Same R 0 , 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 $R_0$ 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 $\eta =0.10$ and distributes colonized admissions across pre-admission colonization ages. Of 1, 000, 000 parameter sets, 45, 250 satisfy cumulative $R^2\ge 0.90$ . The representative calculation is subcritical, with $R_0=0.907957$ ; across accepted sets, the median and 95% behavioral range are 0.917464 and $[0.814593,1.032975]$ , with $92.460\%$ below one. At $Z_{\text{crit}}=5$ , 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 $98.378\%$ of accepted sets. In 200-day conditional simulations, delaying ${\tau }_{\text{crit}}$ 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 $R_0$ , age dependence gives a $7.372\%$ higher peak but a $9.304\%$ lower cumulative burden than constant transmission. Increasing ${\kappa }_{max}$ from 0.6 to 1.4 times baseline reduces $R_0$ by $4.133\%$ . Thus, matching $R_0$ does not preserve timing-sensitive outcomes, but their magnitude and direction depend on the endpoint; over the examined ranges, clearance timing has greater leverage than immune-strength intensification.