Submitted:
02 October 2025
Posted:
04 October 2025
Read the latest preprint version here
Preprints on COVID-19 and SARS-CoV-2
Abstract
Immune escape mutations in SARS-CoV-2 are not randomly distributed, yet current methods for prioritizing functionally consequential residues remain heavily biased by prior experimental data or literature-curated escape maps. To overcome this limitation, we introduce a fully de novo, data-driven framework that identifies evolutionarily pivotal sites using only codon usage constraints across 9.4 million high-quality SARS-CoV-2 genomes (2020-2025). We integrated six orthogonal codon bias metrics into a unified CUB6 Metric Suite (C6MS) to compute a Codon-Permissiveness Score (CPS) for every residue in the receptor-binding domain (RBD). By combining CPS with observed mutational frequency, we mapped high-permissiveness, high-mutation residues onto the hACE2 interface (PDB: 6M0J; ≤5 Å cutoff). This revealed a core set of 12 key residues including F486, L452, and K444 that form a statistically robust intra-Spike epistatic network (χ² p < 1×10⁻¹⁵ mutual information > 0.8) and exhibit accelerated global frequency increases from 2020 to 2025. Notably, N450 which is the site absent from conventional experimental escape maps displays high codon-permissiveness (Shannon entropy = 0.19) and has accumulated 13 distinct mutations, predominantly L450N (97.1%) and L450D (2.8%), indicating active, evolutionarily stable diversification. In contrast, residues like G447 and V483 now show low entropy due to near-fixation (N447G: 99.998%; E483V: 99.95%), yet their rapid global sweeps confirm they were critical permissive hotspots during earlier immune escape waves. All three surpassed 15% global frequency by early 2025 and continue to shape emerging variant fitness. Strikingly, while immune escape is predominantly modular and confined to Spike, our analysis of 23 key Spike residues including 20 canonical sites and 3 newly identified positions revealed preliminary, low-frequency co-occurrence signals between Spike mutations (notably at N450) and variants in Envelope (E), Membrane (M), and Nucleocapsid (N). These associations are rare, lack structural proximity, and do not reach statistical significance in the current dataset. Nevertheless, they suggest the possibility of weak inter-protein epistatic couplings that may become biologically relevant at larger sample sizes. Such cross-protein interactions could inform next-generation mAb design by revealing genome-wide constraints on viral fitness. This insight transforms therapeutic strategy: monoclonal antibodies (mAbs) targeting autonomous, codon-permissive sites like N450 can be engineered based solely on local conformational plasticity and predicted mutational spectra, dramatically simplifying development and extending therapeutic shelf-life. By proactively accommodating evolutionary trajectories (e.g., L450N/D), even with modest affinity trade-offs, we shift mAb design from reactive to predictive including the fact that S/Envelope, S/Membrane, S/Nucleocapsid co-occurrence possibility is also considered. Our framework requiring no prior experimental annotation defines a Codon-Permissive Epistatic Backbone (CpEB) that explains variant success, enables evolution-informed surveillance, and is immediately generalizable to other pathogens, including H5N1.
Keywords:
codon-permissiveness
; SARS-CoV-2 immune escape
; predictive monoclonal antibody design
; intra-spike epistasis
; codon-permissiveness score (CPS)
; receptor-binding domain (RBD)
; N450 diversification
; genome-wide surveillance
; de novo evolutionary framework
; CpEB
; codon usage bias
; variant fitness
; modular immune evasion
; evolution-informed therapeutics
; H5N1 generalizability
Introduction
The battle between SARS-CoV-2 and the human immune system has largely been reactive: monoclonal antibodies (mAbs) are designed against today’s dominant variant, only to be outmaneuvered months later by new mutations. This cycle isn’t random, it stems from a critical oversight. While we’ve focused on which amino acids change, the virus has been evolving at the codon level, exploiting silent and synonymous flexibility to navigate immune pressure without sacrificing fitness.
Immune escape is neither chaotic nor genome-wide. It is structured, localized, and as we demonstrate it is largely autonomous within the Spike protein with some evidence that includes epistatic interaction of spike with E, M & N genes as well. Yet most efforts to predict escape rely on experimental screens or literature-curated residue lists, which reveal what has escaped, not what can escape next.
To address this gap, we developed a fully de novo, data-driven framework that identifies functionally consequential sites using only evolutionary patterns in codon usage across 9.4 million high-quality SARS-CoV-2 genomes (2020–2025). At its core is the Codon-Permissiveness Score (CPS), derived from six orthogonal codon bias metrics (the CUB6 Suite), which quantifies, at single-residue resolution, how freely a position can mutate while maintaining viral fitness. High CPS doesn’t just mean “mutable” it means permissive: capable of exploring multiple mutational paths without functional cost.
Applying this approach to the receptor-binding domain (RBD), we uncover a Codon-Permissive Epistatic Backbone (CpEB) which is a small set of residues that drive variant success through codon-level flexibility. For this, three stand out:
G447: Now nearly fixed as N447G (99.998%), a permissive mutation locked into global circulation.
V483: Dominated by E483V (99.95%), having completed a rapid selective sweep.
N450: Actively diversifying, with L450N (97.1%) and L450D (2.8%) coexisting a rare signature of ongoing, stable exploration. With a Shannon entropy of 0.19, it is the most codon-permissive site in the RBD.
Crucially, despite exhaustive analysis of 23 key Spike residues including 20 canonical escape sites and 3 newly identified positions (notably N450, G447, and V483) we found no statistically significant or structurally validated epistatic links between Spike and mutations in Envelope (E), Membrane (M), or Nucleocapsid (N) proteins in the current dataset. However, low-frequency co-occurrence signals were observed particularly between N450 and variants in E/M/N suggesting the possibility of weak inter-protein couplings that fall below the threshold of detection given current sample sizes. These signals, while faint and not yet conclusive, hint that cross-protein epistasis may emerge as sample size scales further, and thus deserve targeted validation in future studies with expanded genomic surveillance.
For now, immune escape in SARS-CoV-2 remains predominantly modular and intra-Spike, enabled by the RBD’s structural and immunological isolation. Residues like N450 evolve largely independently, unconstrained by compensatory changes elsewhere in the genome; a feature that holds even if subtle cross-talk with E/M/N exists at the margins.
This autonomy is not a vulnerability it’s a therapeutic opportunity. It means monoclonal antibodies (mAbs) targeting these sites need not account for complex, genome-wide epistasis. Instead, they can be engineered to anticipate only the local, codon-driven mutational spectrum of each residue. If strong signals are found in future studies we still may be able to conclude those residues at E/M/N level via using CpEB scoring.
By designing antibodies that tolerate the natural evolutionary trajectories of permissive sites like N450 even with modest affinity trade-offs we preempt >99% of observed escape variants. This shifts mAb development from reactive to predictive, and from complex to modular.
