Unexpectedly Intriguing!
21 January 2021

The last two weeks of COVID-19 data from Arizona suggests the state's second wave of cases has peaked and has begun receding.

The origins of that second wave can be traced back to political campaign activities held throughout the political swing state during the two weekends prior to the 3 November 2020 election. These events triggered an explosion in new coronavirus infections well above and beyond what had previously been a slow and steady rise in the number of coronavirus infections following the September 2020 reopening of high exposure risk businesses in the state.

Arizona's COVID-19 cases grew rapidly as new infections spread beyond the participants in these activities. But that began to change after news reports began focusing on the surge of COVID-19 patients at Arizona Intensive Care Units in early December 2020. We think those reports prompted Arizonans to resume social distancing and good hygiene practices they had previously adopted during the state's first COVID-19 wave in the early summer of 2020, which succeeded in slowing the rate of growth of new cases.

But it wasn't until Christmas came and went, and with it, the period where holiday-related shopping activities and social gatherings contributed to the ongoing spread of COVID-19 cases, that the second wave of COVID-19 cases in Arizona crested and began to recede. At least, that's what the data reported over the last two weeks indicates, which the charts showing Arizona's data for newly confirmed cases by sample collection date, daily ICU bed usage, deaths by death certificate date, and new hospital admissions, covering the period from 3 March 2020 through 19 January 2021. Although the lag from exposure to change in trend for each chart is different, the back calculation method for each confirms the timing of significant events for the progression of SARS-CoV-2 coronavirus infections in Arizona. [Please click on the preceding links or the following images to access full size versions of the charts.]

Arizona COVID-19 Confirmed Cases by Sample Collection Date, 3 March 2020 - 19 January 2021
Arizona COVID-19 ICU Bed Usage, 3 March 2020 - 19 January 2021
Arizona COVID-19 Deaths by Death Certificate Date, 3 March 2020 - 19 January 2021
Arizona COVID-19 New Hospital Admissions, 3 March 2020 - 19 January 2021

Of these charts, only the chart showing Arizona's ICU Bed Usage is fully current. Data for the other three charts are incomplete, where the most recent three weeks shown will be subject to revision during the next few weeks, especially for the most recent dates indicated on the charts.

Notes of Interest

Readers following this series will catch that we're no longer indicating "Event J" on these latest charts. Event J refers to Arizona Governor Doug Ducey's 3 December 2020 restriction on high-attendance events, which proved to have little-to-no impact on the trends for COVID-19 cases in the state.

We are however showing T* to indicate the timing of the Thanksgiving holiday and X* to indicate the Christmas holiday. These events, in and of themselves, had a very noticeable impact on whether Arizonan's sought to be tested for coronavirus infections, which shows up in the chart for confirmed cases. Here, it appears many Arizonans who became sick during these periods delayed seeking COVID-19 tests during the holidays and subsequent weekends, waiting until the following Mondays to be tested.

By contrast, there are slight dips in the data for new hospital admissions and for deaths in the lagging periods associated with the holidays, but these don't greatly deviate from the overall trends.

We've attributed the deceleration and peak in Arizona's second COVID-19 wave to increased social distancing and hygiene practices (standing apart in public, washing hands, etc.). Reports indicate Arizonans have generally maintained a high level of face mask-wearing since the state's first surge in cases in the early summer of 2020, while social distancing and hygiene practices waned. Overall, the evidence suggests social distancing and good hygiene are more effective in avoiding new COVID-19 infections than face masks alone. But we have to emphasize again, the best way to avoid COVID-19 is to avoid contact with political activists.

Unless something changes, we anticipate the next update to this series will confirm Arizona has entered a downward trend for coronavirus infections.

Previously on Political Calculations

We've been covering Arizona's experience with the coronavirus pandemic since the state first became a national hotspot early in the summer of 2020. Here's our previous Arizona coronavirus coverage presented in reverse chronological order, with a sampling of some of our other COVID analysis!


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. [Online Application/Database].

Maricopa County Coronavirus Disease (COVID-19). COVID-19 Data Archive. Maricopa County Daily Data Reports. [PDF Document Directory, Daily Dashboard].

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.

COVID Tracking Project. Most Recent Data. [Online Database]. Accessed 15 December 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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