How to Understand COVID-19 Numbers

How to Understand COVID-19 Numbers Viewed in isolation or presented without context, coronavirus numbers don’t always give an accurate picture of how the pandemic is being handled. Here, ProPublica journalists Caroline Chen and Ash… ProPublica

I’ve watched so many reporters, both at ProPublica and at other outlets, do their best to debunk myths, demystify confusing trends and answer the public’s questions. It is, frankly, exhausting, especially when the same arguments keep coming up again and again. I’m also concerned to see not just laypeople, but local and national leaders, using data out of context to justify their own narratives....So I wanted to step back and, with my colleague Ash Ngu, walk you through some common coronavirus metrics and explain how to interpret them. I hope this will leave you better equipped to assess claims with appropriate skepticism, filter out the garbage and find the real signal amid the noise.

Case Counts Won’t Give You the Full Picture

The first thing I asked experts was: What metric would you recommend I track if I wanted to understand what was going on in my state?

Both Matthew Fox, professor of epidemiology and global health at Boston University, and Youyang Gu, a data scientist best known for his COVID-19 prediction models, advised looking at three measurements together: number of cases, case positivity rates and number of deaths.

“Cases going up or down tells you a fair bit about what’s going on at the moment in terms of transmission of the virus — but it’s only valid if we’re testing enough people,” Fox said.

When there aren’t enough tests available, as was the case in New York in March, the number of cases reported will be an undercount, perhaps by a lot. That’s where case positivity rates come in: that measures the percentage of total tests conducted that are coming back positive. It helps you get a sense of how much testing is being done overall in a region.

“WHO guidelines say we want that to be below 5%,” Fox noted. When a positivity rate is higher, epidemiologists start worrying that means only sicker people have access to tests and a city or region is missing mild or asymptomatic cases. When almost all of the tests come back negative, on the other hand, it’s a good indicator that a locality has enough tests available for everyone who wants one, and public health officials have an accurate picture of all the infections, Fox said.

He gave the example of Massachusetts, where he lives. Currently, daily positive case counts have been steadily falling for the past three months. “The positivity rate is now below 2%, so I feel confident in saying that we know what’s going on, and it’s not that we’re not doing enough testing and we’re missing a lot of positive cases.”

On the flip side, any state where the positivity rate is higher than 10% is “really going to worry me,” Fox said. “That tells me that we’re probably missing a fair number of cases, and you’re not doing enough testing to see what’s going on.”

Fox noted that some states in the Sun Belt, such as Arizona and Florida, have recently had very high positivity rates, even above 20%. “That means we don’t have full visibility.”