Unexpectedly Intriguing!
16 November 2021

Arizona is seeing an upswing in the number of COVID cases in the state. As best as we can tell using the back calculation method, the new uptrend for cases began with a change in the rate of incidence of exposure to the viral infection approximately between 8 October and 11 October 2021. The following chart shows the latest trends for the number of COVID cases, hospital admissions, and deaths indexed to the approximate date of when those infected were initially exposed to the variants of the SARS-CoV-2 coronavirus being transmitted within the state.

Arizona's Coronavirus Pandemic Experience, 15 March 2021 - 15 November 2021

The state's detailed data for hospital admissions and deaths is still as yet too incomplete to tell if they will follow the pattern for the uptrend in cases. The impact of the COVID vaccines combined with improved treatments however has clearly made COVID less serious than it was during 2020.

The period of 8 October through 11 October 2021 was characterized by what might be considered several mass exposure events within the state. Here's a sampling of sporting events that drew thousands in attendance to each:

That's just the sporting events in the state during that period, without including things like tailgate parties or other social gatherings that could constitute significant exposure events.

We were curious to see if Arizona's COVID data could provide more insight into where cases are now arising in the state. The following image is a snapshot of COVID cases by ZIP code in the state, where the darker red the color, the greater the number of cases in the last month.

Arizona Recent COVID-19 Cases by Zip Code (15 October 2021 - 15 November 2021)

The map is fascinating because with a few exceptions it shows the higher levels of new COVID cases in ZIP codes are taking place in regions that might be considered to be the suburbs and exurbs of the Phoenix and Tucson metropolitan areas. Here's the short list of the ZIP codes that became the darkest red during the past month:

  • 85706: 793 cases
  • 85142: 792 cases
  • 85338: 792 cases
  • 85345: 771 cases
  • 85326: 763 cases
  • 85122: 717 cases
  • 85009: 713 cases

It would be interesting to see if those testing positive for COVID-19 infections in these ZIP codes share any common characteristics. Arizona's public data for infections at this level however doesn't provide enough detail to tell.

Previously on Political Calculations

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


Political Calculations has been following Arizona's experience with the coronavirus experience from almost the beginning, because the state makes its high quality data publicly available. Specifically, the state's Departent of Health Services reports the number of cases by date of test sample collection, the number of hospitalizations by date of hospital admission, and the number of deaths by date recorded on death certificates.

This data, combined with what we know of the typical time it takes to progress to each of these milestones, makes it possible to track the state's daily rate of incidence of initial exposure to the variants of the SARS-CoV-2 coronavirus using back calculation methods. Links to that data and information about how the back calculation method works are presented below:

Arizona Department of Health Services. COVID-19 Data Dashboard: Vaccine Administration. [Online Database]. Accessed 15 November 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]. 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.


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