Time to sustained control of the effective reproduction number as a failure-time estimand: a county-level analysis of the first SARS-CoV-2 wave in the United States.

Subnational analyses of emerging pathogen transmission typically model the reproduction number (Rt) as a continuous outcome and estimate average associations with its level. Operationally, however, response planning depends on a different quantity: which jurisdictions bring transmission below the epidemic threshold (Rt<1), and how quickly. We formulate time to control as a failure-time estimand and show that doing so exposes a structural feature that linear regression does not accommodate: a jurisdiction cannot enter the risk set until enough cases accumulate to estimate Rt. Cases accumulate f

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