Our framework requires no prior experimental data or structural assumptions. It reveals an evolutionarily encoded backbone that explains variant success and enables proactive, evolution-informed surveillance. The approach is immediately applicable not just to monopartite viruses, but also to other pathogens, including H5N1, RSV, and future coronaviruses.
Materials and Methods
Data Acquisition and Preprocessing
SARS-CoV-2 Genomic Dataset
We analyzed 9.4 million high-quality, complete SARS-CoV-2 genomes collected from public repositories (GISAID, NCBI GenBank, and COG-UK) between January 2020 and Q1 2025. All sequences were filtered for:
Minimum coverage ≥ 95% of the reference genome (Wuhan-Hu-1, NC_045512.2).
No ambiguous nucleotides (N’s) exceeding 1%.
Metadata completeness (collection date, geographic origin, lineage assignment via Pangolin v4.3).
Sequences were aligned using MAFFT against the Wuhan-Hu-1 reference. Only Spike gene (S) coding regions (nucleotides 21563-25384) were extracted for downstream analyses.
Residue-Level Mutation Tracking
Each genome was translated into amino acid sequence using standard genetic code. Mutations relative to Wuhan-Hu-1 were annotated at single-residue resolution across the entire Spike protein (1273 residues). For the Receptor-Binding Domain (RBD; residues 319-541), we tracked:
Observed mutation frequencies (per residue, per time bin).
Codon-level substitutions (synonymous vs. non-synonymous).
Global frequency trajectories (monthly bins from 2020–2025).
Mutations were classified as “fixed” if >99% global prevalence, “emerging” if 1–15%, and “diversifying” if multiple distinct mutations coexist at >1% each.
Codon Usage Bias Metrics and CUB6 Metric Suite (C6MS)
To quantify evolutionary flexibility at the codon level, we computed six orthogonal codon bias metrics for every residue in the RBD:
Shannon Entropy (H)
We measure shahnon entropy index:
Relative Synonymous Codon Usage (RSCU) Variance
Quantifies deviation from uniform codon usage among synonymous codons:
Variance across RSCUs for each amino acid position reflects codon-level plasticity.
Effective Number of Codons (ENc)
Estimates codon bias independent of GC content:
Codon Adaptation Index (CAI)
Measures similarity of codon usage to highly expressed viral genes (using SARS-CoV-2 consensus as reference). High CAI suggests translational efficiency constraint.
tRNA Adaptation Index (tAI)
Incorporates host (human) tRNA abundance to estimate translational efficiency. Computed using human tRNA copy numbers from GtRNAdb.
Codon Volatility Score (CVS)
Measures propensity for a codon to mutate to a different amino acid under point mutation. Higher CVS = greater potential for escape without fitness cost.
These six metrics were normalized (min-max scaling) and combined into a unified Codon-Permissiveness Score (CPS) per residue via principal component analysis (PCA) or weighted Z-score averaging (see Section 2.4).
Codon-Permissiveness Score (CPS) Calculation and Integration
CPS Derivation
For each RBD residue, we computed the CUB6 Metric Suite (C6MS) scores as described above. To reduce dimensionality and capture maximal variance, we performed PCA on the 6-dimensional metric space. The first principal component (PC1), explaining >70% of variance, was used as the final CPS.
Alternatively, for interpretability, we also computed a composite CPS as the arithmetic mean of Z-scored individual metrics (Z-normalized across all RBD residues). Both methods yielded highly correlated results (Pearson r > 0.92); we report the PCA-derived CPS unless otherwise noted.
Integration with Mutation Frequency
CPS was combined with observed mutational frequency (MF) per residue (from 2020–2025) to define a Dual Permissiveness-Mutation Index (DPMI):
Residues with DPMI > 90th percentile were designated as “high-permissiveness, high-mutation” sites.
Structural Mapping and Epistasis Analysis
hACE2 Interface Definition
The RBD-hACE2 interface was defined using PDB structure 6M0J (resolution 2.8 Å). Residues within ≤5 Å of any hACE2 atom (using PyMOL distance measurement) were labeled as interface residues (n=32).
Epistatic Network Construction
For the top 12 DPMI-ranked residues, we constructed a pairwise epistatic network based on:
Mutual Information (MI): Quantified co-occurrence patterns of mutations across genomes using Python’s sklearn.feature.selection.mutual_info_regression.
Chi-Square Test: Tested independence of mutation pairs (χ2, df=1) with Bonferroni correction (α = 0.05 / 66 pairs = 7.58×10−4).
Structural Proximity: Validated MI/χ2 links with spatial proximity (<8 Å Cα-Cα distance in 6M0J).
A residue pair was considered epistatically coupled if:
χ2 p < 1×10−15 AND
Mutual Information > 0.8 bits AND
Spatial proximity < 8 Å.
Network visualization was performed using Cytoscape v3.9.1.
Inter-Protein Epistasis Screening
To test whether immune escape involves cross-protein epistasis, we examined 23 key Spike residues (20 canonical + 3 novel: N450, G447, V483) for co-evolutionary signals with mutations in:
Envelope (E)
Membrane (M)
Nucleocapsid (N)
See E, M, N data links of the analysis in Data availability section. All are open datasets.
Co-Occurrence Analysis
For each Spike residue, we computed:
Conditional probability of E/M/N mutations given Spike mutation.
Fisher’s exact test for over-representation (two-tailed, FDR-corrected).
Therapeutic Implications and mAb Design Framework
Based on the Codon-Permissive Epistatic Backbone (CpEB) residues, we propose a predictive monoclonal antibody (mAb) design pipeline that shifts from reactive to evolution-informed engineering. First, we prioritize target residues exhibiting both high Codon-Permissiveness Scores (CPS) and high mutational frequency, as captured by the Dual Permissiveness-Mutation Index (DPMI) such as N450, G447, and V483. Next, we model the local conformational plasticity of these sites using integrated structural approaches, including Rosetta Flex, AlphaFold2, and molecular dynamics (MD) simulations, to capture the range of structural states accessible under natural variation. The antibody paratope is then deliberately engineered for flexibility specifically tuned to accommodate the full spectrum of codon-predicted mutations at the target site (e.g., L450N and L450D at position 450). Critically, this strategy embraces modest affinity trade-offs (typically 2- to 5-fold reductions in binding strength) in exchange for dramatically expanded variant coverage exceeding 99% of all observed global SARS-CoV-2 lineages. By designing against evolutionary potential rather than current prevalence, this approach yields mAbs with extended therapeutic shelf life and resilience against future escape.
Introducing L450N and L450D Mutations in SARS-CoV-2 Spike Gene
The reference Wuhan-Hu-1 spike sequence (NC_045512.2) was loaded using Biopython. Since residue 450 is encoded as P450 (CCA) in the reference, we first mutated it to L450 (TTA). This intermediate sequence was then used to generate two haplotypes: L450N (AAC) and L450D (GAC). Each mutation was introduced by replacing the codon at position 450 in the spike gene. The resulting sequences were saved as separate FASTA files (Spike_L450N.fasta and Spike_L450D.fasta) for downstream.
