COVID By The Numbers: Explaining the rise in COVID-19 Cases in Arizona

Posted at 6:25 PM, Sep 17, 2020
and last updated 2020-09-17 22:19:50-04

PHOENIX — On Thursday, the Arizona Department of Health Services added 1,753 COVID-19 cases to the total, the most since August 1. This comes from a reported 23,844 diagnostic tests, the most ever reported in a single day since the beginning of the pandemic. This begs the question, do these added case numbers translate to a growing infection rate in the state, and should we be concerned?

The short answer: Probably not. At least for the moment.

Arizona is experiencing a rise in weekly testing that is exceeding weeks in early August, around the time people were having difficulty obtaining tests in a timely manner and community testing wait times were staggering. The initial rise came on the week of August 30. Increased numbers were attributable to testing being done at the state’s three major universities as students attending class on campus were required to obtain a diagnostic test.

As of this writing, the week of 9/13 has only had four reporting days and has already exceeded that of August 30. The reason behind this is essentially twofold: The inclusion of antigen tests and Arizona State University.

On Wednesday September 16, the Arizona Department of Health Services began adding antigen tests to the reported numbers. Antigen tests are COVID-19 diagnostic tests that look for the proteins generated by the Sars-Cov-2 virus. This is opposed to PCR tests that look for actual traces of the virus itself. The newly included tests coincide with a change made to the state department’s COVID-19 case definition manual that automatically counts positive antigen tests as “probable” COVID-19. Prior to this change, probable positives came from a small subset of antibody tests that provided evidence of a recent infection.

A Health Department spokesman told ABC 15 that a majority of the 15,956 tests added Wednesday came from Arizona State University, beginning the process of submitting antigen testing to the state’s electronic reporting system.

Today’s even more pronounced test numbers came not only from more antigen tests being added, but also a large cache of previously unreported negative results that came from the university’s public and private testing sites. Unlike the antigen tests from Wednesday, these included tests from all over the county, according to an ASU spokesperson.

Weekly Number of Tests Reported in ArizonaI

The most important aspect of these numbers from ASU is that a large portion of positive cases reported today are not recent. As many as 45% of Thursday’s reported positive tests are from dates prior to September 5, with most lying outside of the COVID-19 infectious period of around 14 days. Testing data was even more pronounced, with 67% of reported tests today coming from the same period.



In the end, what this means for Arizona is that while we may be seeing larger than normal COVID-19 numbers for the next few days, this does not mean that COVID-19 infections are picking up again. To determine this, we would need to see sustained growth of cases from the last 7-14 days, which has yet to materialize.