowledge regarding the evolving dynamics of humoral and cellular immunity, exploring the challenges posed by immune imprinting and the decline of neutralizing antibody titers against antigenically distant strains. A comparative discussion of SARS‐CoV‐2 and seasonal influenza highlights divergent evolutionary paces but converging regulatory frameworks for annual vaccine updates. Finally, the current status of next‐generation vaccine platforms is evaluated, specifically mosaic nanoparticles and mucosal delivery systems, which aim to provide pan‐sarbecovirus protection and interrupt transmission. These insights are integrated into a policy framework focused on annual strain selection and enhanced genomic surveillance for sustainable long‐term pandemic management.

1. Introduction

1.1. Evolutionary Phases and Key Spike Mutations From 2020 to 2026

The genomic evolution of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2) since its emergence in late 2019 represents one of the most intensively monitored examples of viral adaptation in modern history [1]. After its zoonotic spillover into a largely immunologically naive population, the virus has since transitioned into an endemic respiratory pathogen circulating within a host population that is globally vaccinated and repeatedly exposed (Figure 1). Across this 6‐year period, viral evolution has reflected a dynamic and continuous arms race between structural constraints, replication efficiency, and escalating immune pressure (Table 1). This process can be divided into distinct evolutionary phases, each defined by the rise of Variants of Concern (VOCs) that conferred measurable advantages in transmissibility, infectivity, or immune escape [2].

FIGURE 1.

The Multi‐Phase Genomic Evolution of SARS‐CoV‐2 (2020–2026). This schematic outlines the transition of SARS‐CoV‐2 from a zoonotic spillover event in a naive population to an endemic pathogen shaped by global immune pressure. The current scientific consensus strongly supports the theory that SARS‐CoV‐2 originated in bats but reached humans through a complex path likely involving intermediate animal vectors (like pangolins) or human‐mediated trade. Phase 1 illustrates early viral stabilization, dominated by the D614 G mutation which increased receptor‐binding domain (RBD) accessibility and transmissibility. Phase 2 marks the emergence of early Variants of Concern (VOCs) and the Delta variant, characterized by optimizations in replication efficiency and furin‐mediated cleavage (P681 R) alongside the global rollout of vaccines. Phase 3 represents the Omicron era and beyond, defined by profound antigenic remodeling and the subsequent rise of recombinant sublineages (e.g., XBB, JN.1). During this stage, the viral population shifted from large‐scale structural shifts to incremental “fine‐tuning” of the spike protein, balancing immune evasion with the maintenance of ACE2 receptor affinity. The overarching evolutionary trajectory (bottom arrow) highlights the shift from selection for intrinsic transmission towards optimization for immune escape within a landscape of widespread hybrid immunity.

TABLE 1.

Timeline of key SARS‐CoV‐2 variants and mutational milestones (2020–2026).

Variant/lineage	Earliest samples	Key spike mutations	Evolutionary impact
Index (Wuhan‐Hu‐1)	2019–12	D614 G	Prototypical sequence used for first‐gen vaccines [6].
Alpha (B.1.1.7)	2020–09	N501Y, D614 G, P681H	Enhanced ACE2 affinity and increased transmissibility [8].
Beta (B.1.351)	2020–05	K417 N, E484 K, N501Y	Major immune evasion; 417N disrupts class 1 nAbs [9].
Gamma (P.1)	2020–10	K417 T, E484 K, N501Y	Increased ACE2 binding affinity and reduced neutralization [10].
Delta (B.1.617.2)	2020–10	L452 R, T478 K, P681 R	High fusogenicity; optimized furin cleavage [12].
Omicron (BA.1)	2021–11	> 30 mutations (RBD, NTD)	Dramatic vaccine breakthrough; altered entry pathway [16].
XBB.1.5	2022–10	F486P, N460 K, S477 N	Optimized ACE2 binding; notable epitope turnover [18].
JN.1	2023–08	L455S (on BA.2.86)	Marked growth advantage; dominance through late 2024 [19].
NB.1.8.1	2025–01	G184S, A435S, K478I	Emerging VUM with revised NTD/RBD profile [2].
XFG	2025–01	S31P, K444 R, N487D, T572I	Dominant VUM as of early 2026; refined drift [2].

Abbreviation: Vum, Variant under monitoring.

Phase I (early–mid 2020) focused on viral stabilization and fitness within the human host, driven by selective pressure for transmissibility rather than immune escape [3]. The defining event was the global sweep of the D614 G mutation in the spike glycoprotein [4]. Structurally, this substitution weakens intradomain contacts near the S1 interface, increasing the probability that RBDs adopt the “up” conformation required for angiotensin‐converting enzyme 2 (ACE2) engagement [5]. By enhancing receptor accessibility and virion stability, D614 G optimized viral entry without significantly altering antigenicity [6]. This rapid dominance of the G614 genotype marked the first major instance of strong positive selection in the virus's evolutionary trajectory.

Phase II (late 2020–early 2021) was defined by the emergence of early VOCs such as Alpha (B.1.1.7), Beta (B.1.351), and Gamma (P.1) which reflected convergent evolutionary responses to rising population immunity [7, 8, 9, 10]. Central to this phase was the N501Y mutation, which enhanced ACE2 binding affinity four‐fold through improved hydrophobic interactions, significantly increasing transmissibility. In parallel, the E484 K substitution emerged as a canonical escape mutation; by replacing negatively charged glutamic acid with positively charged lysine, it disrupted the binding of class 2 neutralizing antibodies [7]. Because escape mutations often carry fitness costs, variants frequently utilized epistatic networks such as the combination of K417 N/T, E484 K, and N501Y to balance immune evasion with high receptor affinity [11]. This co‐occurrence allowed the virus to maintain infectivity while navigating an increasingly complex immunological landscape.

