What Arizona Shows About Vaccine Effectiveness

Posted: Mar 25, 2021 10:37 AM
What Arizona Shows About Vaccine Effectiveness

Source: AP Photo/Rogelio V. Solis

This is an opportune time to take a snapshot of Arizona's high quality COVID-19 data. Beginning on 24 March 2021, the state opened its COVID-19 vaccination program to all Arizonans Age 16 and older. Meanwhile, the state's COVID Data Dashboard has been updated with all available data up through 23 March 2021.

That means we have a nearly ideal setup for seeing how the expansion of the state's COVID vaccination program affects the trends for positive test results, deaths, new hospital admissions, and ICU bed usage for COVID-19. The charts below represent the "before" snapshot of those trends, just before the expansion of Arizona's vaccination program. Please click on the charts to access their full-size versions.

For all charts, we see that the rates of incidence for new COVID cases, deaths, hospital admissions, and ICU bed usage have continued trending downward since our last update two weeks ago. The data is also providing an indication of the effectiveness of the COVID-19 vaccines.

That effectiveness is primarily seen in the much faster rate of decline of deaths from COVID since Arizona's vaccinations of elderly residents began in late December 2020, which began falling faster than the rate that was seen following the peak of Arizona's first wave of deaths attributed to COVID-19 earlier in 2020 in the expected lag period of 17-21 days. In the chart that earlier rate of decline is shown by the heavy black lines. We see the rate of COVID-19 deaths after their effectiveness buildup period falls much more steeply than was seen in the summer of 2020, when no vaccines existed.

That's the most pronounced change, although the data for new hospital admissions and ICU bed usage show similar, but smaller improvements in their trends. The data for positive COVID-19 test results however shows the least difference, which we would not consider significant.

We wondered why we would see that pattern, and found a potential answer in a pre-print medical paper from Public Health England (PHE). PHE finds the vaccines are highly effective in reducing severe cases of COVID-19 in older adults, which is the portion of the population most at risk of death from SARS-CoV-2 coronavirus infections.

Arizona's data would seem to be bearing that out. In reducing the severity of COVID-19 among those who have been vaccinated, the result is reduced deaths and hospitalizations, but not necessarily reduced cases. The vaccines would appear successful in increasing the survivability of those who have been vaccinated.

That brings us to our final chart, which we've taken from Arizona's COVID-19 Data Dashboard for vaccine prioritization. It shows the progress the state has made in vaccinating its high priority groups, in addition to its progress in vaccinating Arizonans by age group:

Targeting the vaccination program to the elderly population has proven successful in improving the trends for COVID-19 in Arizona, especially for deaths.

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 24 March 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]. Updated 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.