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22 September 2020

What effect does going back to school have on the spread of COVID-19 coronavirus infections?

The possible answers to that question have greatly concerned many parents and policymakers around the world. To find out the possible effect, we've turned to data from the state of Arizona where a combination of demographic data from the state's Department of Health Services and its three major public universities provides a window into seeing what that effect might be.

Arizona's universities started their Fall 2020 sessions by delivering course content online in August, but began providing either hybrid or traditional classroom instruction at their campuses in late-August or early September. Since we're mainly interested in how returning to the classroom might affect the spread of COVID-19 among the student age population, we looked at the total number of positive coronavirus tests reported for the Age 0 to 44 population across the state and just by Arizona State University (ASU), the University of Arizona (UA), and Northern Arizona University (NAU) at two points in time: 3 September 2020 and 18 September 2020.

We've presented the results of that exercise in the following chart showing the change in the number of coronavirus cases in the between these two points in time, where we find a mixed picture. Please click here to access a full-size version of the chart. [Update 26 September 2020: We identified an error in our original presentation that undercounted the number of cases in the Age 0-19 portion of the population. The original chart we presented is here, the following chart has been corrected. corrections in our analysis below are indicated with boldface font.]

Corrected - Back to School In Arizona: Change in Number of Reported COVID-19 Positive Test Results for Age 0-44 Age Group Between 3 September 2020 and 18 September 2020

Between 3 September 2020 and 18 September 2020, the total number of positive test results reported by Arizona's Department of Health Services increased by 6,568. Of this figure, 37% of the reported increase in cases originated at Arizona's three major public universities. The other 63% represents the total increase in cases reported in the state for its entire Age 0 to 44 population.

Here is the breakdown for the three public universities:

  • Arizona State University: 627 cases (10%)
  • Univerisity of Arizona: 1,554 cases (24%)
  • Northern Arizona University: 273 cases (4%)

The University of Arizona's high case count stands out because it is utilizing rapid antigen tests, which differ from the established COVID-19 tests performed at the other universities and at testing sites across the state. After long excluding results from these tests in its daily reported case counts, Arizona's DHS began including results from antigen tests in its statewide tallies on 17 September 2020. UA's antigen test results have had issues with false positives.

The reported data is limited because it is silent about where an individual with a positive COVID-19 test result may have been exposed to the SARS-CoV-2 coronavirus. For example, a student's exposure to the viral infection may have taken place in a classroom, a campus facility, a dorm, or even off campus. We should also note that the positive COVID-19 results for university students, faculty and staff members may also include individuals older than Age 44.

That's why breaking out data for a university's faculty and staff may be valuable, since the incidence of cases would come primarily through contact in classroom and on-campus facilities. Here, data from ASU indicates that students account for 99% of the reported cases, while faculty and staff account for just 1% of the new cases reported in our period of interest. Data going back to 1 August 2020 indicates 2% of all ASU's reported cases have been among faculty and staff members.

All these institutions are operating with classes set up to minimize the potential spread of coronavirus infections. The exceptionally low number of new positive test results among faculty and staff suggest those approaches are effective at protecting the health of older individuals who are much more at risk of health complications from COVID-19 than the student-age population, who make up the vast majority of infections on campus.

For all the testing the universities are doing, two of the three are reporting comparatively low test positivity rates, consistent with levels indicating the spread of coronavirus infections is manageable. Both ASU and NAU report their cases are below a 5% threshold.

By contrast, UA reports a 15.5% rate from its antigen tests, prompting the university to tell students on 14 September 2020 to "shelter-in-place" for two weeks. The action is expected to bring the spread of infections back down to manageable levels.

Meanwhile, falling rates of COVID-19 hospitalizations are continuing to be reported for the state. The Age 0-44 demographic is also the least likely to experience health complications from the viral infection requiring admission to hospitals, which may account for why these numbers have not been rising.

Perhaps the most significant factor behind the pattern we see in the incidence of COVID-19 infections at Arizona's major college campuses is whether or not its local community has already had significant numbers of cases. Arizona became a hotbed for infections during June 2020 and peaked in July 2020, with over 60% of its reported concentrated in the Phoenix metropolitan area (where ASU is located). Meanwhile, the Tucson metro area (the home of UA) had a moderate number of cases and greater Flagstaff (home to NAU) had has relatively low numbers.

As we've previously observed, COVID is very much a geographic phenomenon, tending to spread most where it hasn't previously been in great numbers, where local herd immunity hasn't developed. We suspect that dynamic lies behind the high number of cases at UA in Tucson now being recorded, and we fear NAU in Flagstaff may have a surge in cases in its future.

The patterns we've described for Arizona would seem to have direct bearing on the "going back to school" season for college students in other states. Dave Tufte describes what he's observing with a marked surge in COVID-19 cases now taking place in Utah. Looking over the state's data, we think the sharp increase in number of new coronavirus infections in the state may be tied to an initial exposure event coinciding with the late start of classes at Brigham Young University, which was then amplified and spread to students' home towns during the Labor Day holiday weekend a week later. Like Arizona's UA outbreak, it seems to be spreading in areas that haven't previously seen high levels of infections.

That still leaves us with one big question needing more information to be answered. Do high numbers of cases among college students take place in places that have already experienced high epidemic numbers? Arizona's data hints the answer is no, but the sample size of universities running in-person classes across the country is still pretty small.


We've used contemporary news reports to compile the COVID-19 data for the universities, and Arizona's DHS COVID-data dashboard for the state's overall figures for the Age 0-44 population.

Arizona Department of Health Services. COVID-19 Data Dashboard. [Online Application/Database]. Accessed 19 September 2020.

Eltohamy, Farah. University of Arizona reports new daily high for positive COVID-19 tests. AZCentral. [Online Article]. 3 September 2020.

Hansen, Piper J and Myscow, Wyatt. There are 983 positive coronavirus cases within the ASU community. State Press. [Online Article]. 3 September 2020.

Ackley, Madeline. ASU has 112 new COVID-19 cases in past 3 days, whil UA has 678. AZCentral. [Online Article]. 18 September 2020.

Northern Arizona University. Coronavirus updates and resources. [Online Article]. Accessed 18 September 2020.

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