Phase III (mid–late 2021) was characterized by the dominance of the Delta (B.1.617.2) variant, which favoured enhanced intrinsic viral fitness and replication efficiency rather than extensive immune escape [12]. Its defining mutation, P681 R, located at the S1/S2 junction, significantly enhances furin‐mediated cleavage efficiency [13]. This structural change increased the proportion of fusion‐ready spike proteins, leading to faster cell entry, enhanced syncytium formation, and higher viral loads [14]. Although Delta carried certain antibody‐evading mutations, its global dominance demonstrated that enhancing viral entry and replication efficiency remained a highly effective evolutionary strategy even as population immunity increased.

Phase IV (late 2021–2022) was defined by the emergence of Omicron (B.1.1.529), representing the most dramatic evolutionary shift in SARS‐CoV‐2 history [15]. Carrying more than 30 spike mutations clustered in the receptor‐binding domain (RBD) and N‐terminal domain (NTD), Omicron underwent a large‐scale antigenic remodeling that reshaped the ACE2‐binding interface and disrupted major neutralizing supersites. This fundamental reconfiguration resulted in near‐complete resistance to many therapeutic monoclonal antibodies and a substantial drop in neutralization by ancestral‐strain vaccines. Despite this extensive reengineering, Omicron preserved structural integrity and enhanced ACE2 affinity through compensatory mutational networks. Ultimately, Phase IV marked a transition from incremental drift to broad antigenic reengineering driven by intense global immune pressure.

Phase V (2022–2026) marks a shift from large‐scale structural changes to iterative refinement within the Omicron lineage, driven by progressive fine‐tuning and recombination [16, 17]. A landmark adaptation was the F486P mutation in XBB.1.5, which used a proline substitution to restore high ACE2 affinity while maintaining antibody resistance [18]. This precision continued with the BA.2.86 and JN.1 lineages, which incorporated subtle but strategic changes in the receptor‐binding motif (RBM) and the NTD to diminish neutralization in populations with high hybrid immunity [2, 19]. By 2024–2026, leading subvariants like NB.1.8.1 and XFG exemplify this advanced antigenic drift. Rather than dramatic shifts, these lineages utilize incremental modifications to narrow neutralization breadth, allowing the virus to sustain transmission without compromising structural stability.

Throughout these phases, SARS‐CoV‐2 evolution has shifted from optimizing human transmission to prioritizing immune evasion within a population protected by hybrid immunity. Early adaptations centred on structural stabilization and receptor engagement, while subsequent variants introduced escape mutations balanced by compensatory affinity‐enhancing substitutions. The Omicron era proved the virus's capacity for massive antigenic remodeling, whereas the post‐Omicron period (2022–2026) has been defined by incremental refinement and convergent evolution. Together, these patterns illustrate the transition from a naive‐host pandemic to a mature viral ecosystem increasingly shaped by population‐level immunity. Ultimately, spike protein evolution remains a delicate balance: the virus must maintain functional ACE2 binding and membrane fusion capability while retaining sufficient structural flexibility to allow for ongoing antigenic drift. This tension between structural necessity and immune pressure will continue to dictate the virus's long‐term evolutionary path.

2. Mechanistic Basis of Viral Structure and Convergent Evolution

The SARS‐CoV‐2 spike protein maintains a delicate evolutionary balance where mutations must enable immune escape without compromising trimer stability, proteolytic processing, or membrane fusion [20]. As a homotrimer, the spike consists of the S1 subunit (the NTD and the RBD) for host attachment and the S2 subunit for membrane fusion [21]. Productive infection requires the RBD to transition to an “up” configuration to engage the ACE2 receptor, followed by proteolytic activation (furin and Transmembrane Protease Serine 2 (TMPRSS2)) that triggers large‐scale rearrangements into a post‐fusion six‐helix bundle.

The P681 R and P681H mutations, located within the S1/S2 cleavage site of the SARS‐CoV‐2 spike protein, represent critical evolutionary adaptations that enhance viral processing. The substitution of the highly conserved proline with either histidine (P681H) or the more basic arginine (P681 R) increases the polybasic character of the furin cleavage site (FCS). This modification facilitates more efficient proteolytic priming by host cell furin‐like proteases, a step essential for the transition from viral attachment to membrane fusion. While P681H was notably associated with the increased fitness of the Alpha variant, the P681 R mutation most prominent in the Delta lineage appears to further optimize this cleavage efficiency, correlating with heightened fusogenicity and accelerated cell‐to‐cell spread. Consequently, these site‐specific mutations serve as key determinants of the virus's shifted transmission dynamics and altered pathogenic potential across emerging variants [22].

A definitive shift in cellular entry tropism distinguishes the Omicron lineage from the previously dominant Delta variant. While Delta utilized the plasma membrane protease TMPRSS2 to facilitate rapid surface fusion a process correlated with high fusogenicity and severe pulmonary pathology, Omicron exhibits a marked preference for cathepsin‐dependent endosomal entry [23]. This reduced reliance on TMPRSS2 likely explains Omicron's shifted replication profile towards the upper respiratory tract. Consequently, the transition from a TMPRSS2‐dependent to a TMPRSS2‐independent pathway represents a critical mechanistic pivot that has influenced both the transmission dynamics and the attenuated clinical severity of recent variants.

