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Arizona's Plunging COVID-19 Caseloads and the Vaccines

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In two weeks since our previous update, Arizona’s number of COVID-19 cases, hospital admissions (including ICU bed usage), and deaths have been plummeting.

That good news prompted us to put together an animation showing the results of the back calculation method we’ve been using to identify the events that led to significant turning points in the trends for COVID-19 in Arizona. The animation cycles through the charts we’ve created using Arizona’s high quality data for the number of daily positive COVID-19 test results by date of sample collection, the number of daily COVID-19 hospitalizations by date of hospital admission, and the daily number of COVID-19 deaths according to death certificate data.

Animation: Arizona - Daily Number of Positive COVID-19 Test Results, New Hospital Admissions, and Deaths, 1 January 2020 - 17 February 2021

Full size versions of the animation and the three individual charts may be found by clicking the four links presented above.

The continued downward trend is exciting, where we were hoping to start seeing the effects of the COVID vaccines that began being administered in the state beginning in December 2020. As of 17 February 2021, over 1.33 million doses have been administered in Arizona, with 332,070 Arizonans having had both the first and second doses of the vaccines developed by Pfizer and Moderna. The following chart shows the daily progress in Arizona’s COVID-19 vaccination program:

Arizona: Number of COVID-19 Vaccine First and Second Doses Administered, 1 December 2020 - 17 February 2021

The inset of the chart presents the age distribution of the state’s vaccination program, which has initially prioritized first responders, health care workers, teachers, and the elderly. The population of Arizonans below Age 20 doesn’t include individuals who belong to these prioritized groups, which offers an opportunity for an interesting natural experiment.

Because Arizona’s Age 0-19 population has so far been excluded from the state’s COVID-19 vaccination program, we should be able to see effects of the vaccine in preventing coronavirus by comparing the incidence of COVID-19 in this age group with others. With over 528,000 Arizonas Age 65 or older having received at least one dose of the COVID-19 vaccines as of 17 February 2020, or the equivalent of about 40% of the population of this age demographic group in the state assuming that everyone in this group has had one dose of the vaccine, we thought it would be interesting to compare these two groups.

The next chart shows eight months worth of the rolling 7-day moving average for Arizona’s age demographic data indicating the numbers of those who have tested positive for COVID-19 by age group, from 17 June 2020 through 17 February 2021. This data is different from the data for positive COVID-19 test results we presented earlier, because the daily data used for this chart reflects the daily reported number of cases, which can span many days worth of sample collections. We will assume this data generally follows the similar patterns indicated by the more precise data that tracks the date of test sample collections.

Arizona: Rolling 7-Day Moving Average of Positive COVID-19 Test Results by Age Group, 17 June 2020 through 17 February 2021

Until recently, there have generally been more Age 0-19 individuals with confirmed COVID-19 cases than seniors (Age 65+), but with the exception of the ‘back to school’ surge in cases for the Age 0-19 population in the period from mid-August to late-September, the pattern for the incidence of cases for the two age groups has been generally similar.

Our next step was to calculate the simple ratio of the number of seniors (Age 65+) testing positive for COVID-19 to the number of Age 0 to 19 year olds testing positive in Arizona. In the next chart, that ratio is presented as a percentage, where values greater than 100% indicate more seniors than Age 0-19 year olds, and values less indiate more Age 0-19 individuals tested postive for COVID-19 than seniors.

Arizona: Rolling 7-Day Moving Average for Age 65+ Positive COVID-19 Cases as Percentage of Age 0-19, 17 June 2020 to 17 February 2021

This presentation makes it easy to see the back to school surge for the Age 0-19 year olds, where the number of individuals infected by the SARS-CoV-2 coronavirus caused the ratio to plunge in August-September 2020, before recovering then stabilizing in October 2020. After that, the ratio skewed in favor of seniors with the state’s second COVID-19 surge, which the back calculation method traces back to political events that occurred in the weeks ahead of the 3 November 2020 elections.

The ratio then went on to peak on 29 December 2020, just as Arizona’s COVID-19 vaccination program began providing inoculations to Arizonans Age 75 or older. The ratio then dips, but stabilizes at an elevated level into February, when the ratio spikes above the 100% threshold once again, as relatively more seniors than Age 0-19 year olds have been testing positive for COVID-19.

That’s a disappointing result for considering the effectiveness of the COVID-19 vaccines, because it means the decline in positive test results in unvaccinated Arizonas is falling faster than the decline in positive test results for seniors.

That observation suggests that getting past the 2020 holiday season and its increased levels of social mixing is primarily responsible for Arizona’s decline in COVID-19 cases, hospital admissions, and deaths.

There are other ways we might parse the data to see if we can tease out any effects from the vaccines on the trajectory of COVID-19 in Arizona, which we’ll look closer at in our next update. For now however, we cannot meaningfully attribute any part of Arizona’s plunging COVID-19 caseload to the introduction of the vaccines.

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. [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.

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 18 February 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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