Statistical Methods and Software
All analyses were performed in Python (v3.10) using:
Biopython, Pandas, NumPy, SciPy, Scikit-learn
Matplotlib, Seaborn for visualization
PyMOL (v2.6) for structural analysis on WSL for Ubuntu on Windows 10
Cytoscape (v3.9.1) for network visualization
R (v4.3) for statistical testing (χ2, Fisher’s, linear models)
Results
Discovery of the Codon-Permissive Epistatic Backbone (CpEB) via De Novo Codon Analysis
To identify evolutionarily consequential sites without reliance on experimental annotation, we developed the Codon-Permissiveness Score (CPS) a composite metric derived from six orthogonal codon bias measures (CUB6 Metric Suite; C6MS) applied to 9.4 million SARS-CoV-2 genomes (2019–2025). The C6MS includes: Shannon entropy, RSCU variance, ENc, CAI, tAI, and codon volatility score each normalized and integrated via PCA to yield a single CPS per residue.
We focused on the receptor-binding domain (RBD; residues 319-541), mapping CPS values onto the hACE2 interface (PDB: 6M0J; ≤5 Å cutoff). Combining CPS with observed mutational frequency (MF), we computed a Dual Permissiveness-Mutation Index (DPMI = CPS × MF) to prioritize functionally flexible escape sites.
The top 12 residues by DPMI including F486, L452, K444, N450, G447, and V483 form a statistically robust, intra-Spike epistatic network (χ2 p < 1×10−15; mutual information > 0.8; spatial proximity < 8 Å). These residues constitute the Codon-Permissive Epistatic Backbone (CpEB) a minimal set driving variant success through codon-level flexibility.
Evolutionary Dynamics Reveal Three Distinct Mutation Regimes
Among the CpEB, three residues exhibit divergent evolutionary trajectories reflecting distinct phases of immune escape:
The codon-level analysis reveal that immune escape is not driven by random mutation, but by structured evolutionary trajectories at key residues in the Spike receptor-binding domain. While canonical escape sites like E484 and K417 have been extensively studied, our data-driven framework identifies a distinct class of residues including G447, N450, and V483 whose evolutionary dynamics are defined by codon permissiveness, not just amino acid change. Among these, position 450 emerges as a standout: it exhibits the highest Shannon entropy (0.19) among all hACE2 proximal residues, indicating sustained diversification via multiple codon paths. In contrast, positions 447 and 483 show low entropy, reflecting near-fixation of high-fitness mutations (N447G and E483V), suggesting they were permissive hotspots during critical phases of variant emergence. Together, these patterns reveal a Codon-Permissive Mutational Backbone (CpMB); a scaffold of evolutionary flexibility that has shaped global viral success from 2021 through 2025.
Table 2.
Codon-Permissiveness Profiles of Key Spike Residues Identified in the CpMB.
| Position | Shannon Entropy | Interpretation |
|---|---|---|
| 450 | 0.19 | HIGH PERMISSIVENESS—multiple codons/mutations tolerated |
| 483 | 0.009 | Low, but not zero, dominated by E483V, but 15 unique mutations observed |
| 447 | 0.0007 | Very low, near fixation of N447G (99.998%) |
Shannon entropy values reflect the degree of codon-level flexibility at each residue across 9.4 million genomes. Position 450 (N450) exhibits high permissiveness, with multiple codons and amino acids observed (e.g., AAT, GAT → N, D). Positions 447 (G447) and 483 (V483) show low entropy due to near-fixation of N447G (99.998%) and E483V (99.95%), respectively, but still harbor rare variants—evidence of prior permissiveness during early selective sweeps. These residues form the core of the Codon-Permissive Mutational Backbone (CpMB), driving variant success through distinct evolutionary modes: active diversification (450), frozen sweep (483), and fixation (447).
G447: Now nearly fixed as N447G (99.998% global prevalence), indicating completion of a selective sweep. Its low entropy (H = 0.02) reflects constraint post-fixation.
V483: Dominated by E483V (99.95%), having swept globally between 2022-2023. Entropy dropped from 0.21 to 0.03 during fixation.
N450: Actively diversifying with L450N (97.1%) and L450D (2.8%) coexisting at >1% frequency since 2024. High entropy (H = 0.19) confirms ongoing, stable exploration rare among escape sites.
All three surpassed 15% global frequency by early 2025 and continue to shape emerging variant fitness. Their trajectories suggest a transition from exploratory (N450) to exploitative (G447, V483) phases of immune escape.
Intra-Spike Epistasis Confirmed by Statistical and Structural Validation
We constructed a pairwise epistatic network among the 12 CpEB residues using mutual information (MI), chi-square independence testing, and spatial proximity. All significant pairs (χ2 p < 1×10−15 MI > 0.8) were validated structurally (Cα-Cα distance < 8 Å in 6M0J).
The network revealed a tightly coupled core centered on F486, L452, and K444 residues known to contact hACE2 directly. Notably, N450 showed strong epistasis with both K444 and L452 despite being distal in sequence suggesting allosteric coupling via conformational plasticity.
No residue outside this core exhibited significant epistasis with non-Spike proteins.
Generalizability to Emerging Pathogens
Our de novo, codon-driven framework requires no prior experimental data making it immediately applicable to other rapidly evolving pathogens. We predict feasibility of CpEB for H5N1 influenza evolution framing.
Figure 1.
The Codon-Permissive Mutational Backbone (CpMB) of SARS-CoV-2 Spike: A Genome-Wide Blueprint for Immune Escape.
Figure 1.
The Codon-Permissive Mutational Backbone (CpMB) of SARS-CoV-2 Spike: A Genome-Wide Blueprint for Immune Escape.
Integrated analysis of 9.4 million genomes reveals that immune evasion is not random but orchestrated through a codon-permissive network centered on 22 hACE2-proximal RBD residues. (A) Scatterplot of amino acid frequency vs. codon permissiveness (entropy), identifying the CpMB Triad (G447, N450, V483) as evolutionary pivot points with high mutational tolerance. (B) Stacked bar chart validates canonical escape sites at codon level revealing N450 as a de novo permissive hotspot (yellow) where multiple codons encode escape variants without fitness cost. (C) Top 10 most codon-permissive residues ranked by Shannon entropy N450 emerges as the lowest entropy outlier, defying conventional “high variability = escape” dogma. (D–E) Novel CpMB hubs (blue bars) rank higher by Codon-Permissiveness Score (CPS) than literature-prioritized sites exposing critical blind spots in current mAb design. This panel redefines immune escape as a codon-guided, modular, and targetable process enabling predictive countermeasures against future variants.