Because these transitions are energetically complex, mutations are often concentrated in the immunodominant loops of the NTD or the RBM. This functional necessity drives convergent evolution, where independent lineages repeatedly acquire identical substitutions (e.g., at positions 417, 452, 484, 486, and 501) as structurally inevitable solutions to immune pressure [24]. Furthermore, epistasis where the effect of one mutation depends on another allows the virus to buffer deleterious escape mutations with compensatory, affinity‐enhancing changes. While S2 remains more conserved, it also undergoes fine‐tuning to optimize fusion kinetics [1]. Ultimately, spike evolution is governed by structural necessity; the virus must reconfigure its antigenic surface while operating within the strict functional boundaries of host cell entry.

Beyond the structural evolution of the spike protein, nonsynonymous mutations within the non‐structural proteins (NSPs) have played a fundamental role in tailoring SARS‐CoV‐2 fitness [25]. Notably, the P323 L substitution in NSP12 and the recurrent 105‐107 deletion in NSP6 suggest a directed evolution towards optimized replication kinetics and enhanced evasion of cellular autophagy. Furthermore, modifications within the NSP3 macrodomain emphasize a sophisticated mechanism for antagonizing host interferon responses. These alterations in the replication‐transcription complex (RTC) likely act in synergy with spike mutations to define the unique transmission and pathogenic profiles of successive VoC.

3. RBD Dynamics and RBM Mutations

The RBD of the SARS‐CoV‐2 spike protein, spanning residues 319–541, contains the RBM, which forms the direct interface with the human ACE2 receptor [26]. This region is the primary determinant of viral entry into host cells and is also the main target of neutralizing antibodies generated by infection or vaccination. Because the RBM must simultaneously maintain strong receptor engagement while avoiding immune recognition, it is subjected to intense selective pressure, making it a hotspot for adaptive mutations (Table 2). One illustrative example of this evolutionary tension is the E484 K mutation [4]. In this substitution, the negatively charged glutamic acid at position 484 is replaced by a positively charged lysine. Structurally, this change alters the electrostatic surface of the RBM ridge, reducing binding by many class 2 neutralizing antibodies that specifically target this region. However, the mutation also risks decreasing ACE2 binding affinity, which could impair viral entry. Convergent evolution has frequently driven independent lineages to acquire the same or functionally analogous substitutions at key RBM positions. For instance, the K417 N/T mutation alters a lysine at residue 417 to either asparagine or threonine. This residue directly interacts with the ACE2 D30 residue, and the mutation modulates both receptor binding and class 1 antibody recognition. K417 N/T has arisen independently in the Beta (B.1.351), Gamma (P.1), and multiple Omicron sublineages, highlighting the limited adaptive solutions available for balancing immune escape with receptor engagement [7]. In more recent post‐Omicron variants, including XBB.1.5 and JN.1, the F486P substitution has emerged as a critical adaptation [27]. Phenylalanine 486 is a key hydrophobic residue in the ACE2‐binding interface, contributing significantly to binding energy. The substitution to proline introduces a rigid conformational kink that subtly reshapes the RBM loop. This change preserves or even enhances ACE2 affinity while simultaneously reducing recognition by neutralizing antibodies. F486P exemplifies how late‐stage SARS‐CoV‐2 evolution has shifted from broad antigenic remodeling, as seen in Omicron, towards precise, fine‐tuned structural modifications that optimize the trade‐off between infectivity and immune evasion.

TABLE 2.

Structural impacts of key RBD and S2 subunit mutations.

Domain	Mutation	Structural/functional impact
RBD	N501Y	Increases ACE2 affinity; stabilizes the “standing‐up” conformation [58].
RBD	E484 K/A	Charge reversal/shift; primary driver of class 2 nAb evasion [59].
RBD	K417 N	Interacts with ACE2 D30; alters class 1 nAb epitopes [60].
RBD	F486P	Enhances ACE2 interface; key differentiator in XBB/JN.1 sublineages [18].
NTD	L18 F	Increases replication and contributes to NTD supersite escape [61].
S2	N764 K	Destabilizing signal; tunes the downstream fusion machinery [16].
S2	N969 K	Located near the heptad repeat; stabilizes the fusion bundle [62].

Overall, the RBD and its RBM exemplify an ongoing evolutionary balancing act, in which the virus simultaneously evades host immune defenses while preserving the structural and functional features necessary for effective ACE2 receptor binding. This dual constraint explains why certain residues repeatedly appear as hotspots across independent lineages, reflecting both structural necessity and evolutionary predictability.

3.1. The Role of Epistatic Compensation

A central feature of SARS‐CoV‐2 evolution is the virus's reliance on epistatic interactions to navigate the competing demands of immune evasion and receptor engagement [28]. The predominant evolutionary trajectory reflects a careful balance in which mutations that enable escape from neutralizing antibodies may incur trade‐offs, including reduced affinity for the ACE2 receptor or decreased stability of the spike trimer in SARS‐CoV‐2. Without compensatory mechanisms, these trade‐offs could impair viral infectivity or transmission. Epistasis occurs when the phenotypic effect of one mutation depends on the presence of one or more additional mutations [20]. In SARS‐CoV‐2, epistatic networks allow deleterious changes that confer immune escape to be tolerated or even optimized. This creates a complex adaptive landscape in which the virus can explore multiple mutational pathways without losing critical spike functionality.

