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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 March 26, 2021.
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 March 26, 2021
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. Updates can be accessed at https://publichealth.arizona.edu/news/2021/covid-19-forecast-model.
For the week ending March 21st, 3893 Covid-19 cases were diagnosed in Arizona (Figure 1). This represents a 12% decrease from last week’s initial tally of 4445 cases and marks the tenth straight week of decline. The prior week’s tally was upwardly revised by 1% (39 cases) to 4484 cases this week. The outbreak remains evenly distributed by age (Figure 2).
Figure 1. Newly Diagnosed Covid-19 Cases in Arizona and Number of Individuals Undergoing Covid-19 Diagnostic Testing March 1, 2020 through March 21, 2021.
Arizona is transitioning from a period of substantial risk to one of moderate risk. While continuing small improvements are possible, it is becoming more likely that improvements will stall or reverse owing to more transmissible variants (e.g., B.1.1.7) and/or further normalization of business and social activities (e.g., EO 2021- 06). Hospital capacity remains adequate to meet Arizona’s needs; however, the backlog of non-Covid care has yet to be fully addressed as evidenced by unseasonably high hospital occupancy.
While residents and businesses should continue to follow public health recommendations, normalization of low-risk activities is reasonable. New cases are now being diagnosed at a rate of 53 per 100K residents per week; and, this rate is declining by approximately 8 cases per 100K residents per week. For reference, September 8th marked the fall nadir between the summer and winter outbreaks at 38 per 100K per week. Unvaccinated Arizonans who are at risk of developing severe disease (e.g., age or comorbid conditions) or who simply wish to be vaccinated should remain sheltered as much as feasible as viral activity remains widespread.
Note: Data for this report was updated Friday, March 26 allowing 4 full days to adjudicate cases and keep week-over-week backfill <10%. This allows more interpretable comparisons and graphics. All comparisons are week-over-week changes. No update is planned for April 2nd, instead updates will now occur every other week.
Figure 2. Newly Diagnosed Covid-19 Cases in Arizona by Age Group March 1, 2020 through March 21, 2021.
Test positivity among those undergoing traditional nasopharyngeal PCR testing was essentially unchanged this week, at 8% the week ending March 21st (Figure 3). Positivity is now within the recommended 5 – 10% for optimal public health practice.
Figure 3. Weekly Number Patients Undergoing Traditional Nasopharyngeal PCR Testing and Associated Percent Positivity March 1, 2020 – March 21, 2021.
As of March 26th, 581 (7%) of Arizona’s 8591 general ward beds were occupied by Covid-19 patients, a 17% decrease from the previous week’s 700 occupied beds (Figure 4 and Figure 5 Panel A). Another 1049 (12%) beds remained available for use. The number of available beds is higher than the previous week’s 978 beds.
Covid-19 occupancy has dropped by 89% from its January 11th peak of 5082 ward patients. The summer-fall nadir was 468 occupied beds on September 27th.
Figure 4. Arizona Daily Covid-19 General Ward and ICU Census April 20, 2020 – March 26, 2021.
As of March 26th, 181 (11%) of Arizona’s 1734 ICU beds were occupied with Covid-19 patients, a 0% decrease from the prior week’s count of 182 patients (Figure 4 and Figure 5 Panel B). An additional 252 (15%)
ICU beds remained available for use. This is similar to the prior week’s 259 available beds. ICU occupancy has fallen 85% from its January 11th peak of 1183 occupied beds. The summer-fall nadir was 114 occupied beds on September 22nd.
Figure 5. Covid-19 Occupancy as a Percent of Listed General Ward (A, left) and ICU (B, right) Capacity in Arizona April 20, 2020 – March 26, 2021.
Arizona hospital occupancy remains above seasonal trends. Improvements in ward and ICU occupancy remain stalled at ≥85% occupancy (Figure 6). This indicates that medically necessary procedures that were previously postponed are being scheduled at higher than seasonal amounts to address the backlog of care. It will still take several more months to resolve. Occupancy will need to fall <70-75% before conditions will be back to “normal.” As capacity constraints are lessened, care practices should return to those prior to the outbreak ensuring all patients will receive optimal care. Hospitals will remain crowded through April before returning to pre-outbreak levels assuming continued reductions in viral transmission.
Figure 6. Observed Excess Non-Surge General Ward and ICU Capacity April 20, 2020 – March 26, 2021.
The week ending January 17th remains Arizona’s deadliest with 1065 deaths (Figure 7). Covid-19 deaths are expected to fall below 200 per week by the week ending March 14th or 21st.
Figure 7. Weekly Arizona Covid-19 Deaths March 1, 2020 – March 21, 2021.
Pima County Outlook
For the week ending March 21st, 462 Pima County residents were diagnosed with Covid-19, a 16% decrease from the 548 cases initially reported last week (Figure 8). Last week’s initial tally was upwardly revised by <1% (4 cases) to 552 cases.
Figure 8. Covid-19 Cases and Individuals Undergoing Diagnostic Testing in Pima County March 1, 2020 – March 21, 2021
New cases are being diagnosed at a rate of 44 cases per 100K residents per week and are declining by approximately 9 cases per 100K residents per week. For reference, October 9th marked a nadir between the summer and winter outbreak at 46 cases per 100K residents per week. Trends are similar across the various age groups (Figure 9).
Figure 9. Covid-19 Cases by Age Group in Pima County from March 1, 2020 – March 21, 2021.
- This week saw a tenth straight week of meaningful declines in Covid-19 cases and hospital occupancy. Absolute levels of SARS-CoV-2 viral transmission are below 50 new cases per 100,000 residents per week threshold in many Arizona counties.
- As of March 21st, new cases were being diagnosed at a rate of 53 cases per 100,000 residents per week (Figure 10 below). This rate is declining by 8 cases per 100,000 residents per week.
- While Arizona has one of the lowest viral transmission rates in the US according to the CDC, it remains the 6th hardest hit state overall.
- All residents should continue to wear a mask in public, avoid large social gatherings, maintain physical distance from non-household contacts, avoid >15 minutes contact in indoor spaces, especially if physical distancing is inadequate and adherence to face masks is low.
- While residents and businesses should continue to follow the recommended public health mitigation efforts, normalization of lower risk activities is reasonable throughout the state.
- The test positivity rate for traditional nasopharyngeal PCR testing is now <10% which is within the recommended 5 – 10% range for optimal public health practice.
Figure 10. 7-Day Moving Average of Arizona Covid-19 Cases by Date of Test Collection Mar 1, 2020 – March 21, 2021.
- In all communities, absolute levels of transmission and test positivity have reached levels that warrant a return to in-person instruction using a hybrid mode or reduced attendance (see CDC Recommended Mitigation Strategies for K – 12 Learning Modes). In many communities, transmission has fallen below 50 cases per 100K residents per week which marks the threshold for in-person instruction by the CDC.
- Hospital Covid-19 occupancy continues to decline in the ward and ICU. Access to care however, remains somewhat restricted as the backlog of medically necessary non-Covid procedures is addressed.
- Arizona is still reporting a large number of weekly deaths and this count may underestimate true fatalities. The week ending January 17th will be Arizona’s deadliest with >1000 deaths. Arizona’s weekly tally of deaths ranks it 6th in the nation while its overall rank remains 6th since the outbreak began.
- According to the CDC, 20.4% of Arizona adults have received at least 2-doses of vaccine while another 14.5% have received 1-dose. The ADHS Dashboard is reporting slightly higher levels.
Forecast reports to date, available as PDFs
Download PDF to view additional charts of Arizona counties, available in appendix of report.
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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