This is a short post on the critical distinction between the Case Fatality Rate (CFR) of a disease such as the SARS-COV-2 coronavirus thought to cause COVID-19 and the Infection Fatality Rate (IFR), also sometimes known as the actual mortality rate or lethality. This remains a source of confusion and perhaps deliberate obfuscation several months into the crisis.
The case fatality rate or CFR is the number of deaths attributed to the disease usually among those diagnosed with the disease divided by the number of diagnosed cases according to some diagnostic criterion, for example a “positive” RT-PCR (Reverse Transcriptase-Polymerase Chain Reaction) test.
The infection fatality rate or IFR is the number of deaths attributed to the disease divided by the actual number of people infected, which generally includes mild or asymptomatic infections that are not diagnosed. Most diseases have many mild or asymptomatic infections. This is not unusual and is the case for the coronavirus SARS-COV-2.
The CFR generally reflects those with more serious infections who seek medical attention, go to a hospital emergency room, etc. It is generally biased, usually higher than the IFR for most diseases, and also can vary a lot depending on the availability of tests and on other causes unrelated to the genuine lethality of the disease.
A disease that kills everyone who exhibits symptoms and no one who has no symptoms even though actually infected can have a case fatality rate (CFR) of 100 percent and and an infection fatality rate (IFR) of nearly 0.0 percent.
For example, an exotic disease that produces distinctive green and purple spots in those it kills — easily identifiable even without advanced tests like RT-PCR — but in fact kills only 100 people out of a United States population of 330 million even though most are infected for some reason.
Although there are a number of subtleties in the definition and computation of these numbers that I have omitted for clarity, the infection fatality rate (IFR), ideally broken down by age, medical conditions, and other risk factors, is key to evaluating the proper public health response to an outbreak of an infectious disease. Not the case fatality rate or CFR.
(C) 2020 by John F. McGowan, Ph.D.
About Me
John F. McGowan, Ph.D. solves problems using mathematics and mathematical software, including developing gesture recognition for touch devices, video compression and speech recognition technologies. He has extensive experience developing software in C, C++, MATLAB, Python, Visual Basic and many other programming languages. He has been a Visiting Scholar at HP Labs developing computer vision algorithms and software for mobile devices. He has worked as a contractor at NASA Ames Research Center involved in the research and development of image and video processing algorithms and technology. He has published articles on the origin and evolution of life, the exploration of Mars (anticipating the discovery of methane on Mars), and cheap access to space. He has a Ph.D. in physics from the University of Illinois at Urbana-Champaign and a B.S. in physics from the California Institute of Technology (Caltech).