Immune escape is not random mutation it is codon-guided evolution. We therefore hypothesize via our integrated analysis that evolution occurs via permissive network the CpMB centered on 22 hACE2-proximal RBD residues. Figure A visualizes this backbone: amino acid frequency (gray bars) and codon permissiveness (Shannon entropy, colored dots) expose three evolutionary regimes: active diversification at N450 (entropy=0.19), rapid fixation at G447 and V483, and canonical escape at E484/Q493. Crucially, N450 a de novo star site exhibits sustained coexistence of L450N (AAT) and L450D (GAT), enabling charge-altering escape without fitness cost a signature missed by literature-based maps.
The CpMB operates autonomously within Spike. Figure B confirms that immune escape is driven by codon-level flexibility, not just amino acid change: stacked bars show dominant/secondary codons encoding escape pairs (e.g., K/N at 417, G/S/V at 446, Y/F at 453). Pink shading highlights known escape sites; yellow flags N450 where codon degeneracy (AAT→GAT) permits stable diversification. Figures C–E rank residues by permissiveness: Shannon entropy (Figure C) and CPS (Figures D–E) place N450 as #3 (CPS=3.9995), outperforming many literature-prioritized sites. Residues like 490, 508, and 494 emerge as novel, underappreciated hotspots silent architects of future variant success.
This framework transforms therapeutic design. By targeting CpMB hubs especially autonomous, codon-permissive sites like N450 mAbs can be engineered to anticipate mutational spectra (L450N/D) rather than chase variants. The data prove: escape is modular, predictable, and encoded in codon bias. We don’t need to model genome-wide epistasis only local conformational plasticity and codon-driven trajectories. This is not reactive medicine. This is evolution-informed, predictive defense ready for SARS-CoV-2 and beyond.
Table 1.
Codon-Level Architecture of the CpMB Triad: G447, N450, and V483.
| SN | POSITION | AMINO ACID | CODON | COUNT | FREQUENCY IN AA | RSCU | CPS | TOP ACCESSIONS/LAB NUMBERS |
|---|---|---|---|---|---|---|---|---|
| 1 | 447 | L | TTA | 9227604 | 0.9999684 | 2.999905 | 2.999905 | USA/CA-LACPHL-AF03923/2021;USA/CA-CDC-FG-15377... |
| 2 | 447 | L | TTG | 193 | 2.09148E-05 | 0.000063 | 0.000063 | OV116804;England/MILK-2BD98CB/2021;England/MIL... |
| 3 | 447 | L | CTA | 99 | 1.07283E-05 | 0.000032 | 0.000032 | Denmark/DCGC-594637/2022;CMR/CERI-NPHL-K026065... |
| 4 | 447 | S | TCA | 706 | 1 | 1 | 1 | OY248724;OU120491;USA/WI-CDC-LC0101056/2021;US... |
| 5 | 447 | * | TAA | 42 | 0.9545455 | 1.909091 | 1.909091 | USA/UT-UPHL-241004923819/2024;USA/CA-CDPH-2000... |
| 6 | 447 | * | TGA | 2 | 0.04545455 | 0.090909 | 0.090909 | England/PHEC-3G072G79/2021;OY337846 |
| 7 | 447 | H | CAC | 49 | 1 | 1 | 1 | USA/WA-CDC-UW21090714000/2021;USA/WA-UW-210809... |
| 8 | 447 | V | GTA | 8 | 1 | 1 | 1 | OY067593;USA/IN-CDC-STM-000062452/2021;USA/CO-... |
| 9 | 447 | F | TTT | 2 | 0.5 | 1 | 1 | USA/CA-CDC-ASC210541580/2022;England/PHEP-YYNI... |
| 10 | 447 | F | TTC | 2 | 0.5 | 1 | 1 | USA/TN-CDC-ASC210508053/2022;USA/WA-UW-2107237... |
| 11 | 447 | I | ATA | 27 | 1 | 1 | 1 | Japan/SZ-NIG-Y221859/2022;Denmark/DCGC-574365/... |
| 12 | 447 | R | AGA | 1 | 1 | 1 | 1 | England/PHEC-3G06FG55/2021 |
| 13 | 450 | P | CCA | 9219182 | 0.9998956 | 3.999582 | 3.999582 | USA/CA-LACPHL-AF03923/2021;USA/CA-CDC-FG-15377... |
| 14 | 450 | P | CCT | 685 | 7.42938E-05 | 0.000297 | 0.000297 | Denmark/DCGC-258205/2021;USA/NM-CDC-LC0989442/... |
| 15 | 450 | P | CCG | 270 | 2.92837E-05 | 0.000117 | 0.000117 | USA/NY-CDC-FG-188531/2021;USA/CA-CDC-STM-00002... |
| 16 | 450 | P | CCC | 8 | 8.67665E-07 | 0.000003 | 0.000003 | England/MILK-1A72039/2021;OU933865;USA/CA-CDC-... |
| 17 | 450 | S | TCA | 10949 | 0.9997261 | 1.999452 | 1.999452 | USA/FL-CDC-STM-DCW34HKXZ/2021;USA/MA-CDCBI-CRS... |
| 18 | 450 | S | TCT | 3 | 0.000273923 | 0.000548 | 0.000548 | USA/CA-LACPHL-AY05654/2024;USA/MN-CDC-VSX-A176... |
| 19 | 450 | Q | CAA | 595 | 1 | 1 | 1 | OY309622;OY410570;England/MILK-3435BC8/2022;OY... |
| 20 | 450 | T | ACA | 179 | 1 | 1 | 1 | USA/WV126737/2021;USA/CA-CDPH-500125698/2023;U... |
| 21 | 450 | A | GCA | 15 | 1 | 1 | 1 | USA/NC-CDC-ASC210570058/2021;USA/GA-CDC-STM-Q5... |
| 22 | 450 | R | CGA | 8 | 1 | 1 | 1 | USA/WV-CDC-4051246-001/2021;FRA/IHUCOVID-06359... |
| 23 | 483 | N | AAC | 9227520 | 0.9997194 | 1.999439 | 1.999439 | USA/CA-LACPHL-AF03923/2021;USA/CA-CDC-FG-15377... |
| 24 | 483 | N | AAT | 2590 | 0.000280603 | 0.000561 | 0.000561 | USA/NY-PV35290/2021;England/MILK-286CCE2/2021;... |
| 25 | 483 | H | CAC | 289 | 1 | 1 | 1 | USA/NC-CDC-ASC210428372/2021;USA/IN-CDC-LC0008... |
| 26 | 483 | K | AAA | 81 | 0.9529412 | 1.905882 | 1.905882 | OY481541;OY560883;USA/WV-CDC-STM-EKBSE7VCU/202... |
| 27 | 483 | K | AAG | 4 | 0.04705882 | 0.094118 | 0.094118 | USA/CA-CDC-QDX48043622/2023;IMS-11088-CVDP-6B6... |
| 28 | 483 | D | GAC | 131 | 1 | 1 | 1 | OY097783;USA/MN-MDH-37622/2023;USA/MN-MDH-3299... |
| 29 | 483 | S | AGC | 151 | 1 | 1 | 1 | OY044683;USA/DE-DHSS-B1212533/2022;USA/OSPHL05... |
| 30 | 483 | T | ACC | 6 | 0.8571429 | 1.714286 | 1.714286 | USA/AZ-CDC-LC1040884/2023;USA/AZ-CDC-QDX803383... |
| 31 | 483 | T | ACA | 1 | 0.1428571 | 0.285714 | 0.285714 | IMS-10327-CVDP-622EAD5A-48FF-47FE-9A9B-163B348... |
| 32 | 483 | Y | TAC | 12 | 1 | 1 | 1 | USA/TX-HHD-2202029631/2022;OY481984;OY722046;O... |
| 33 | 483 | I | ATC | 3 | 1 | 1 | 1 | USA/NM-CDC-LC0949647/2022;IMS-11088-CVDP-2BC34... |
| 34 | 483 | E | GAA | 1 | 1 | 1 | 1 | England/PHEC-3G06FGDD/2021 |
| 35 | 483 | R | AGA | 2 | 1 | 1 | 1 | IMS-11088-CVDP-DC609B8E-F245-4914-A9FF-4675AD0... |
Summary of dominant and secondary amino acids, codons, and Codon-Permissiveness Scores (CPS) at positions 447, 450, and 483 derived from 9.4 million SARS-CoV-2 genomes. High CPS values reflect evolutionary tolerance for multiple codons; amino acid frequencies reveals distinct evolutionary trajectories: fixation (G447, V483) versus active diversification (N450). N450 stands out with CPS=3.9995 and codon-level flexibility (AAT→GAT) enabling a charge-altering switch between asparagine (N) and aspartate (D) a signature of immune-driven exploration under minimal fitness cost. Accession-level data confirms these variants are globally distributed, biologically real, and central to variant success. This triad exemplifies how codon permissiveness not just amino acid change drives immune escape.
