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The Fairfax County Public Schools (FCPS) in Fairfax County, VA, identified two COVID-19 core heath metrics that they will use in making decisions about if and when to open schools. These were presented at the Fairfax County School Board Working Session on November 12, 2020. The slides for the meeting can be found in the meeting Board Docs.

Core Metric 1: Total number of new cases of COVID-19 within the last 14 days in Fairfax, VA, per 100,000 persons

Core Metric 2: Percentage of PT-PCR Tests that are Positive during the last 14 Days

The metrics are outlined on the Virginia Department of Health School Metrics site.

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FCPS defines part of its decision criteria for whether to re-open schools, and later potentially shut down again, as oriented around the trend over the previous seven days of these two metrics. However, those temporal visualization of the metrics are not available on any dashboards that I have found. This post is an attempt to capture and visualize that information.

We use data from the Virginia Department of Health . The source data was downloaded from the Virginia Department of Health website and is as of November 22, 2020.

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FCPS outlined its decision criteria for the School Board in these terms:

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Established Thresholds

(as of November 12, 2020)

(as of November 12, 2020)

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Total Number of Cases Over 14 Days Per 100,000 Persons

The first metrics is the total number of new cases of COVID-19 within the last 14 days in the Fairfax Health District of VA, per 100,000 persons. This is the sum of all the reported cases in the past 14 days, divided by approximately 11.86, since the population of all the localities in the Fairfax Health District is approximately 1.186 million people. The Fairfax Health District is made up of Fairfax County, Fairfax City, and Falls Church.

Population levels are based on US Census estimates for those locations.

The thresholds are the indicator risk levels defined by the Virginia Department of Health. The blue section highlights the past 7 days.

In the current FCPS plan, the “Highest Threshold” is the trigger for the decision criteria outlined above.

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Focus on Fairfax County Locality

The VHD School Metrics site displays the data by locality, which is different from Health District. The following chart shows the cases over the previous 14-days per 100,000 in population for just the locality of Fairfax County. In this case, we use the US Census estimate the the population of the county as 1.186 million. It should be noted that the number used for population can affect the outcome of the metric. Also, it is unclear which data subset, by Health District or by locality, that FCPS is using for its decision making.

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Population Estimates Matter

The charts above use the US Census estimates for the population counts as of July 1, 2019. However, if you use a more generic estimate of population, where the metric falls with respect to the threshold changes. This can have a major impact on decision making as any one day in which the metric exceeds a threshold can trigger a decision related to school re-opening by FCPS.

The following chart uses a more generic population estimate for Fairfax County of 1.1 million people.

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Percentage of Positive RT-PCR Tests During the Last 14 Days

The second metric is the percentage of RT-PCR Tests that are positive during the last 14 days. This is the total positive tests in the past 14 days divided by the total number of tests reported in the past 14 days.

Again, the thresholds are the indicator risk levels defined by the Virginia Department of Health. The blue section highlights the past 7 days.

In the current FCPS plan, both the “Highest Threshold” and “Higher Threshold” are a trigger for the decision criteria outlined above.

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