Unexpectedly Intriguing!
19 November 2020

One week ago, we projected the state of Arizona would soon arrive at a critical juncture in its experience with the coronavirus pandemic. With the rate of ICU bed usage in the state for COVID-19 patients now surpassing a key threshold, that time has now arrived.

That state of affairs may be seen in a chart tracking Daily COVID-19 ICU Bed Usage in Arizona, where the number of beds occupied by COVID-19 patients now exceeds the level would be considered easily sustainable.

Daily COVID-19 ICU Bed Usage in Arizona, 3 March 2020 - 18 November 2020

Arizona's hospitals still have available ICU bed capacity, so the situation in the state isn't as critical as other areas that are currently experiencing a significant surge in cases. What exceeding this threshold means is that Arizona hospitals need to begin more actively managing their ICU beds usage to accommodate the rising numbers of COVID-19 patients. Ideally, those measures will involve increasing their ICU bed capacity. Unlike many states, Arizona saw two new hospitals open this month in its major metropolitan areas, which will provide some additional breathing room.

Sharp eyed readers will note we've added a new event to this chart. Event I marks an increase in the trend for COVID-19 ICU bed usage that coincides with political events that took place in the state during the period from 23 October 2020 through 25 October 2020. Using the back calculation method to identify the period in which the incidence of coronavirus exposures points to this period as a significant event. The latest update to our chart tracking daily new COVID-19 hospital admissions in Arizona identifies each of the major events associated with a changes in the risk of coronavirus exposure among Arizona's population.

Daily COVID-19 New Hospital Admissions in Arizona, 3 March 2020 - 18 November 2020

The data for this latter chart is still incomplete, where the ICU bed usage chart has proven to be a good real time indicator of the progression of SARS-CoV-2 coronavirus infections within the state. We anticipate the rolling 7-day moving average will soon confirm the surge in COVID-19 hospitalizations.

Data for positive COVID-19 test results in Arizona already confirms a surge in new cases, pointing once again to the period of 23 October 2020 through 25 October 2020 as the period in which the incidence of new infections increased. The following chart of daily newly confirmed COVID-19 cases in Arizona shows the latest surge:

Daily COVID-19 Confirmed Cases by Sample Collection Date in Arizona, 3 March 2020 - 18 November 2020

Meanwhile, since it has the greatest lag between the incidence of exposure and observed change in trend, a chart of deaths attributed to COVID-19 in Arizona does not yet confirm a change in trend. We project the rolling 7-day moving average of coronavirus deaths will show a change in trend taking place in the period from 9 November through 13 November 2020 as these deaths are reported in the weeks ahead.

Daily COVID-19 Deaths in Arizona, 3 March 2020 - 18 November 2020

We track Arizona's COVID-19 data because the state provides high quality, relatively detailed data that makes it possible to use the back calculation method to identify when the rate of incidence of coronavirus infections has changed for the state's residents. To better show how that method works, we put together the following chart to track the incidence of COVID-19 infections among the various demographic age groups reported by Arizona's Department of Health Services. The chart focuses on the time from 9 August 2020 through 18 November 2020, which covers Arizona's 'back-to-school' period for its state universities.

In this chart, we're identifying trend-changing events by number instead of letter, so here's the basic summary:

  1. 20-22 August 2020: In-person classes begin at state universities.
  2. 4-6 September 2020: Social mixing during Labor Day weekend.
  3. 13-15 September 2020: University of Arizona imposes lockdown measures on its students (the outbreak of cases in the state during this time was concentrated at the UA campus). After this third significant event affecting the incidence of new coronavirus cases in Arizona, it's important to note the divergence that takes place among demographic age groups in the state. The Age 0-19 group sees a falling number of cases, as Arizona schools have measures that are largely effective in limiting the spread of new cases. But the Age 20-44 group sees an increasing number of cases as compared to all other age groups, where this group is the most likely to frequent high exposure risk venues, such as bars, gyms, and other businesses for which public health officials have established specific operating requirements.
  4. 23-25 October 2020: Political rallies centered around the occasion of the 24 October 2020 National Vote Early Day, less than two weeks ahead of the U.S. elections.

The current upward trend in cases and what we can identify as contributing factors to it using the back calculation method suggests the most effective approach state and local government officials can take to reverse its adverse trend would be to restrict the operation of high exposure risk businesses in local communities as the ICU beds usage within them nears 95% of capacity. Arizona has already demonstrated a decentralized approach can be highly effective in coping with a surge in cases, without unnecessarily imposing economic hardships on the state's residents in areas where the number of cases and burden on loal hospital resources is relatively low.

Local officials could also mandate wearing masks at public venues within their jurisdictions, though we think this option would provide little benefit. That is because most areas in the state already have relatively high rates of compliance with wearing masks inside local businesses, where there is little evidence to suggest a statewide mandate would significantly alter the trend. The current situation differs from the situation that applied during the summer when Arizona became a national hotspot for COVID-19 infections, when the rate of mask wearing was very low prior to the state governor's order allowing local officials to impose mask wearing requirements for their residents. Starting from an already higher level of mask wearing, any additional benefit that might be realized is much smaller.

With the elections now in the past, removing that contributor to the risk of virus exposure, Arizona's next challenge in its coronavirus pandemic experience will be to address the social mixing that will take place during the Thanksgiving holiday. We'll present our next follow up after the holidays to see how the state fared.

Previously on Political Calculations

Here's our previous Arizona coronavirus coverage, with a sampling of some of our other COVID analysis!


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 10 November 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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