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 Ngu offer insight on how to navigate the figures.
It’s the middle of the summer, and the coronavirus has not gone away.
When the pandemic first began, some had hoped that there’d be a lull during the summer, with the heat knocking the virus into submission, but it has continued its march across America, with outbreaks flaring across the southern and southwestern states. Arguments have also become part of the daily discourse, with people debating over case counts and death tolls, how the trends should be interpreted and whether the reported numbers can even be trusted.
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. At the same time, I see members of the public who are sincerely confused, trying to find a way through all the numbers and charts being thrown around, asking: “How concerned should I be right now? How bad are things, really?”
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.
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.”
(Graphics by Ash Ngu)
Deaths are also an important metric, because this is what most people care about when it comes to the virus’s ultimate impact, Gu said. “There are instances where you have clusters of infections but they’re mostly in young people, so you see a spike in cases, but not in deaths, because they’re all low-risk individuals — but if they were all to transmit it to their parents or older, high-risk individuals, or if the virus started going around long-term care homes, that’d be a high cause for concern.”
Computational epidemiologist Maimuna Majumder also recommends tracking deaths. Even though deaths lag behind new cases, typically by three weeks to a month, “it’s a good indicator for just how serious of a burden this pandemic is causing, not only on our health care system, but also on the general public’s mental health and well-being.”
Hospitalization data is another way to track the impact of the pandemic that has less of a lag than reported deaths. After the U.S. Department of Health and Human Services told hospitals this month to stop reporting data to the Centers for Disease Control and Prevention and report directly to HHS instead, some of these numbers have become more erratic
, according to The Covid Tracking Project
. Currently, all the states except for Hawaii and Kansas are reporting COVID-19 hospitalization data. Continue Reading…