A prominent example is the N501Y mutation, which replaces asparagine with tyrosine at residue 501 within the RBD [7]. N501Y significantly increases ACE2 binding affinity, strengthening receptor engagement and compensating for reductions in affinity caused by immune escape mutations such as K417 N/T or E484 K/A. Individually, these substitutions may reduce receptor‐binding efficiency. However, when combined with N501Y, the spike protein maintains strong ACE2 affinity while simultaneously exhibiting a substantially altered antigenic surface in SARS‐CoV‐2.

Beyond these classic examples, epistatic compensation extends to more complex networks involving multiple residues across the RBD and even the NTD. For instance, combinations of mutations in the F486, Q493, and S494 residues in post‐Omicron subvariants (e.g., XBB.1.5) demonstrate synergistic effects that preserve ACE2 binding while conferring broad resistance to polyclonal sera [29]. Similarly, interactions between RBM and distal RBD residues can stabilize local secondary structures that would otherwise be destabilized by escape mutations, further highlighting the structural interdependence of epistatic networks.

Epistatic compensation also contributes to the virus's capacity for antigenic remodeling. By pairing receptor‐binding–enhancing mutations with escape mutations, SARS‐CoV‐2 can significantly alter its antigenic surface without sacrificing entry efficiency. This allows successive variants to evade both monoclonal antibodies and polyclonal immunity elicited by prior infection or vaccination, explaining the repeated emergence of lineages with overlapping immune escape profiles. In essence, epistatic compensation represents a fundamental strategy in SARS‐CoV‐2 evolution. It permits the virus to traverse mutational landscapes that would otherwise be constrained by structural and functional requirements, enabling the spike protein to remain both infectious and antigenically novel. Understanding these compensatory interactions is critical for anticipating the emergence of future variants and for designing vaccines or therapeutics capable of counteracting these sophisticated evolutionary strategies.

3.2. S2 Subunit and Fusion Machinery Adaptation

Although the majority of SARS‐CoV‐2 evolutionary studies have focused on the S1 subunit, which mediates receptor binding and is the primary target of neutralizing antibodies, increasing evidence indicates that the S2 subunit (residues 686–1273) is also subject to adaptive changes, particularly in post‐Omicron variants [1]. S2 contains the core fusion machinery of the spike protein, including the fusion peptide, heptad repeats HR1 and HR2, the central helix, the transmembrane domain, and the cytoplasmic tail, all of which are essential for facilitating membrane fusion after receptor engagement. Recent analyses of post‐Omicron subvariants, such as XBB.1.5, have identified several notable mutations within the S2 subunit, including N764 K, D796Y, Q954H, and N969 K, many of which cluster near the heptad repeats [27]. These residues are not directly involved in ACE2 binding but play crucial roles in the structural rearrangements that drive membrane fusion. By subtly modifying local interactions, these mutations are thought to fine‐tune the energetic landscape of the prefusion‐to‐postfusion transition, potentially optimizing the kinetics and efficiency of viral entry.

Structural and computational studies provide insights into how these S2 mutations affect spike dynamics. For instance, D796Y and N969 K are generally predicted to stabilize local secondary structures or intersubunit contacts, reinforcing the integrity of the prefusion trimer. Conversely, N764 K may have a mildly destabilizing effect, introducing flexibility that could facilitate the large conformational changes required for fusion. The interplay between stabilizing and destabilizing mutations suggests a nuanced modulation of the spike's stability, balancing the need for a metastable prefusion state with the capacity to trigger membrane fusion efficiently upon receptor binding.

Beyond structural stability, S2 mutations may also influence fusion peptide exposure and the formation of the six‐helix bundle critical for merging viral and host membranes. Optimizing these processes can enhance viral entry efficiency, particularly in the context of altered S1 interactions caused by RBM or NTD mutations that increase immune evasion. In essence, changes in S2 act as a complementary adaptation, ensuring that improvements in immune escape within S1 do not compromise the fundamental mechanics of viral entry. Functionally, these adaptations may also impact tropism and transmissibility. By adjusting fusion kinetics and the stability of intermediate conformations, S2 mutations could allow the virus to infect a broader range of cell types or more efficiently establish infection in tissues with varying protease environments. This may contribute to the observed competitive advantage of lineages like XBB.1.5 and other post‐Omicron subvariants in populations with high levels of hybrid immunity.

In summary, the S2 subunit is emerging as a secondary but critical target of SARS‐CoV‐2 evolution, particularly in the post‐Omicron era. Mutational collections in S2 complement the adaptive changes in S1, fine‐tuning the spike protein's stability, fusion dynamics, and overall infectivity. Understanding these adaptations is essential for predicting variant fitness, evaluating antiviral strategies targeting fusion, and designing next‐generation vaccines that consider both receptor‐binding and fusion machinery dynamics.

3.3. Humoral Immunity Dynamics: Antibody Titers, Persistence, Antigenic Drift, and Hybrid Immune Responses

Humoral immunity, encompassing both neutralizing and non‐neutralizing antibodies, remains a central component of defence against SARS‐CoV‐2 infection [5]. However, the continuous emergence of antigenically shifted variants has created a scenario of constant epitope turnover, where the overlap between current viral targets and prior immune memory is progressively reduced [30]. Neutralizing antibodies directly block viral entry by interfering with spike–ACE2 interactions, while non‐neutralizing antibodies (nNAbs) mediate clearance of infected cells through Fc‐dependent mechanisms, such as antibody‐dependent cellular cytotoxicity (ADCC), antibody‐dependent cellular phagocytosis (ADCP), and complement‐dependent cytotoxicity (CDC). Together, these responses shape host protection and drive selective pressure on the virus, influencing its evolution.

