Same R₀, Different Risk: Colonization-Age Timing and Heterogeneity Drive ICU MRSA Establishment and Burden

Intensive care unit (ICU) control of methicillin-resistant Staphylococcus aureus (MRSA) occurs over short horizons, whereas R₀ R 0 summarizes average transmission at the importation-free disease-free state without encoding timing. We couple within-host bacterial and immune dynamics to an ICU model structured by colonization age, generating transmission and progression kernels that vary with colonization age. A two-stage framework uses a branching process to quantify finite-threshold crossing and a deterministic ward model to quantify conditional burden after the threshold is reached. Calibration fixes η =0.10 η = 0.10 and distributes colonized admissions across pre-admission colonization ages. Of 1, 000, 000 parameter sets, 45, 250 satisfy cumulative R²≥ 0.90 R 2 ≥ 0.90 . The representative calculation is subcritical, with R₀=0.907957 R 0 = 0.907957 ; across accepted sets, the median and 95% behavioral range are 0.917464 and $$[0.814593,\,1.032975]$$ [ 0.814593 , 1.032975 ] , with $$92.460%$$ 92.460 % below one. At Zcrit=5 Z crit = 5 , exact threshold-reaching probabilities are 0.114659 for age-dependent transmission and 0.117074 for matched constant transmission. Their accepted-set difference range crosses zero, so this contrast has no robust direction. By contrast, age-dependent transmission reaches 50% of reproductive potential earlier in $$98.378%$$ 98.378 % of accepted sets. In 200-day conditional simulations, delaying τ crit τ crit from 5 to 15 days increases the age-dependent peak total MRSA burden from 5.629 to 7.298 patients and cumulative total burden from 683.8 to 1398.0 patient-days. At the 9-day baseline and equal R₀ R 0 , age dependence gives a $$7.372%$$ 7.372 % higher peak but a $$9.304%$$ 9.304 % lower cumulative burden than constant transmission. Increasing κ ₘₐₓ κ max from 0.6 to 1.4 times baseline reduces R₀ R 0