What is Case Fatality Rate (CFR)?

Case Fatality Rate (CFR) measures the severity of a particular disease by defining the total number of deaths as a proportion of reported cases of a specific disease at a specific time.

Case Fatality Rate

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How is CFR Determined?

CFR is presented as a percentage (0%-100%) or a ratio (between 0-1) and measures the number of confirmed deaths among the number of confirmed diagnosed cases of a particular disease at a given time.

This is different from general mortality rates which are the number of general deaths that occur within a population (normalized to the population) at any given time.

For example, if 15 people die from seasonal flu (in that particular winter) out of 1000 confirmed diagnoses, then the CFR would be 1.5% or 0.015. This is not the percentage of deaths from seasonal flu in the whole population, as that would be far lower.

However, CFR (as with most epidemiological statistics) heavily relies on confirmed cases and deaths and may not accurately reflect the total picture within the population due to unreported cases and subsequent deaths.

CFR can vary considerably for the same disease across different cities and countries. Much of this can depend on the population's characteristics – such as average age, as well as access to robust and free healthcare systems, levels of previous immunity (e.g. vaccinations), and treatment strategies.

The infection fatality rate (IFR) refers to the proportion of people being infected by a disease-causing agent (in the case of COVID-19, the SARS-CoV-2 virus), including asymptomatic and undiagnosed infections (such as in care homes), who die from the disease; and is much lower than the CFR. The IFR is often very difficult to establish due to the variable estimates of asymptomatic and undiagnosed infections within a population.

CFR of Common Diseases

Specific CFRs of particular diseases if left untreated or unvaccinated for include:

  • Bubonic Plague – 60%
  • Ebola – 90%
  • Naegleriasis – 98-99%
  • Seasonal Influenza – 0.1%
  • HIV/AIDS – 80%
  • Dengue Fever – 26%
  • Malaria – ~0.3%
  • Typhoid – 10-20%
  • SARS – 9-11%
  • MERS – 34.4%
  • COVID-19 – ~2.1%* (variable estimates due to ongoing pandemic – see below)

CFR and COVID-19

SARS-CoV-2 Virus

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The coronavirus disease-2019 (COVID-19) pandemic caused by the SARS-CoV-2 virus (first identified in Wuhan, China) brought the world to a standstill with national lockdowns in force throughout the world to combat cases and fatalities.

Worldwide confirmed cases have exceeded 109 million (as of 17th February 2021), with approximately 2.42 million deaths from COVID-19 resulting in a worldwide death-to-case ratio (the number of deaths divided by the number of diagnosed cases) of 2.22% of all confirmed cases, although this varies considerably.

This is congruent with a recent average from January 2020 to January 2021 including some non-specific treatment but no vaccination found it to be ~2.1%

Back at the beginning of the pandemic, several nations experienced high numbers of deaths relative to the affected population (higher CFR). After its outbreak in March 2020, Italy had an overall CFR of around 14.3% for COVID-19, compared to China’s CFR of 5.3% at that time. There are many factors that could cause stark differences like these, such as the mean age differences - in Italy, people aged 70 or over account for 37.6% of the population, compared to China where only 11.9% are over 70.

It is important to note that testing for COVID-19 has increased significantly since the beginning of the pandemic and so theoretically more cases should be recognized and the proportion of deaths will hence decrease, making CFR lower. Treatments have also improved and vaccinations are rolling out, again affecting the CFR and making it lower than it would be had the disease been left  

The following uses data taken from the COVID-19 Case Fatality Analysis (referenced below) as of 17 February 2021. In the USA, 499,991 deaths and 28381220 cases give a CFR of ~1.76%. S… In the UK, there were approximately 4,058,468  confirmed cases with 118,195 deaths resulting in a CFR of ~2.91%.

The variability of such estimates depends on the levels of testing (reflecting a more accurate diagnosis and prevalence of the disease within a population), as well as lockdown measures and adherence to such measures.

The emerging evidence suggests that the overall CFR or COVID-19 is around 2.2% depending on population and demographic characteristics. This is much lower than the CFR for SARS and MERS, but higher than seasonal influenza.

Biases

There are many biases within calculating CFR, some of which have been mentioned.

The beginnings of outbreaks are likely to present a higher CFR due to factors such as low testing and those with pre-existing health conditions being reported more than others. Thus, a higher proportion of reported cases are fatal and CFR is high.

Furthermore, a global CFR encompasses a broad range of conditions. Different countries will have different detection methods, different definitions of cases and deaths due to the disease, different healthcare and different resources. There are also varying population profiles across the world (such as sex, ethnicity, gender and health conditions).

Another limitation comes from the fact that it is unlikely that all cases or deaths due to a widespread disease will be detected or correctly designated. Delays in recordings of deaths and cases can also skew CFR.

Summary

In summary, CFR measures the severity of a particular infectious disease within a particular environment at a specific time. Specifically, it refers to the total number of deaths due to a specific disease as a percentage of the total number of confirmed diagnoses of that disease. Different diseases have different CFRs, and many biological, social, political, and environmental factors can alter the CFR within a population.

A disease can be very infectious (spreads easily) but have a low CFR due to the nature of the disease as well as the presence of vaccinations and treatments, whilst other diseases are not as infectious (not as easily spread) but are more deadly.

New disease outbreaks (such as COVID-19) can lead to significantly higher CFR at first, but a lower CFR in subsequent peaks due to factors like increased testing and the development of potential vaccinations and effective treatments.

References

Further Reading

Last Updated: Feb 17, 2021

Dr. Osman Shabir

Written by

Dr. Osman Shabir

Osman is a Postdoctoral Research Associate at the University of Sheffield studying the impact of cardiovascular disease (atherosclerosis) on neurovascular function in vascular dementia and Alzheimer's disease using pre-clinical models and neuroimaging techniques. He is based in the Department of Infection, Immunity & Cardiovascular Disease in the Faculty of Medicine at Sheffield.

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