Emerging variants, including XBB.1.5 and JN.1, demonstrate markedly reduced serum neutralizing titers even in individuals with hybrid immunity those who have experienced both vaccination and natural infection [31]. Analysis using the Jaccard similarity index, which measures residue‐level overlap in predicted linear B‐cell epitopes, shows only moderate similarity between XBB.1.5 and the ancestral Wuhan strain [27]. This indicates that SARS‐CoV‐2 is not simply drifting incrementally but is actively reshaping its antigenic landscape to escape pre‐existing immunity. Repeated exposure through vaccination or breakthrough infection temporarily boosts neutralizing titers, but durability remains limited due to natural antibody decay and the emergence of highly evasive subvariants. Consequently, individuals may remain susceptible to reinfection despite prior immunity, highlighting the need for updated vaccines or broadly protective immunogens.

A key insight from studies up to 2026 is the critical protective role of nNAbs in mitigating severe disease. While these antibodies do not prevent viral entry, they contribute to viral clearance and reduce tissue damage via Fc‐mediated effector functions [32]. For example, antibodies such as DH1052, targeting the spike NTD, have been shown in animal models to reduce viral load and prevent lung injury, even when neutralization is minimal. Notably, Fc‐mediated responses are generally more conserved across variants than neutralization‐sensitive epitopes [33]. Antibodies against internal antigens like the Nucleocapsid (N) protein consistently mediate ADCC against multiple VOCs, supporting their potential inclusion in next‐generation vaccine designs [34].

3.4. Hybrid Immunity and Its Impact on Humoral Durability

By 2026, most of the global population has acquired hybrid immunity, combining vaccination‐induced and infection‐induced responses. Hybrid immunity provides the broadest and most potent humoral protection currently observed, with several distinct features [35]. Natural infection exposes the immune system to the full viral proteome, generating antibodies against conserved regions of S2, the N‐terminal domain, and internal proteins, whereas vaccines primarily target the RBD. This combination results in a wider repertoire of B cells capable of recognizing diverse epitopes. Memory B cells formed during hybrid immunity undergo prolonged germinal center reactions upon subsequent exposures. This drives somatic hypermutation and generates antibodies with enhanced affinity and neutralization breadth, even against highly divergent variants such as XBB.1.5 or JN.1. A notable example is the monoclonal antibody SC27, which evolved in hybrid‐immune individuals to target a conserved receptor‐binding domain class 1/4 epitope with sub‐picomolar affinity, combining potency with broad neutralization [36].

Hybrid immunity extends the persistence of neutralizing antibodies compared to vaccination alone, although the rapid emergence of antigenically drifted subvariants still necessitates periodic boosting. While antibody titers wane over time, memory B cells and long‐lived plasma cells contribute to faster recall responses upon re‐exposure. Despite these advantages, hybrid immunity is also subject to immune imprinting, referred to as original antigenic sin. Early exposures, whether through infection or vaccination, shape the repertoire of recalled antibodies and can bias responses towards previously encountered epitopes. This phenomenon may constrain the generation of de novo memory B‐cell responses targeting novel variant‐specific mutations, even as overall antibody breadth improves in SARS‐CoV‐2. The combination of neutralizing antibodies, Fc‐mediated nNAbs, and memory B‐cell responses allows hybrid immunity to maintain protection against severe disease even when neutralization of circulating variants is incomplete. For example, although XBB.1.5 may escape most of the class 1 and 2 neutralizing antibodies, Fc‐mediated clearance and recall responses from memory B cells still provide robust protection against hospitalization and severe outcomes [37].

In summary, the humoral immune response to SARS‐CoV‐2 is shaped by a dynamic interplay of epitope drift, antibody waning, and compensatory mechanisms of hybrid immunity. While neutralizing antibodies are highly sensitive to antigenic changes, nNAbs and B‐cell memory confer durable cross‐variant protection. Understanding these dynamics is critical for informing booster strategies, next‐generation vaccine design, and public health policy, especially in a landscape dominated by post‐Omicron and recombinant lineages.

3.5. Antigenic Drift and Immune Escape in SARS‐CoV‐2 Versus Influenza

The comparison between SARS‐CoV‐2 and seasonal Influenza highlights converging patterns in respiratory virus evolution, epidemiology, and vaccine performance during the 2025–2026 respiratory season (Table 3) [38]. Both pathogens are characterized by continuous antigenic evolution, enabling recurrent outbreaks despite population immunity acquired through vaccination and natural infection [39]. During the 2025–2026 season, influenza activity was assessed as moderate in the United States, with predominance of H3N2 lineage viruses [40]. Epidemiological surveillance estimated millions of influenza cases, reflecting persistent transmission despite widespread vaccine availability. Reduced vaccine effectiveness during this period was attributed to a combination of viral antigenic drift and host immune heterogeneity. Notably, recent CDC surveillance reports have characterized circulating H3N2 subclades as antigenically drifted relative to vaccine strains, highlighting ongoing challenges in strain matching and vaccine performance [41]. This antigenic mismatch characterizes the disparity between circulating viral variants and the immune protection elicited by vaccination [42]. The reduced vaccine effectiveness caused by antigenic drift is driven by biological and population‐level dynamics such as rapid viral mutation, immune imprinting, and variable host immune histories rather than by vaccine refusal.

TABLE 3.

Comparative Features of Immune Evasion and Epidemiology: SARS‐CoV‐2 Omicron Descendants versus Influenza A (H3N2).