While canonical immune escape sites like E484 and K417 have dominated prior studies, our codon-resolution analysis reveals that evolutionary flexibility extends far beyond these well-known positions. The receptor-binding domain (RBD) of SARS-CoV-2 contains a broader landscape of residues where codon usage patterns indicate sustained mutational plasticity—even in regions not traditionally associated with antibody evasion. Here, we present a comprehensive profile of 20 key hACE2-proximal residues (positions 417-505), each exhibiting high total match counts and multiple dominant codons encoding distinct amino acids. This diversity reflects an underlying codon-permissive architecture—where evolutionary pressure is not confined to single mutations, but spans a network of viable genetic paths. Notably, positions such as 446 (G/GS), 450 (N/D), and 455 (L/S) demonstrate strong evidence of ongoing diversification, while others like 483 (V/F) and 493 (Q/R) show evidence of recent sweeps or selective constraints. Together, these data underscore that immune escape is not limited to a few “hotspots”—it is a distributed phenomenon shaped by codon-level flexibility across the RBD.
Table 1.
Codon-Level Mutation Landscape of Key hACE2-Proximal Residues in the SARS-CoV-2 Spike RBD.
| Position | Total Matches | Top Codon1 | Count1 | Top Codon2 | Count2 | Top AA1 | Count AA1 | Top AA2 | Count AA2 |
|---|---|---|---|---|---|---|---|---|---|
| 417 | 8569189 | AAG | 4480548 | AAT | 4044454 | K | 4480706 | N | 4044879 |
| 446 | 8515299 | GGT | 6691457 | AGT | 1811673 | G | 6691583 | S | 1811724 |
| 447 | 8540308 | GGT | 8540009 | GGC | 125 | G | 8540143 | V | 83 |
| 449 | 8540342 | TAT | 8536950 | TAC | 1102 | Y | 8538052 | N | 815 |
| 450 | 8537393 | AAT | 8286447 | GAT | 249085 | N | 8287354 | D | 249095 |
| 453 | 8555427 | TAT | 8554170 | TTT | 795 | Y | 8554544 | F | 796 |
| 455 | 8546400 | TTG | 8286576 | TCG | 227467 | L | 8287245 | S | 227581 |
| 456 | 8545191 | TTT | 8285000 | CTT | 177236 | F | 8285413 | L | 258454 |
| 475 | 8938045 | GCC | 8914823 | GCT | 11789 | A | 8927257 | V | 9980 |
| 476 | 8936563 | GGT | 8934299 | AGT | 1804 | G | 8934510 | S | 1817 |
| 477 | 8928312 | AAC | 4616646 | AGC | 4277450 | N | 4621974 | S | 4297994 |
| 483 | 8708540 | GTT | 8703086 | TTT | 2624 | V | 8704199 | F | 2626 |
| 484 | 8936676 | GCA | 4335563 | GAA | 4260627 | A | 4337357 | E | 4260912 |
| 486 | 8955710 | TTT | 7215798 | GTT | 1093074 | F | 7216229 | V | 1093104 |
| 487 | 8965131 | AAT | 8961752 | GAT | 2415 | N | 8962456 | D | 2415 |
| 489 | 8959273 | TAC | 8945444 | TAT | 13483 | Y | 8958927 | H | 191 |
| 493 | 8950300 | CAA | 6029269 | CGA | 2825257 | Q | 6029484 | R | 2825435 |
| 494 | 8958293 | TCA | 8938768 | CCA | 16711 | S | 8939818 | P | 16716 |
| 498 | 8842093 | CGA | 4519230 | CAA | 4322081 | R | 4519641 | Q | 4322219 |
| 500 | 8856327 | ACT | 8854444 | ACC | 1165 | T | 8855722 | A | 214 |
| 501 | 8857981 | TAT | 5261026 | AAT | 3592163 | Y | 5261420 | N | 3592755 |
| 502 | 8860900 | GGT | 8860205 | GGC | 348 | G | 8860635 | A | 61 |
| 505 | 8848803 | CAC | 4520230 | TAC | 4326615 | H | 4521399 | Y | 4327315 |
Summary of codon usage and amino acid frequency at 20 functionally critical positions within the RBD (positions 417-505). For each residue, the top two codons and their counts are listed, along with the corresponding amino acids and frequencies. High total match counts (>8 million) reflect extensive sampling across 9.4 million genomes. Multiple dominant codons per position (e.g., 446: GGT/GCT → G/S; 450: AAT/GAT → N/D) indicate codon permissiveness and potential for diversification. Positions with low amino acid entropy (e.g., 483: V/F) may represent sites undergoing fixation or selection for specific functional traits. This dataset provides a foundation for identifying novel escape signatures and engineering mAbs resilient to future variants.
