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COVID-19 Forecast Model, Arizona and Pima County

Researcher Analyzes Arizona COVID-19 Spread Models for Decision-Makers

The following information regarding the spread of COVID-19 in Arizona was prepared by Joe Gerald, MD, PhD, a researcher at the Mel and Enid Zuckerman College of Public Health (MEZCOPH) at the University of Arizona. This information has also been reviewed by other MEZCOPH faculty.

This data were obtained from the Arizona Department of Health Services COVID-19 webpage and reflect conditions in Arizona as of May 22, 2020.

This information is intended to help guide our response to the outbreak. It is not intended to predict how this pandemic will evolve. Rather, this model extrapolates what might occur if current conditions remain unchanged. As regional authorities and healthcare providers respond, their actions are expected to mitigate the worst consequences of this pandemic.




COVID-19 Disease Outbreak Forecast
Arizona State and Pima County

Updated May 22, 2020


Disclaimer: This information represents my personal views and not those of The University of Arizona, the Zuckerman College of Public Health, or any other government entity. Any opinions, forecasts, or recommendations should be considered in conjunction with other corroborating and conflicting data.

COVID-19 Disease Outbreak Forecast For the week ending Sunday, May 17, weekly case counts aggregated by test collection date show the number of newly diagnosed cases in Arizona declined from the prior week (Figure 1). While a welcomed signal of improvement, increased testing makes it difficult to judge how the decline in case counts correlates with underlying trends in viral transmission. These data also precede any potential impact from the May 15 expiration of Arizona’s “stay at-home” order.

For the week ending May 17, 40757 individuals provided 49365 PCR samples of which 5.4% were positive (Figure 2, Panel A). In addition, 22692 individuals provided 31615 serology samples of which 3.1% were positive (Figure 2, Panel B). The serology results likely represent an upper bound of state-wide prevalence.

COVID-19 Disease Outbreak Forecast

As of May 22, 775 deaths have been announced in Arizona; however, reporting lag makes it difficult to interpret these counts in real-time. For example, ADHS announced 144 new deaths during the week ending May 17, but only 55 of these deaths occurred during this period (Figure 3). The week ending May 10 is the first to clearly show the impact of ADHS’s new practice of using case tracking and death certificate data to identify total deaths. Even with a 1 – 3 week lag in reporting and verification, deaths appear to be slowly increasing with the largest number of deaths (124) occurring the week ending May 10.

COVID-19 Disease Outbreak Forecast

The Centers for Disease Control and Prevention (CDC) is now aggregating various model projections to provide a consensus view of the trajectory of new COVID-19 deaths nationally and across the 50 states (Figure 4). These models predict that cumulative deaths will continue to increase at roughly the same trajectory for the next 6 weeks. A non-governmental source for similar model aggregation can be found at FiveThirtyEight. Comparatively, Arizona falls somewhere in the middle of the pack with regard to the number and trajectory of COVID-19 deaths as tabulated by the New York Times (Figure 6 following page).

COVID-19 Disease Outbreak Forecast

COVID-19 Disease Outbreak Forecast

COVID-19 Disease Outbreak Forecast
As of May 22, 784 (7.7%) of Arizona’s 10247 general ward beds were occupied by patients with suspected or confirmed COVID-19 infection; 1,734 (16.9%) additional beds were available for use. With regard to ICU beds, 309 (13.5%) of 2285 ICU beds were occupied by patients with suspected or confirmed COVID-19 infections; 364 (15.9%) additional beds were available for use. According to the ADHS dashboard, excess capacity in general ward beds has declined from 32% to 17% since March 26. Over the same period, excess capacity in ICU beds has declined from 38% to 16%.

While total COVID-19 hospitalization has slowly increased from April 20 – May 22, the total number of ward and ICU beds devoted to COVID-19 care decreased from 1133 beds last week to 1093 beds this week (Figure 7). Unfortunately, excess capacity as indicated above has been simultaneously declining mostly due to greater non-COVID use.

Pima County Outlook
COVID-19 Disease Outbreak Forecast
For the week ending Sunday, May 17, weekly case counts aggregated by test collection date show a decline the number of newly diagnosed cases in Pima County from the prior week (Figure 8). While changes in test capacity making it challenging to understand the true pace of viral spread, there is greater evidence that newly diagnosed cases are declining in Pima County since the week ending April 19.


Summary,

  • Now that social distancing restrictions have been lifted, a greater frequency and intensity of human interactions will tend to facilitate viral spread. If this scenario is realized, increases in newly reported cases, hospitalizations, and ICU utilization would be detectable by early June.
    • Absolute levels of community-driven viral transmission remain high as evidenced by substantial numbers of newly reported cases.
    • For many locales, social distancing restrictions are likely needed to prevent reported cases, hospitalizations, and ICU utilization from increasing.
    • Hospital utilization has slowly increased over the past 4 – 6 weeks for COVID and non-COVID related care with excess capacity declining from approximately 30% to 15%. While adequate capacity exists to care for some increase in severely ill patients, additional viral spread will narrow that safety margin.
  • COVID-19 testing capacity (PCR and serology) has meaningfully increased over the past several weeks; however, the PCR test positive rate remains above 3% indicating capacity is not adequate to meet clinical and public health demands. This 3% target reflects testing practices in countries that had a more robust public health response and were more effective in controlling viral spread.
  • Several important changes have occurred in the past week that still make it difficult to assess the underlying trends in viral transmission. First, testing capacity has expanded identifying milder disease that would have previously remained undiagnosed. Second, ADHS added COVID-relevant causes of death from death certificates to the case definition of COVID deaths. Lastly, Arizona’s “stay-at-home” order expired May 15 likely increasing social interactions with the potential to transmit infection.

See Appendix for COVID-19 case counts by county.

Next update scheduled for May 22.


Forecast reports to date, available as PDFs

March   17   |   22   |   28

April   8   |   13   |   23   |   28

May   1   |   8   |   15   |   22


Appendix

Content created by: Joe K. Gerald, MD, PhD (Associate Professor, Zuckerman College of Public Health, geraldj@email.arizona.edu) with gratitude to Patrick Wightman, PhD, MPP from the UA Center for Population Health Sciences for assistance with data analysis.




Based on what we know now about this pandemic, we support guidelines for social distancing to slow the spread of the virus and urge everyone to follow the recommendations provided by the Centers for Disease Control and Prevention (CDC) to protect yourself, your family, your neighbors, and your employees. Please heed the recommendations as provided by the CDC, found at the following website: https://www.cdc.gov/coronavirus/2019-ncov/prepare/prevention.html

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