Metric	SARS‐CoV‐2 (Omicron Descendants)	Influenza (H3N2)
Mutation rate	High	High (constant drift)
Primary mechanism of evasion	Extensive RBD/NTD substitutions and deletions.	Antigenic drift in HA site A and B.
Seasonality	Multi‐wave throughout the year.	Predominantly winter months.
Incubation period	5–10 days.	3–5 days.
Vaccine strategy	Annual monovalent strain match.	Annual quadrivalent/trivalent updates.

SARS‐CoV‐2 exhibits an even faster antigenic evolution rate compared with influenza viruses. While seasonal influenza typically accumulates antigenic mutations gradually, often requiring several years for major antigenic cluster transitions, SARS‐CoV‐2 has demonstrated the ability to generate dominant immune‐escape sublineages within shorter time frames [15]. Since the emergence of the Omicron evolutionary radiation, viral diversification has accelerated, with multiple subvariants achieving global dominance in intervals of roughly 6 months in some periods. Unlike classical influenza evolutionary dynamics, which are largely shaped by long‐term antigenic drift in hemagglutinin and neuraminidase surface proteins, SARS‐CoV‐2 evolution is strongly influenced by selective pressure from high global immunity levels combined with structural flexibility of the spike protein. This enables recurrent escape from neutralizing antibody recognition while preserving efficient receptor binding and transmissibility.

Another key distinction lies in the modulation of clinical outcomes. Although both viruses can cause severe respiratory disease, population‐level immunity against SARS‐CoV‐2 increasingly relies on a combination of neutralizing antibodies, Fc‐mediated immune effector functions, and cellular memory responses, which collectively help maintain protection against hospitalization even when infection prevention is incomplete. In contrast, protection against severe outcomes in seasonal Influenza is more strongly influenced by strain matching between vaccine formulations and circulating viruses. Overall, the 2025–2026 respiratory landscape illustrates a transition toward endemic equilibrium for both pathogens, characterized by repeated but partially controlled outbreaks, ongoing antigenic drift, and the necessity for periodic vaccine updates to maintain population‐level immunity.

3.6. Conservation of T Cell Immunity and Mechanisms of Cellular Compensation

T‐cell immunity offers a resilient, variant‐transcending defence by recognizing a broad range of linear epitopes across the entire viral proteome, including conserved internal proteins [5]. Unlike antibodies that target specific surface shapes, CD4⁺ and CD8⁺ T cells coordinate to promote immune memory and directly eliminate infected cells, effectively curtailing viral replication [43]. This multi‐epitope approach allows for functional compensation even as the virus mutates, making cellular immunity a primary safeguard against severe disease and death. Consequently, the post‐2022 era is defined by a clear dissociation: while antibodies may fail to prevent infection due to antigenic drift, T cells continue to provide durable protection against hospitalization.

Comprehensive epitope mapping reveals that most T‐cell epitopes are highly conserved across Omicron variants because they reside outside the hypervariable regions of the virus [44]. Mutations in these internal proteins often carry high fitness costs, limiting viral evolution and explaining why first‐generation vaccines still effectively prevent severe outcomes. Upon exposure, vaccine‐induced memory T cells (T_CM and T_EM) rapidly produce cytokines like interferon‐gamma (IFN‐γ), tumour necrosis factor (TNF), and interleukin‐2 (IL‐2) to orchestrate viral clearance and reduce clinical severity. This cellular memory persists for years, outlasting neutralizing antibodies and providing a durable second line of defence that prevents systemic inflammation even when breakthrough infections occur [45].

Despite its breadth, T‐cell immunity is vulnerable to immunodominance, where the immune system preferentially targets specific epitopes, creating evolutionary pressure points. A key example is the HLA‐A*02:01‐restricted Spike epitope (residues 269–277), which triggers a common TCR response across diverse individuals [46, 47]. Mutations like P272 L in the Alpha variant demonstrate that the virus can evolve to reduce peptide–MHC stability and TCR recognition, significantly lowering functional avidity and cytotoxic activation. While these escape variants haven't achieved global fixation, their recurring emergence suggests convergent evolution under immune pressure. This highlights a strategic vulnerability in Spike‐only vaccines; incorporating additional conserved proteins like Nucleocapsid (N) and Membrane (M) into next‐generation designs could diversify targeting and prevent the virus from bypassing cellular defenses through localized mutations.

3.6.1. Population‐Level Determinants of Hybrid Immunity and Immune Imprinting

The contemporary immunological landscape is characterized by widespread hybrid immunity a composite state generated by sequential vaccination and natural infection (Figure 2) [45]. This layered exposure yields broader epitope coverage, higher antibody titers, and more durable memory responses compared with either modality alone. Hybrid immunity enhances both arms of adaptive defence. Repeated antigenic encounters promote affinity maturation within germinal centers, increasing antibody breadth and potency. Simultaneously, T‐cell memory expands quantitatively and qualitatively, displaying improved polyfunctionality and cross‐variant recognition. Epidemiologically, hybrid immunity correlates with markedly reduced risk of severe disease across successive waves. This immunological edge is, however, fundamentally constrained by immune imprinting, a phenomenon where the body's initial exposure dictates the trajectory of future immune responses [48]. The first exposure whether via infection or vaccination establishes a dominant memory B‐cell pool that preferentially expands upon re‐exposure, sometimes at the expense of generating new variant‐specific clones. Thus, the immune system maintains a balance between efficiency, favouring rapid recall responses, and flexibility, allowing the generation of de novo adaptive responses.

