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COVID-19 data: What the numbers mean and how to tell if the coronavirus is spreading

Ryan Blethen, The Seattle Times on

Published in Health & Fitness

SEATTLE -- Months into the COVID-19 pandemic, the public is used to seeing a cavalcade of numbers and charts that show how the coronavirus that causes COVID-19 is spreading or how it's affecting a given place.

This data is crucial for informing decisions about how to respond to the crisis and keep ourselves and others safe.

But the more numbers floating around, the more potential for misinterpretation -- especially when epidemiological concepts such as R0 were unknown to most people before 2020.

We're here to help. Below, you'll find explanations of common virus stats and what they tell us -- individually and in combination with one another -- about the state of the pandemic.

The total cases

These are the data points you may be most accustomed to hearing about: the number of cases (people who have tested positive) and deaths confirmed each day, and the total cases that have occurred in your county or state.

 

Keep in mind that these numbers are influenced by how many people are being tested. If a place is reporting few cases but also is barely testing anyone, the low numbers could be due to a lack of testing and don't necessarily mean the virus isn't present. Some people who have the virus don't show symptoms, which makes it hard to get an accurate case count if a place is only testing people who feel sick -- as many parts of the United States were doing early in the pandemic because test kits were hard to come by.

Daily counts and cumulative totals are graphed separately. On a chart of total cases or deaths, look at how steeply the line is moving upward. The steeper the slope, the faster the total is increasing.

When charts show cases and deaths per day, look for a line that shows the overall trend. Each point on this line represents the average daily count from the previous 14 days. This average helps us more easily understand how things are trending over time, without our perception being muddled by one day here or there when the count was especially high or low.

The number of cases includes people who have died. Epidemiologists look at what percentage of infected people have died to see how lethal a disease is, but spotty testing has made this hard to do accurately with COVID-19.

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