The canonical immune escape sites are not merely amino acid substitutions—they are codon-permissive hotspots where evolutionary pressure is reflected in multiple viable genetic paths. This stacked bar chart visualizes the cumulative frequency of the top two codons at each hACE2-proximal residue, confirming known escape signatures (e.g., E484K, Q493R) while highlighting novel permissiveness at N450—a residue that diversifies via L450N and L450D, encoded by AAT and GAT. The dominance of secondary codons at key positions underscores a deeper principle: escape is not random, but structured by codon usage bias.
Figure 2.
Structural Mapping of the Codon-Permissive Epistatic Backbone (CpEB) Triad onto the Wild-Type SARS-CoV-2 Spike RBD-hACE2 Interface.
Figure 2.
Structural Mapping of the Codon-Permissive Epistatic Backbone (CpEB) Triad onto the Wild-Type SARS-CoV-2 Spike RBD-hACE2 Interface.
All panels depict the wild-type (WT) Spike protein (PDB: 6M0J) without mutations. (A) Full complex showing Spike (cyan) docked to hACE2 (green). Arrow 1 points to the N-terminal domain (NTD); arrow 2 indicates the receptor-binding domain (RBD). (B) Highlighted region encompasses residues 319-541 (the RBD), shown in green for clarity. Arrow 3 identifies this RBD segment. (C) Focus on the CpEB triad: G447 (arrow 4), N450 (arrow 5), and V483 (arrow 6), shown as magenta spheres to emphasize their spatial clustering at the RBD-hACE2 interface. (D) Zoomed view isolating the triad: arrow 7 points to the 483 residue; arrows 8 and 9 highlight individual residues N450 and G447, respectively, revealing their proximity to ACE2 (green) and conformational plasticity. This panel confirms that immune escape is driven by codon-permissive residues localized to the functional interface enabling modular, predictable evolution without cross-protein epistasis.
Discussion
The Codon-Permissive Mutational Backbone: A New Lens on Immune Escape
For over four years, the field has mapped SARS-CoV-2 immune escape through the lens of amino acid substitutions—K417N, E484K, L452R—treating each as an isolated event in a reactive arms race. Our study fundamentally shifts this paradigm. By analyzing codon usage patterns across 9.4 million genomes, we reveal that immune escape is not driven by point mutations alone, but by structured evolutionary flexibility at the codon level—a phenomenon we term the Codon-Permissive Mutational Backbone (CpMB).
At its core, the CpMB is defined by residues that—due to their position, structural context, and codon degeneracy—can explore multiple mutational paths without collapsing viral fitness. The triad of G447, N450, and V483 exemplifies this principle in three distinct modes: fixation (G447), sweep (V483), and active diversification (N450). These are not random hotspots. They are evolutionarily privileged positions—sculpted by selection to serve as launchpads for immune escape.
N450, with its Shannon entropy of 0.19 and codon-level flexibility (AAT → N, GAT → D), is the most consequential. It is not merely mutating—it is experimenting. Unlike E484—where mutations are frequent but often transient or fitness-costly—N450 maintains high global frequency while diversifying. This is not noise. This is evolutionarily stable exploration—a signature of a residue under strong, sustained immune pressure, yet buffered by codon-level redundancy. It was missed by experimental escape maps because those maps measure what escapes now—not what can escape tomorrow. Our framework measures the latter.
While residues like 444 and 486 rank highest in raw entropy, they reflect codon degeneracy or tolerance for stop codons—not immune-driven escape. In contrast, N450 though lower in entropy—represents a de novo star: a residue where codon permissiveness directly enables antibody evasion through biologically consequential, charge-altering mutations (N/D). This makes it the most actionable target for predictive mAb design.”
Why Codon-Level Analysis Matters
Prior studies have reported codon usage in SARS-CoV-2—but almost always at the gene level, or as global averages. Our innovation is position-resolved codon analysis at atomic scale (≤5 Å). This allows us to ask: Which specific residues are evolutionarily permissive—and why?
At G447, near-fixation of N447G (99.998%) masks a deeper truth: this residue was once highly permissive—its codon TTA (Leu) was replaced almost entirely by GGT (Gly), with RSCU = 2.9999—the highest in the RBD. This was not drift. This was directed evolution—a high-fitness solution so successful it became invariant.
At V483, E483V dominates (99.95%), but 15 unique mutations were observed—evidence of a permissive landscape now winnowed by selection. Again—low entropy now ≠ low importance then.
Only by measuring codon usage at the residue level can we reconstruct these evolutionary histories—and predict future ones.
Translational Impact: From Reactive to Predictive mAb Design
Current monoclonal antibodies fail because they are designed reactively against today’s dominant variant, not tomorrow’s inevitable mutation. Our framework flips this script.
By engineering mAbs to tolerate the codon-permissive spectrum of autonomous sites like N450—even with mild affinity trade-offs, we preempt >99% of observed evolutionary trajectories. We don’t need to predict which mutation will win. We design against all of them.
This is not speculation. Our data show that N450’s mutational paths are constrained by codon degeneracy which is not randomness. AAT and GAT are the only codons viably encoding N and D at this position. An mAb engineered to accommodate both—or better, to lock the residue in a conformation agnostic to N/D—would remain effective for years, not months.
This transforms mAb design from reactive to predictive and from complex to modular.
Limitations and Future Directions
Our study is intentionally scoped to the hACE2 interface—residues within 5 Å mostly targeting 2 to 5 Å region of receptor contact. We do not claim the CpMB explains all immune escape only the most direct, antibody-relevant kind. Other mechanisms glycan shielding, conformational masking, non-RBD epitopes remain important and warrant separate frameworks.
We acknowledge that our dataset reflects well-documented global disparities in SARS-CoV-2 genomic surveillance. But its history now. Sequencing efforts were heavily concentrated in high-income regions particularly the United States, the United Kingdom, and Western Europe which together account for the majority of submissions to public repositories like GISAID and NCBI during 2020–2025. In contrast, many regions remain underrepresented: parts of Asia (e.g., India, Southeast Asia), the Middle East (e.g., Israel), and especially the African continent contributed far fewer sequences. While this geographic bias is a limitation of global surveillance as a whole, it does not undermine our core findings. The scale of our dataset (9.4 million genomes) and the consistency of codon-level signals across dominant lineages provide robust statistical power to detect evolutionarily consequential patterns. Moreover, because our framework relies on mutational frequencies and codon usage features shaped by selection rather than sampling alone the identified Codon-Permissive Epistatic Backbone (CpEB) reflects biological reality in the globally circulating virus, not merely artifacts of regional sequencing intensity.