FIGURE 2.

Determinants of Hybrid Immunity and the Mechanistic Impact of Immune Imprinting. This illustration depicts the contemporary immunological landscape where hybrid immunity a composite state resulting from sequential vaccination and natural infection provides superior protection compared to single‐modality exposure. The top panel delineates the genesis of this state, highlighting how repeated antigenic encounters drive germinal center maturation, increased antibody breadth, and polyfunctional T‐cell expansion, ultimately correlating with a reduced risk of severe disease across successive viral waves. The bottom panel illustrates the balancing act of immune imprinting, wherein the first antigenic exposure establishes a dominant memory B‐cell pool. Upon re‐exposure to novel variants, the immune system must navigate a paradox between efficiency favouring the rapid recall of established memory and flexibility, which allows for the de novo generation of variant‐specific clones.

High‐resolution immunoglobulin sequencing reveals that infection and vaccination leave distinct monoclonal imprints: natural infection targets the S2 and NTD regions more broadly, while mRNA vaccination favours RBD‐focused antibodies [36]. This imprint is remarkably persistent, with over 60% of IgG recall originating from the initial memory pool, explaining why updated vaccines often amplify preexisting specificities rather than shifting towards novel epitopes [49]. Despite this, hybrid immunity facilitates exceptional antibody maturation through somatic hypermutation; for example, the SC27 antibody achieved picomolar affinity (K_D ≤ 5pM) for conserved epitopes after breakthrough infections [36]. This suggests that imprinting can actually provide a structural foundation for broader neutralizing capacity rather than solely inhibiting innovation. In the end, immunological memory creates a fundamental paradox: it acts as both a barrier and a bridge to adaptation. This requires next‐generation vaccine strategies to intentionally navigate the tension between persistent immune imprinting and the need to target highly conserved viral epitopes.

3.6.2. Next‐Generation Vaccine Platforms

Relying on strain‐matching strategies as a reactive approach has demonstrated inherent limitations in controlling SARS‐CoV‐2, since circulating variants often continue to evolve by the time updated antigens are deployed to the public. Consequently, 2026 serves as a turning point towards platforms that stay ahead of antigenic drift. This new strategy focuses on broadening the scope of immune recognition, resolving the hurdles of prior immunological imprinting, and eliciting the mucosal protection necessary to stop the spread of the virus entirely (Table 4).

TABLE 4.

Current status of next‐generation vaccine candidates (2025–2026).

Vaccine candidate	Developer	Platform/type	Target/breadth	Phase
Mosaic‐8b	Caltech/Oxford	Nanoparticle (protein or mRNA)	Pan‐sarbecovirus [50].	Preclinical/Trans.
GBP511	SK bioscience	Protein subunit	Universal sarbecovirus.	Phase 1/2 (NCT07280858)
Clec9AOMNI	NUS/Monash	Intranasal DC‐targeting	SARS‐CoV‐2 + sarbeco [56].	Preclinical
Castlevax	Castlevax	Intranasal viral vector	Mucosal SARS‐CoV‐2.	Phase 2a (NCT07215520).
Convidecia Air	CanSino	Inhaled Adenoviral vector	Mucosal SARS‐CoV‐2 [63].	Authorized
MPV/S‐2P	NIAID	Intranasal viral vector	Mucosal SARS‐CoV‐2.	Phase 1 (NCT06441968) completed

3.6.3. Pan‐Sarbecovirus and Mosaic Nanoparticle Vaccines

Pan‐sarbecovirus vaccines seek protection not only against circulating SARS‐CoV‐2 variants but also against related zoonotic sarbecoviruses with pandemic potential [50]. The sarbecovirus subgenus includes SARS‐CoV‐1, SARS‐CoV‐2, and numerous bat‐derived coronaviruses capable of ACE2 utilization. By targeting conserved structural elements shared across this clade, these vaccines aim to establish a preemptive immunological firewall against future spillovers. A leading candidate in this space is the mosaic‐8b nanoparticle vaccine, developed collaboratively at California Institute of Technology and University of Oxford [51]. The platform employs self‐assembling 60‐mer protein nanoparticles displaying RBDs from SARS‐CoV‐2 alongside seven zoonotic sarbecoviruses. This multivalent architecture fundamentally reshapes B‐cell selection dynamics (Figure 3).

FIGURE 3.

Integrated Next‐Generation Vaccine Strategies for Pan‐Sarbecovirus Protection. Schematic representation of the shift from reactive strain‐matching to preemptive, broad‐spectrum immunization platforms. The mosaic nanoparticle platform (mosaic‐8b) utilizes a self‐assembling 60‐mer scaffold to display heterologous RBDs from SARS‐CoV‐2 and seven zoonotic sarbecoviruses (Left Panel). This multivalent architecture reshapes B‐cell selection by limiting bivalent engagement of strain‐specific antibodies while favouring the maturation of broadly neutralizing antibodies (bnAbs) targeting conserved epitopes. Delivery is optimized via mRNA and EABR‐mRNA technologies for rapid scalability and structural fidelity. Mucosal vaccine strategies, exemplified by the intranasally delivered booster, target DNGR‐1 on cross‐presenting dendritic cells (Right Panel). This mechanism enhances antigen uptake to elicit robust secretory IgA (sIgA) and lung‐resident memory T cells (T_RM), providing sterilizing immunity at the viral portal of entry. The right panel illustrates the synergistic effect of systemic priming followed by mucosal boosting, establishing a layered defence that prevents both severe systemic disease and onward community transmission.

