It can be confusing to interpret all of the numbers people are using to describe the COVID-19 pandemic. Here are the three basic types of numbers officials are measuring and some examples:
- Counts. The number of known cases and number of deaths.
- Percentages (sometimes called proportions). Percentage of people with COVID-19 who are hospitalized.
- Rates. Technically, rates are percentages with a defined period of time. But certain measurements are often called “rates” even without a defined period of time, including incidence rate, case fatality rate, and mortality rate.
Each of these numbers helps us understand COVID-19’s impact on a community, state, or country in different ways.
Counts are most frequently reported during an ongoing epidemic. Two examples are: a) the number of known cases of COVID-19 in the United States (608,377 on April 14th), and b) the number of known deaths (25,922 on April 14th). Counts allow public health officials to understand how big the pandemic is and to plan for health care services like how many hospital beds will be needed. Since there are widespread reports of test shortages and difficulty accessing testing, there are probably more cases of COVID-19 than have been reported.
Percentages help us put counts into context by looking at the counts relation to a larger group of people. For example, 10 hospitalizations out of 100 COVID cases is 10% of the population, and it’s much different than 10 hospitalizations out of 1000 COVID cases, or 1% of the population.
Tests per capita (tests relative to the US population) is another important proportion. The US has tested more people than South Korea (1,760,000 versus 467,000 as of a few days ago). However, Korea is a smaller country, so the US has tested a smaller percentage of its population than South Korea (0.6% versus 0.9%).
Incidence rates (number of cases per 100,000 people) measure the number of new cases per population over the course of the epidemic. Iowa has 1,048 cases (33 cases per 100,000 population) compared to 3,335 cases in Virginia (39 cases per 100,000 population). Looking at case counts alone might give the idea that COVID-19 is much less common in Iowa than Virginia, but comparing the incidence rates shows the states are similar.
The case fatality rate describes the percentage of deaths out of the total number of known cases). California’s case fatality rate of 2.5%, means 2.5% of those diagnosed with COVID-19 have died. Case fatality rate is difficult to calculate during a pandemic because of the long period of time between when people get sick and when they die. So we have to wait until the pandemic is over to get the correct numbers. Furthermore, when the total number of known cases is underestimated (because of limited testing), the case fatality seems higher than it actually is.
Mortality rate (percentage of deaths out of the population of a community, state, our country, not just people confirmed to have COVID-19). California’s mortality rate is 1.1 COVID-19 deaths per 100,000 residents, or 0.001% of the state population.
Each of these numbers is important, but each provides different information, so it’s important to understand the definitions.
Science review by: JAB, ERS
Last update: April 14, 2020 at 11:05am ET