Unexpectedly Intriguing!
08 April 2021

One month ago, we demonstrated COVID-19 was in retreat in Arizona, though with some signs the rate of improvement was beginning to slow. Since then, enough data has filled in to indicate that positive trend has nearly stalled. We've identified two significant events using the back calculation method that appear to have contributed to that change.

The quartet of charts below are based on Arizona's high quality data for the numbers of new cases by test sample collection date, hospitalizations by date of admission, deaths by date as recorded on death certificates, and ICU bed usage.

Arizona Positive COVID-19 Test Results by Date of Sample Collection, 1 January 2020 - 6 April 2021
Arizona New COVID-19 Hospitalizations by Date of Admission, 1 January 2020 - 6 April 2021
Arizona New COVID-19 Deaths by Date of Death Certificate, 1 January 2020 - 6 April 2021
Arizona COVID-19 ICU Bed Usage, 10 April 2020 - 6 April 2021

The two new significant events changing the trends for COVID-19 infections in these charts are identified by the letters M and N in each of these charts. Event M reflects a change in the established downward trend that took place on 14 February 2021 for cases, 17 February 2021 for hospitalizations and ICU bed usage, and on 23 February 2021 for deaths. Using the established lags of 9-11 days for cases, 11 to 14 days for hospitalizations and ICU bed usage, and 17-21 days for deaths, that puts the timing of the significant events that altered the rate of incidence of exposure to the SARS-CoV-2 coronavirus in Arizona between 3 February and 6 February 2021.

Within this period, we identified two actions by the Biden-Harris administration with the potential to slow what had been a strong trend of improvement. First, the Biden-Harris administration's action to terminate a deal with Arizona to deport immigrants who had been charged or convicted of crimes on 4 February 2021. Second, the Biden-Harris administration's suspension of border fence construction and other Trump-era policies restricting border crossings resulted in an uncontrolled surge of unaccompanied minors and others crossing the border from Mexico into Arizona.

That surge has come at a time when Mexico is experiencing much higher rates of COVID-19 infections than the U.S. Though like Arizona, Mexico's rate of COVID infections has been falling after peaking in late January 2021, the higher rate of infections of people crossing the border from Mexico into Arizona, both lawfully and not, has meant Arizona's rates of improvement for COVID cases, hospital admissions, deaths and ICU bed usage has slowed as result of the influx.

But not stalled or reversed, which brings us to Event N. Here, only the data for cases is complete enough to indicate a change in trend, but that is enough to point to the timing of Arizona Governor Doug Ducey's 8 March 2021 lifting of capacity limits on high-exposure risk businesses (gyms, water parks, restaurants, etc.) as a significant factor affecting Arizona's COVID trends. In this case, causing what had been a trend of improvement in the state's COVID cases to bottom.

It's interesting to compare Arizona's current trend for coronavirus cases with the bottoming of its trend following its first surge in cases. The main difference between the two periods is the higher level of cross border migration into Arizona. The rolling 7-day moving average for new cases has bottomed at a level about 40% higher than it did in the earlier period.

It will take more time to see if the other datasets follow suit. We'll look to see what happened in our next update to this series, along with determining if any new events contributed to changing the direction of Arizona's COVID trends.

Previously on Political Calculations

Here is our previous coverage of Arizona's experience with the coronavirus pandemic, presented in reverse chronological order.


We've continued following Arizona's experience during the coronavirus pandemic because the state's Department of Health Services makes detailed, high quality time series data available, which makes it easy to apply the back calculation method to identify the timing and events that caused changes in the state's COVID-19 trends. This section links that that resource and many of the others we've found useful throughout the coronavirus pandemic.

Arizona Department of Health Services. COVID-19 Data Dashboard: Vaccine Administration. [Online Database]. Accessed 7 April 2021.

Stephen A. Lauer, Kyra H. Grantz, Qifang Bi, Forrest K. Jones, Qulu Zheng, Hannah R. Meredith, Andrew S. Azman, Nicholas G. Reich, Justin Lessler. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application. Annals of Internal Medicine, 5 May 2020. https://doi.org/10.7326/M20-0504.

U.S. Centers for Disease Control and Prevention. COVID-19 Pandemic Planning Scenarios. [PDF Document]. Updated 10 September 2020.

More or Less: Behind the Stats. Ethnic minority deaths, climate change and lockdown. Interview with Kit Yates discussing back calculation. BBC Radio 4. [Podcast: 8:18 to 14:07]. 29 April 2020.


About Political Calculations

Welcome to the blogosphere's toolchest! Here, unlike other blogs dedicated to analyzing current events, we create easy-to-use, simple tools to do the math related to them so you can get in on the action too! If you would like to learn more about these tools, or if you would like to contribute ideas to develop for this blog, please e-mail us at:

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