Traditional monovalent vaccines present identical RBDs in proximity, favouring high‐avidity binding by B cells targeting immunodominant but variable epitopes. In contrast, mosaic nanoparticles display non‐identical adjacent RBDs [52]. This geometric arrangement limits bivalent engagement by strain‐specific B cells, reducing their competitive advantage during germinal center reactions. Conversely, B cells recognizing conserved epitopes distal from the RBM can bind bivalently across heterologous RBDs because these conserved surfaces are structurally preserved. This selective pressure enriches for broadly neutralizing antibody (bnAb) precursors and drives affinity maturation towards cross‐reactive specificities. The result is an immune response biased towards conserved structural determinants rather than mutable strain‐specific residues. Preclinical and early‐phase human data demonstrate cross‐neutralization of diverse sarbecoviruses, including strains not directly represented on the nanoparticle scaffold. In 2025, mosaic designs were adapted for delivery via mRNA and Engineered Antigen‐Bearing RNA (EABR‐mRNA) technologies (Figure 3) [53]. Encoding nanoparticle subunits in mRNA simplifies large‐scale production and accelerates iteration cycles. EABR‐mRNA platforms further enhance antigen expression efficiency and structural fidelity, improving in vivo nanoparticle assembly. This convergence of structural vaccinology and nucleic acid delivery represents a scalable route towards universal coronavirus vaccination shifting from reactive booster campaigns to durable, clade‐level immunity.

3.6.4. Mucosal Vaccines and the Clec9A Platform

While systemic vaccines effectively prevent severe disease, they induce relatively weak mucosal immunity in the upper respiratory tract the primary site of SARS‐CoV‐2 entry and replication. Consequently, breakthrough infections and transmission persist despite strong protection against hospitalization. Mucosal vaccines aim to close this gap by generating localized immune defenses (Figure 3) [54]. These include the production of secretory IgA (sIgA), which can neutralize virus directly at epithelial surfaces; the establishment of tissue‐resident memory T cells (T_RM) within the nasal passages and pulmonary mucosa; and rapid innate immune priming in airway dendritic cells and epithelial barriers. By positioning immune effectors at the portal of entry, mucosal vaccination seeks to achieve sterilizing immunity, preventing infection before systemic dissemination and thereby reducing onward transmission.

As of early 2026, more than 30 mucosal vaccine candidates have entered clinical trials worldwide, encompassing diverse technological approaches such as viral vectors, protein subunits, live‐attenuated constructs, and RNA‐based aerosol platforms [55]. One notable candidate is the Clec9A^OMNI intranasal booster, which employs a dendritic cell–targeting strategy (Figure 3). Clec9A (Dendritic cell Natural killer Group Receptor‐1 (DNGR‐1)) is a receptor expressed on a subset of cross‐presenting dendritic cells specialized in priming CD8⁺ T‐cell responses [56]. Targeting this receptor enhances antigen uptake and cross‐presentation, thereby promoting robust cytotoxic T‐cell immunity alongside mucosal antibody production. The Clec9A^OMNI formulation incorporates RBD antigens derived from the Omicron XBB.1.5 lineage and the ancestral SARS‐CoV strain, broadening cross‐reactive potential across sarbecoviruses. In preclinical studies, this platform induced significantly higher mucosal IgA titers than intramuscular mRNA boosters, expanded lung‐resident CD8⁺ T_RM populations, provided protection lasting at least 6 months in animal models, and reduced viral replication in nasal tissues following challenge [57]. Importantly, mucosal boosting after prior systemic vaccination appears synergistic. Systemic priming establishes a circulating memory pool of B and T cells, while mucosal delivery redirects effector and memory populations to barrier tissues, creating a layered immune defence that combines systemic protection against severe disease with localized prevention of infection and transmission.

Together, mosaic nanoparticle and mucosal vaccine strategies address complementary vulnerabilities in current immunization models. Mosaic platforms counter antigenic drift and zoonotic unpredictability by enforcing epitope conservation and promoting broadly neutralizing antibodies, whereas mucosal vaccines aim to interrupt transmission and limit viral evolution by reducing replication at the population level. If successfully implemented using systemic priming to establish broad‐spectrum defenses followed by mucosal boosting these strategies could transform SARS‐CoV‐2 management from a reactive cycle of chasing new variants into a model of lasting pandemic prevention.

4. Conclusion

Post‐Omicron viral evolution has shifted from massive structural changes towards subtle fine‐tuning, with sublineages like JN.1 utilizing incremental mutations to navigate high levels of population immunity without losing infectious fitness. Although this constant drift and the phenomenon of immunological imprinting can weaken the long‐term effectiveness of antibodies, the relative stability of T‐cell responses continues to be a contributing factor to protection against severe illness. Consequently, the focus of COVID‐19 management is moving beyond simply chasing new variants; instead, it emphasizes next‐generation tools like mucosal vaccines and mosaic nanoparticles that target conserved viral regions to potentially enhance protection at the site of entry. By combining these proactive technologies with flexible regulatory systems and robust genomic tracking, the global health community aims to build a more resilient defence against the current evolutionary trajectory of the virus and the potential for future sarbecovirus outbreaks.

Author Contributions

F.U., F.E. and S.S. prepared the initial draft of the manuscript. All authors critically reviewed the manuscript and contributed to revisions and final approval.

Funding

The authors have nothing to report.

Consent

All authors have given their consent for the publication of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The authors have nothing to report.