Unexpectedly Intriguing!
09 September 2021

Arizona's third COVID wave has peaked and begun to recede.

Since our last update three weeks ago, the number of cases and new hospital admissions in the state has reached their peaks and begun to fall. Meanwhile, the state's data for deaths is still too incomplete over the last few weeks to confirm the pattern seen for cases and hospitalizations, but can be expected to follow.

In the following chart, we've aligned the rolling seven-day averages for the number of COVID cases, new hospital admissions, and deaths to the approximate date of exposure for the Arizonans affected by the spread of the various strains of the SARS-CoV-2 coronavirus. The chart is presented in linear scale, where you can see the third wave of infections begin to accelerate at the end of June 2021, which coincides with the mass exposure event of the Phoenix Suns advancing to the finals in the NBA playoffs. [Note the vertical scale of the chart is presented in logarithmic scale.]

Arizona's Experience During the Coronavirus Pandemic, 15 March 2020 - 6 September 2021, Logarithmic Scale

The data indicates the incidence of initial exposure to the active variants of SARS-CoV-2 coronavirus in Arizona's third COVID wave peaked in the period between 11 August 2021 and 15 August 2021.

The data also confirms that COVID-19 has become less risky for Arizonans, with the number of new hospital admissions and deaths falling below the trendlines (shown as the dashed yellow lines for the third wave) that would be expected from the number of cases based on the patterns observed during the state's experience of its first two coronavirus waves in 2020. That change corresponds to the state's high rate of vaccination for its most vulnerable elderly population, where COVID tends to be less serious for those in younger demographic groups, whose rate of vaccination has been lower.

There is a point of concern in that the state's ICU Bed Usage for COVID patients is still in a rising trend. We've put togther the following animation to cycle through charts illustrating Arizona's data for COVID cases, new hospital admissions, deaths, and ICU Bed Usage, showing each frame for five seconds. The vertical scale on each of these charts is presented in linear scale.

Animation: Arizona's Experience During the Coronavirus Pandemic, COVID Cases, New Hospital Admissions, Deaths, and ICU Bed Usage, 15 March 2020 - 6 September 2021, Linear Scale

The data confirms Arizona's third COVID wave peaked and began to recede at levels far lower than occurred during the state's first two waves.

Arizona is interesting because other than news reports communicating the rising number of hospitalizations and ICU bed usage at the time the incidence of cases and hospital admissions peaked, we have not been able to identify any state-level interventions that might explain the timing of its third peak.

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 September 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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