This article is written by Jennifer M. Miller, PhD, Independent Scholar and Apolitical Insider Fellow. This article has been edited by Miguel A. Rozo, Apolitical Insider Fellow and Policy Advisor at Global Affairs Canada, on democracy and elections

Does more coronavirus testing mean more cases of coronavirus? Taken at face value, this statement is both obviously false and obviously true.

How can that be?

It is obviously false, because people will contract the coronavirus whether or not testing is done. Yet, when it comes to the various charts, graphs, and dashboards that public managers use to evaluate our progress, it is true that increases in reported cases often result from increased testing. This effect was especially visible in the early days of the pandemic, when we first overcame shortages in testing kits.

Case counts met with scepticism

Given that the number of new cases reported each day depends so much on how many tests are performed, there is the possibility that governments who are trying to demonstrate success in fighting the pandemic will suppress the number of cases reported by reducing the number of tests performed.

People are aware of this possibility, and thus somewhat sceptical of daily case counts. The stage was set for scepticism when, with the world’s eyes on the fate of cruise ships, US president Donald Trump opposed bringing the Grand Princess back to shore because of the effect on the nation’s case count.

In a recent discussion of US state government responses to the pandemic, someone asked “Why do we keep reporting cases, when we know this measure can be unreliable or even manipulated? Why don’t we rely more on reports of hospitalisations, ICU admissions, and deaths?”

To understand the importance of reporting new cases, we need to consider how new cases serve as a leading indicator of the things that we actually want to measure.

A leading indicator is something we can see before the phenomenon with which we are most concerned. In the case of Covid-19, perhaps we are most concerned with counting persons suffering from severe illness, since these are the patients who could potentially overwhelm medical facilities. Here again, new cases may not seem like the best indicator to track. After all, most cases of coronavirus are not severe and governments do not routinely track every cold and flu.

Despite these concerns, there are two features of the coronavirus pandemic that make it critically important to report new cases. Those features are exponential growth and disease progression.

Exponential growth and disease progression

First, because of how the coronavirus spreads exponentially, infections can increase rapidly from a small number of new cases to a large number of new cases. A local outbreak can go from a manageable number to an unmanageable crisis over the course of just a few days.

In New York City, cases doubled from 1,035 to 2,123 from March 15 to 16. This doubling set the stage for daily cases to top 6,000 per day at the city’s peak in late March and early April.

To interpret cases as a leading indicator of Covid-19, it is important to understand the level of testing in the community

Second, because of how Covid-19 progresses in infected patients, new cases can be valuable as a leading indicator. We can think of Covid-19 as having three stages: pre-symptomatic, symptomatic, and recovery. For some people infected with the coronavirus, the symptomatic stage becomes serious. Some of those infected require hospitalisation and even intensive care. The most serious cases result in death.

Case counts as a leading indicator

In the UK, new cases reached a peak on 5 April at 6,199, only surpassed in the report on 1 May, which was based on significantly expanded testing. The number of hospitalised patients due to Covid-19 reached a peak on April 12th, then began to decrease. The number of patients on mechanical ventilation beds also peaked on April 12th, but did not start to decrease until about a week later.  The highest number of daily deaths was recorded on April 21st.

The pattern of these dates shows how the number of cases serves as a leading indicator to help officials anticipate the demand severe illness will place on hospitals. The high case counts in early April signalled the devastating health crisis that strained health systems later that month.

The time to stop the virus is when case counts begin to climb. If we wait until hospitalisation numbers climb to take action, we will have lost important days to stop the virus’s exponential growth

To interpret cases as a leading indicator of Covid-19, it is important to understand the level of testing in the community. New cases will be most effective as a leading indicator if levels of testing are relatively consistent. Also, the pattern of dates seen for the UK is not seen in every community. Especially during early days of the outbreak, testing was focused on hospitalised patients.

If patients are only tested in hospitals, reported cases and hospitalisations will increase at the same time. For case counts to serve effectively as a leading indicator, testing must remain widely and consistently available to all of those potentially exposed to the virus.

Lessons for public managers

Public managers should advocate for reporting coronavirus case counts as a leading indicator and consistent, widespread testing.  Case reporting will help them anticipate the impact of the disease in their communities and implement appropriate policy responses. The time to stop the virus is when case counts begin to climb. If we wait until hospitalisation numbers climb to take action, we will have lost important days to stop the virus’s exponential growth. — Jennifer M. Miller

(Picture credit: Unsplash)

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