Unexpectedly Intriguing!
23 June 2020

Nearly two weeks ago, in our final weekly update on the progression of the coronavirus pandemic in the United States, we noted that three states were seeing significant increases in the number of new COVID-19 cases: Arizona, Utah, and Oregon. Since then, one of these three states has broken away from the pack for the rate at which it is confirming new cases.

We won't beat around the bush any more. It's Arizona. Here's an interactive chart of the rolling 7-day average of newly confirmed COVID-19 cases per 100,000 residents per day for all 50 states and the District of Columbia from 17 March 2020 through 22 June 2020, where you can easily see Arizona's surge.

Before we continue, take a moment to review the chart to compare Arizona's trajectory with previous leaders for population-adjusted newly confirmed SARS-CoV-2 coronavirus infections, such as New York, New Jersey, Rhode Island, and the District of Columbia.

We wanted to point that out before showing the next interactive chart, which tracks the rolling 7-day average number of deaths attributed to COVID-19 per 100,000 residents per day for the 50 states and the District of Columbia over the same period of time as the first chart.

In this second chart, we can see that Arizona is having a very different experience than the previous leaders for the number of COVID-19 deaths per 100,000 residents, with Arizona's daily average death counts not echoing the trend being set by the state's exploding number of newly confirmed cases as happened elsewhere in the U.S.

That's because unlike New York and other states that copied the Cuomo administration's disturbing policy of forcing nursing homes to admit coronavirus-infected patients, which then ran through those facilities like "fire through dry grass", Arizona's state officials have not, which has kept the state's total count of COVID-19 deaths much lower.

And even though Arizona is now seeing increases in the rate at which state residents are being found to be infected with the SARS-CoV-2 coronavirus that are comparable to what the states that have previously been epicenters of COVID-19 infections in the United States have experienced, we think it is unlikely that the state will experience a similar level of deaths attributable to COVID-19.

That's because of the age demographics of the state's confirmed coronavirus infections, which over the last month, have shifted considerably with younger residents becoming the most likely to become infected. The following chart shows that transition, where we're comparing the number of new infections confirmed by age group for the period from 7 May 2020 through 12 May 2020 with the data for 18 June 2020, which we selected because the number of cases for the Age 65+ population is very similar and because Arizona's Department of Health Services doesn't make time series data for its COVID-19 age demographics available, which meant we could only work with what little data we could get.

Changing Age Demographics for Arizona's Newly Confirmed COVID-19 Cases

As you can see, over the last month and half, the age demographic profile for Arizona's confirmed COVID-19 cases has shifted heavily toward younger residents, whom the Centers for Disease Control indicates are far less likely to die from the infection.

But why is Arizona experiencing such a large increase in the number of new coronavirus cases in the first place? To answer that question, we're going to a forward-calculation method and a backward-calculation method.

For the forward calculation method, we'll start with the established observation that half of those who test positive for COVID-19 will develop symptoms between 5 and 6 days after having been exposed to enough of the virus to become infected by it. Then, because coronavirus testing in Arizona is now taking anywhere from 7 to 10 days to indicate results, which if we use 6 days from exposure to infection as our base before considering the lag to report test results, it takes 13 to 16 days for test results to be reported after the initial exposure.

The backward calculation method is linked to the amount of time a patient who goes on to succumb to COVID-19 dies after they've developed symptoms. Here, an early study found the typical period before death might occur could range between 15 and 22 days, with a median period of 18.5 days after the patient's initial onset of symptoms. Combined with the median time from exposure to the onset of symptoms, that gives an estimated period from infectious exposure to death of anywhere from 20 to 28 days after they were initially exposed.

The next chart shows the rolling 7-day average number of newly confirmed cases and deaths per day in Arizona shows when these methods would predict a change in trend for both might occur following an exposure event. For our analysis, we've used the lifting of Arizona's statewide lockdown order on 15 May 2020 and the George Floyd protests that occurred in Phoenix and other cities in the state between 29 May 2020 and 3 June 2020 as our candidate exposure events.

Daily Newly Confirmed COVID-19 Cases and Deaths in Arizona, 3 March 2020 through 21 June 2020

Noting that the vertical scales on the two charts are very different from one another, we see that the initial upturn in Arizona's incidence of newly confirmed COVID-19 infections and deaths is consistent with the lifting of the state's lockdown order on 15 May 2020. What this outcome suggests is that Arizona had a large number of latent cases percolating within its population that, once businesses began to reopen and the stay-at-home order for residents was lifted, resulted in the resumption of the spread of COVID-19 infections in the state. Combined with the fact the age demographics of who was becoming infected shifted heavily toward younger residents during this time, the two observations suggest the activities in which these individuals engaged immediately following the lifting of the lockdown orders is responsible for the sudden rise in the number of cases in the state.

There's a second exposure event to consider. Over the six days from 29 May through 3 June 2020, numerous protest events took place in Phoenix and other cities around Arizona following the tragic death of George Floyd as he was being arrested in Minneapolis, Minnesota on 25 May 2020. Once again, crowds of predominantly younger people participated in these activities, which older Arizonans generally avoided, concerned with the risk of becoming infected with the coronavirus.

Here, we see the rate at which new COVID-19 infections were confirmed have accelerated following the forward calculation method's 13-16 day lag for a change in trend to appear after this multi-day event, as would be expected.

Meanwhile, we cannot yet confirm the effect with the trend for deaths because not enough time has yet passed for it to become evident with the longer lag of the backward calculation method, although here, because of the age demographics of who participated in the protests skewing so heavily toward younger Arizonans who are much less likely to die from COVID-19, we would anticipate a considerably smaller change in trend than what occurred for the numbers of newly confirmed cases.

This data also suggests that Arizona's statewide lockdown policy from 1 April 2020 through 14 May 2020 was less than successful for the state. During the lockdown, hospitals and other medical facilities throughout Arizona operated well below their capacity, providing little benefit for the great economic cost of the closure of businesses and the resulting loss of jobs. Had the state instead limited these kinds of restrictive policies to more local levels of government within the state, allowing residents to live more regular lives as local conditions warranted, it might have avoided the rush that became a new coronavirus wave when the lockdown was lifted.

But then, that's true for many states which had previously avoided significant cases or deaths from the coronavirus. The good news is that because they learned from the worse mistakes made in the states that realized both high rates of cases and deaths, they will be much more likely to avoid having a similar magnitude of deaths. Without deaths to accompany the number of cases, their coronavirus experience will be much more like a seasonal influenza event, albeit a bad one.

25 June 2020 update: A closer look at COVID-19 deaths in Arizona.


The COVID Tracking Project. Coronavirus numbers by state. [Online Data, Multiple Formats]. Accessed 22 June 2020.

U.S. Census Bureau. Annual Estimates of the Resident Population for the United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2019 (NST-EST2019-02). [Excel Spreadsheet]. Last updated 30 December 2019. Accessed 14 March 2020.

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