Whereas some regions have lower representation in our dataset of 9+ million genomes while several regions are underrepresented or do not have any representation at all. We acknowledge this but the focus of this study is not as much on temporal in the sense that while more than 9 million genomes filled much of gap and we had enough variations at hand we therefore could then proceed with our analysis. We hope to add more to our sample size of 9 million + dataset as and if more sequences become available. The chance of more SARS-CoV-2 sequences seems gloomy for the fact that there is declining sequencing efforts rightly so as the there is no pandemic emergency of COVID-19 now and the global efforts to eradicate and sequencing effort has been at such scale globally that probably COVID-10 can never show up ever again except for localized eruptions such as the seasonal endemics and secondly resources should face more towards the similar level may be more hazardous H5N1 also that this study has been done without any sort of funding from any funding body. Our goal is not to model an idealized virus, but the one that actually spread with all its real-world imperfections. The scale of our dataset (9.4M genomes) ensures statistical power even in underrepresented regions.
Future work should extend the CpMB framework to other pathogens—H5N1, RSV, future coronaviruses—where codon permissiveness may similarly predict escape.
Conclusion
We did not set out to disprove epistasis. We set out to find what drives immune escape and let the data lead us. What we found is may be valuable than a network: a principle. Because we have aggregated many principles into one generalized conclusive hypothesis so in the sense of AI training models we have this study trained on many principles just to reach the final conclusion.
Immune escape is not a genome-wide conspiracy. It is a local, codon-level negotiation between mutation, structure, and selection. And residues like N450—autonomous, permissive, predictable—are the keys to mastering it.
The central challenge in antiviral antibody design has always been this: viruses evolve faster than we can react.
SARS-CoV-2 turned that challenge into a global crisis rendering monoclonal antibodies obsolete within months… sometimes weeks… of their deployment. The prevailing assumption? That escape was complex, a genome-wide dance of compensatory mutations, epistatic networks, and structural trade-offs. We were wrong.
Our data reveal a simpler, more elegant truth:
Immune escape in SARS-CoV-2 is not complex. It is modular. It is local. And most importantly it is predictable.
By analyzing codon usage patterns across 9.4 million genomes without relying on a single experimental assay or literature-curated escape map we identified the Codon-Permissive Epistatic Backbone (CpEB): a small set of residues in the Spike RBD that act as evolutionary launchpads for immune evasion.
At its heart sits N450 a residue with high Shannon entropy (0.19), actively diversifying under selection (L450N at 97.1%, L450D at 2.8%), and encoded by flexible codons (AAT, GAT) that allow mutation without fitness cost. Crucially, N450 evolves independently, unconstrained by Envelope, Membrane, or Nucleocapsid. Exhaustive analysis of 23 key Spike residues, including 20 canonical escape sites and 3 newly identified positions, confirms: no statistically significant or structurally validated epistatic coupling exists beyond Spike.
It means we no longer need to model cross-gene networks, anticipate global haplotype shifts, or chase lineage-defining constellations. We need only understand the local, codon-driven mutational spectrum of autonomous sites like N450 and engineer antibodies that anticipate them.
An mAb designed to tolerate both L450N and L450D even with modest affinity trade-offs would remain effective against >99.9% of observed evolutionary trajectories. Not because it’s broadly neutralizing but because it’s evolutionarily preemptive.
G447 and V483 complete the triad showing us that permissiveness doesn’t always mean diversity. Sometimes, it leads to fixation.
G447: Now nearly fixed as N447G (99.998%) a permissive mutation locked into global circulation.
V483: Dominated by E483V (99.95%) having swept the globe in record time.
Their low entropy today is not evidence of constraint it’s evidence of completion. They were permissive. They were selected. They won.
Together, these residues reveal a deeper truth:
Immune escape is not random. It is structured. It follows rules, the rules written not in protein structures or antibody assays, but in the silent mathematics of codon degeneracy.
Our framework, the CUB6 Metric Suite, the Codon-Permissiveness Score (CPS), the CpEB, provides the first fully de novo, data-driven method to read those rules. No prior experiments. No structural assumptions. Just genomes millions of them whispering evolutionary logic through codon bias.
This is not just a tool for SARS-CoV-2. It is a universal strategy immediately extensible to not only mono partite but also to H5N1. Wherever codon permissiveness exists, escape will follow. And wherever escape is autonomous, design can be predictive. We are no longer chasing variants. We now know how most of monopartite play including SARS-CoV-2. The era of reactive antibody design is over. The age of predictive, evolution-informed therapeutics begins now.
Rationale
Rationale: Methodological Foundations and Validation Framework
Our approach was designed to overcome the limitations of prior immune escape prediction methods, which rely heavily on experimental data, literature-curated residue lists, or structural assumptions. Instead, we built a fully de novo, data-driven framework grounded in evolutionary genomics and codon-level biophysics, requiring no prior annotation, no escape maps, and no structural modeling.
Data Curation: Ensuring Representativeness and Minimizing Artifacts
To ensure robustness and global representativeness, we curated 9.4 million high-quality SARS-CoV-2 genomes from GISAID, NCBI GenBank, and COG-UK (Jan 2019–Dec 2025). Sequences were filtered to exclude: Ambiguous nucleotides exceeding 1% (N’s), Incomplete metadata (missing collection date, geographic origin, or lineage assignment), Genomes with <95% coverage of the Wuhan-Hu-1 reference (NC_045512.2).
These filters minimized sequencing artifacts and ensured temporal and geographic diversity, critical for detecting true evolutionary signals rather than sampling biases.
Codon Usage Bias Metrics: Capturing Orthogonal Dimensions of Permissiveness
We selected six orthogonal metrics to quantify codon-level flexibility at single-residue resolution:
| Metric | Biological Interpretation |
| Shannon Entropy (H) | Measures codon usage diversity—low H = constrained; high H = permissive |
| Relative Synonymous Codon Usage (RSCU) Variance | Quantifies deviation from uniform synonymous codon usage—higher variance = greater plasticity |
| Effective Number of Codons (ENc) | Estimates overall codon bias independent of GC content—lower ENc = stronger constraint |
| Codon Adaptation Index (CAI) | Reflects translational efficiency relative to highly expressed viral genes—high CAI = selection for speed/accuracy |
| tRNA Adaptation Index (tAI) | Incorporates host (human) tRNA abundance—measures translational compatibility with human cells |
| Codon Volatility Score (CVS) | Predicts propensity for mutation to alter amino acid identity—higher CVS = greater escape potential |
These metrics were chosen to capture distinct, non-redundant facets of codon biology, ensuring that our composite score reflects true evolutionary permissiveness, not just one aspect of bias.
Codon-Permissiveness Score (CPS): Integration via PCA with Biological Justification
To integrate these six metrics into a unified Codon-Permissiveness Score (CPS), we applied Principal Component Analysis (PCA) after min-max scaling each metric to [0,1] to ensure equal weighting. The first principal component (PC1) explained >70% of total variance, indicating a dominant axis of codon permissiveness that aligns with evolutionary flexibility.
For interpretability, we also computed a weighted Z-score average of the six metrics, yielding results highly correlated with PCA-derived CPS (Pearson r > 0.92). Both approaches identified the same top residues, validating robustness.
Why PCA?
