Severe Acute Respiratory Syndrome Coronavirus 2 Serial Interval Variation, Montana, USA, March 1–July 31, 2020

I n support of efforts in response to the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the pathogen causing novel coronavirus disease (COVID-19), the scientifi c community has attempted to predict its transmission trends, often through disease modeling.However, diseasespecifi c parameter estimates for SARS-CoV-2 vary greatly.These parameters include the serial interval (SI), or the duration between onset of symptoms in connected primary and secondary cases, which is crucial in estimating epidemic reproduction numbers (R 0 ) and assessing the effects of nonpharmaceutical interventions (NPIs) on transmission (1).Recent studies report SARS-CoV-2 SIs ranging from 2.97 to 7.5 days, with estimates representing primarily densely populated and urban settings (Table 1; Figure 1).The rural United States was relatively untouched in early epidemic waves, but major outbreaks followed in subsequent waves, so it is unknown whether rural-and urban-based transmission differ.Our objective was to report and compare SARS-CoV-2 SI values for Montana, USA, a primarily rural population, with other global and urban estimates.The study was defi ned as a public health surveillance activity by the University of Montana Institutional Review Board. The StudyWe acquired COVID-19 data, reported by local health jurisdictions, from the Montana Department of Public Health and Human Services; we obtained 45,102 case records as of November 15, 2020.We examined a subset of cases with symptom onset dates during March 1-July 31, 2020 (n = 4,793), as well as secondary cases resulting from primary infections during that period, regardless of onset date.We selected this period because all reported cases were PCR positive, all NPI stages were represented (pre-shelter-in-place [pre-SIP], shelter-in-place [SIP], and reopening phase 1 and phase 2), and the proportion of identifi able transmission chains among cases was relatively high (March-June 39%-44%; July 11%) compared with later periods (August-November 0%-2%).We assessed the records to identify all epidemiologic links.We defi ned links as cases having contact with another reported case, when viral infection through accepted modes of transmission was plausible.Linked records (n = 1,005) were organized into pairs and designated as primary or secondary cases.When appropriate, cases were listed as primaries for multiple secondary cases; however, cases were limited to 1 secondary designation.For some secondary cases, 1 specifi c primary was not clearly defi ned.To estimate a serial range in these situations, we assigned upper and lower bounds using the shortest and longest SIs from all possible primaries.We excluded records when we could not determine an epidemiologic link or transmission direction.We identifi ed 583 pairs, with 466 primary and 583 secondary cases.We gave temporal markers to pairs on the basis of the primary case's symptom onset date, consistent with forward-looking SIs (2), and grouped them by the corresponding statewide NPI: pre-SIP, March 1-27; SIP, March 28-April 25; phase 1, April 26-May 31; phase 2 (June), June 1-30; and phase 2 (July), July 1-31.We divided phase 2 into 2 subperiods to account for changing incidence trends.

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1. 3.

   Improved inference of time-varying reproduction numbers during infectious disease outbreaks · 2019

2. 4.

   Serial Interval of COVID-19 among Publicly Reported Confirmed Cases · 2020

3. 7.

   Serial interval of SARS-CoV-2 was shortened over time by nonpharmaceutical interventions · 2020

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