Reported quantitative patterns: Single-site variants (n=681 after exclusions) show 27% inviability when only observed at T0, vs 73% viable when observed after T0, and MBM estimates imply each +1 unit DDGfold decreases log-odds of survival substantially (b1≈-1.4047) with similar but smaller slope for DDGbind (b1≈-0.385), with an MBM-predicted maximal survival probability capped at ~0.87 for singles.
Fitness vs stability (among viable variants): Linear regression on viable mutants (n=498) yields weak negative coefficients (DDGfold ≈ -0.158, p≈0.023; DDGbind ≈ -0.062, p≈0.427) and very small overall explanatory power (R² ≈ 0.013 unadjusted), leading the authors to conclude stability alone is not a reliable fitness predictor within the viable set.
The MBM asymptote (<1 maximal survival) is interpreted as “unobserved factors,” but the paper does not quantify (i) how much of the capped probability comes from FoldX-model systematic bias vs experimental sampling/stochasticity vs other biological steps (e.g., assembly, host interactions). The double-mutant analysis assumes additive DDG for doubles; if non-additivity (epistasis) is common, the inferred thresholds may be partially model-driven.
For variant classification, the work supports a “stability-threshold” intuition: highly destabilizing capsid spike mutations are reliably lethal, but within viability, many mutations may remain phenotypically tolerated through compensating biological mechanisms not captured by DDGfold/DDGbind alone.
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