Multicollinearity among codon metrics is common (e.g., entropy and ENc often correlate). PCA reduces dimensionality while preserving maximal variance avoiding overfitting and ensuring biological signal dominates.
This framework builds upon our prior work on CpG/UpA dynamics and codon permissiveness across 9.4M genomes (Bhatti, 2025; DOI: 10.5281/zenodo.17173874), extending it from dinucleotide context to residue-level evolutionary capacity.
Statistical Rigor in Epistasis Detection: Justifying Extreme p-Values
The reported χ2 p-values (<1×10−15) reflect the immense statistical power afforded by our dataset size not an overstatement, but a mathematical inevitability when testing independence across 9.4 million genomes.
In small datasets, such p-values would be implausible. But here, even minute deviations from expected co-occurrence become detectable e.g., a 0.01% excess co-occurrence translates to ~940 extra cases, easily significant at p < 10−15.
To ensure biological relevance not just statistical significance we imposed three stringent criteria for defining epistatic linkage: χ2 p < 1×10−15 surviving Bonferroni correction for 66 pairwise tests (α = 7.58×10−4).
Mutual Information > 0.8 bits indicating substantial informational coupling between mutations (typical range in protein co-evolution: 0.1–1.5 bits).
Spatial proximity < 8 Å Cα-Cα distance in PDB 6M0J confirming structural plausibility of direct or allosteric interaction.
These filters collectively ensure that reported epistatic links are not artifacts of sample size but reflect genuine, evolutionarily constrained functional networks.
Validation Tool: All epistatic pairs were visually inspected in PyMOL to confirm spatial clustering distant residues (>8 Å) were excluded, as they are unlikely to co-evolve due to direct physical constraints.
Inter-Protein Epistasis Screening: Confirming Modularity
To test whether immune escape requires genome-wide compensatory changes, we screened for epistasis between 23 key Spike residues (including 3 novel CpEB sites) and mutations in Envelope (E), Membrane (M), and Nucleocapsid (N) proteins.
Using Fisher’s exact tests across 9.4M genomes, we found no statistically significant co-occurrence (FDR-corrected p > 0.05) between any Spike escape mutation and E/M/N variants. Structural analysis using cryo-EM structures (PDB: 7T9K, 6XRA) confirmed no direct contacts or allosteric pathways linking CpEB residues to E/M/N—supporting the autonomy of Spike-driven escape.
Why This Matters: It means mAb design need not account for complex cross-protein networks—only local conformational plasticity and predicted mutational spectra.
Therapeutic Design Pipeline: From CpEB to Predictive mAbs
Our CpEB-based pipeline enables rational, predictive antibody engineering:
Target residues with high CPS and DPMI (e.g., N450, G447).
Model local conformational flexibility using Rosetta or AlphaFold2.
Engineer paratope tolerance for predicted mutational spectra (e.g., L450N/D for N450).
Accept modest affinity trade-offs (~2–5 fold) to achieve >99% variant coverage.
This shifts mAb development from reactive (chasing variants) to predictive (anticipating trajectories)—dramatically extending therapeutic shelf-life.
Supplementary Material
The following supporting information can be downloaded at the website of this paper posted on Preprints.org.
Ethical Compliance
This study is entirely computational and based on publicly available, anonymized SARS-CoV-2 genomic sequence data. No human subjects, animal experiments, or personally identifiable information were involved. All analyses were performed in silico using open-access datasets from GISAID, NCBI GenBank, and COG-UK, in compliance with their data use agreements. Therefore, no ethical approval was required.
Data Availability Statement
All underlying codon frequency tables, CPS scores, and co-occurrence matrices are publicly archived on Zenodo/Figshare & will be made public upon acceptance of the manuscript. Codon and amino acid frequencies at hACE2 interface residues: https://doi.org/10.5281/zenodo.17173874 Structural protein co-evolution network (S, E, M, N): https://doi.org/10.5281/zenodo.17054335 Global haplotype landscape of Spike contact residues: https://doi.org/10.5281/zenodo.17161330 RBD mutation constellations and mAb escape risk: https://doi.org/10.5281/zenodo.17208971 High-risk VYD2311 escape haplotype detection: https://doi.org/10.5281/zenodo.17203862 Codon Adaptation Index (CAI) profiling: https://doi.org/10.5281/zenodo.17053652 tRNA Adaptation Index (tAI) profiling: https://doi.org/10.5281/zenodo.17052660 Chi-square and mutual information profiling of Spike clusters: https://10.5281/zenodo.15951243 Codon-permissive residues at hACE2 interface (240M genomes): https://10.5281/zenodo.17167437.
Conflict of Interest
The author declares no conflict of interests.
Abbreviations/Dictionary Panel
All abbreviations used in this study are defined below for clarity and reproducibility.
CUB6 Metric Suite (C6MS)
A unified framework comprising six orthogonal codon usage bias (CUB) metrics computed per residue to quantify evolutionary permissiveness:
| Abbreviation | Full Name | Biological Interpretation |
| RSCU | Relative Synonymous Codon Usage | Measures preference among synonymous codons; higher variance = greater plasticity |
| CAI | Codon Adaptation Index | Reflects translational efficiency relative to highly expressed viral genes |
| CPB | Codon Pair Bias | Quantifies bias in adjacent codon pairs; influences translation speed/fidelity |
| ENC | Effective Number of Codons | Estimates overall codon bias (lower ENC = stronger constraint) |
| PR2 | Parity Rule 2 Asymmetry | Measures strand-specific nucleotide bias (A≠T, G≠C); correlates with mutational pressure |
| GC3 | GC Content at Third Codon Position | Indicates selection for/against GC-rich codons; affects stability and mutation rate |
Collectively, these six metrics form the CUB6 profile—a comprehensive signature of codon-level evolutionary flexibility.
CPS—Codon-Permissiveness Score
A composite metric derived from PCA of the CUB6 suite, quantifying how freely a residue can mutate while maintaining viral fitness. High CPS = high permissiveness.
DPMI—Dual Permissiveness-Mutation Index
Defined as:
DPMI=CPS × Mutational Frequency
Used to prioritize residues that are both evolutionarily flexible and frequently mutated.
CpEB—Codon-Permissive Epistatic Backbone
A core set of residues (e.g., F486, L452, K444, N450, G447, V483) that drive variant success through codon-level flexibility and intra-Spike epistasis.
MI—Mutual Information
A measure of informational dependence between two variables (here, mutation states at two residues). Values > 0.8 bits indicate strong co-evolutionary coupling.
FDR—False Discovery Rate
A multiple testing correction method (Benjamini-Hochberg) used to control the proportion of false positives among significant results.
PCA—Principal Component Analysis
A dimensionality reduction technique used to integrate the six CUB metrics into a single CPS, capturing >70% of variance in codon permissiveness.
PDB—Protein Data Bank
Structural database used for mapping residues onto 3D structures (e.g., 6M0J for RBD-hACE2 interface; 7T9K for full Spike trimer).
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