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.

In 2,956 US counties during the first SARS-CoV-2 wave (Jan 22–Jul 15, 2020), only 171 (5.8%) achieved a first three-day sustained mean Rt below 1.

• Why it matters: Planning depends on when jurisdictions reach transmission control (Rt<1), not on average Rt levels. Standard linear approaches miss that counties can only be assessed after enough cases accumulate to estimate Rt.
• What they did: The study defined “control” as the first three-day sustained reduction of mean Rt below one and modeled time to this event using a mixed-effects Cox model with calendar-time left truncation and state-level Gaussian frailties.
• The result: Higher log population density was linked to a lower control rate (HR 0.54; 95% CI 0.44–0.67), while higher policy stringency was linked to a faster control rate (HR 1.50; 95% CI 1.09–2.05). This supports using rate-to-target metrics for prospective intervention allocation.

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