# COVID-19: Case Fatality Rate (CFR) & Infection Fatality Rate (IFR)

by Sean Elliott, MD

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00:07 Another number, which you you'll hear, another estimate I should say, is the “Case fatality rate,” which actually should be, “Case fatality ratio.” This this estimates the mortality ratio, among documented cases and it's calculated by taking the number of deaths, divided by the number of documented cases.

00:28 As noted, we should not really consider that a rate, but unfortunately, rate, is the usage or the term, which most individuals in the social and common media and literature have used and so, you'll hear us use rate.

00:41 Ratio of course, is the more accurate statistical number.

00:46 This number the “CFR” is not a constant and it's also a very poor measure, during an epidemic.

00:53 You could pause this this session right now, just think about reasons for that.

00:57 But of course, what you're coming to come up with, is that, the CFR will typically overestimate the true death rate, why is that? Because our denominator the total number of cases, is imperfect, we don't know the number of asymptomatic cases, we don't know the number of true cases that were not tested and not confirmed and so our “n” our denominator, is likely far larger than what we are using to calculate the CFR and again, as we say here most death by infection are also noted going to be noted because they may not be diagnosed or they may be attributed to something else or they may be wrongly attributed to infection with SARS-Coronavirus II, causing COVID-19.

01:45 The crude mortality rate is a measure of the proportion of the entire population that die from a particular disease.

01:51 This is very different from the case fatality rate, which only factors disease individuals into the equation and discounts the healthy population.

02:00 So again, perhaps a more accurate estimate of the mortality rate, is the infection fatality rate the “IFR.” And this is the number of deaths due to infections, divided by the total number of infected people.

02:15 So, inserting here, is going to be a better attempt, to confirm that deaths and infections are specifically due, to the disease of interest, in this case COVID-19.

02:26 So, the IFR, should be a better measurement, it will almost always be lower than the CFR, just due to the imperfections in calculating the CFR.

02:38 The IFR, will include both documented and undocumented or asymptomatic cases and it's going to be typically estimated at the end of a pandemic.

02:51 Although of course estimates will occur throughout the epidemic, but it is largely looking in retrospect, that one can accurately calculate the IFR.

03:00 Why is it more accurate? Because it requires documentation of the infection, typically, through antibody studies, the IFR is going to vary with the distribution of other factors, comorbidities, so age, health issues, pre-existing health issues, other qualities of the infected individuals, as well as qualities of the medical care they receive or have access to.

03:24 So, you can imagine then that that for COVID-19, that the, true mortality rates have and will vary greatly, across different countries and age groups.

03:34 The CFR currently is being estimated at 1.2% with COVID 19.

03:39 The IFR is closer to 0.5%.

03:42 We must remember that this may be higher in specific populations, for example, in communities in which 20% of the population is over 85 years of age.

03:52 The IFR may reach 4%.

03:54 Neither the CFR nor the IFR can account for the full burden of COVID 19 because they don't include mortality, which is indirectly caused by the pandemic.

04:03 An example of this would be delayed care for other medical conditions Perhaps the health system itself is overburdened, certainly, we're seeing this in many parts of the world, were, the sheer vast numbers of COVID-19 patients, has overwhelmed the medical setting, clinics, hospitals, emergency departments and there is insufficient medication, or healthcare personnel, ventilators, IV fluids and thus, those who do make it into the healthcare system, encounter an overburden system, which has a very decreased quality of care.

04:39 Social determinants of health, this is a very difficult and yet a very important factor which will impact on both CFR and IFR.

04:48 So those individuals who lost their jobs because of all the social closures.

04:52 Decreased social interactions, causing perhaps mental health variabilities, maintenance of education, maintenance and mental health et cetera.

05:02 All these have the ability to intervene or impact on the CFR or the IFR.

05:09 So, as an example, again in real time, these are still estimates, but the fatality rates, in individuals with COVID-19 who also have comorbidities and you see them listed here.

05:23 Cardiovascular disease, the mortality rate, the fatality rate, over 10%, those with diabetes, especially those with insulin dependent diabetes, 7.3%, those individuals with chronic respiratory disease, 6.3%, death rate from COVID-19, pre-existing hypertension or other cardiovascular disease, 6.0%, those with cancer immunodeficiencies, almost 6.0%, those who had no pre-existing conditions, so zero comorbidities, age, obesity, any other medical conditions, that the estimate of the fatality rate with COVID-19 is just 0.9, while that is still much higher, than it is for influenza, where the estimate is 0.1%, yet that is far less significantly horrible, than as we see with cardiovascular disease.

06:12 So, it is through using these statistical predictive numbers, that we can both follow as well as predict areas of the world, which are going to be negatively affected by the COVID-19 pandemic and it is certainly the wish and the concern, to continue to use these successfully to drive resources and to do a better job of impacting on COVID-19.

06:36 case fatality rates from China in February of 2020 which show how preexisting medical conditions can impact the disease outcomes.

06:44 So far, age has proven to be the strongest risk factor for severe COVID 19 outcomes, including death.

06:50 The number of preexisting conditions has also been shown to be associated with worse outcomes.

06:55 And finally, with each new virus variant, the symptom profile immunogenicity and treatment resistance of the virus can all evolve to result in worsening outcomes.

### About the Lecture

The lecture COVID-19: Case Fatality Rate (CFR) & Infection Fatality Rate (IFR) by Sean Elliott, MD is from the course Coronavirus.

### Included Quiz Questions

1. It overestimates the death rate since the number of documented infectious cases is smaller than the number of true infectious cases.
2. It is calculated by the number of documented cases divided by the number of deaths.
3. It is lower than the infection fatality rate (IFR).
4. It includes both documented and asymptomatic cases.
5. It is more accurate than the infection fatality rate (IFR).
1. Cardiovascular disease
2. Cancer/immunosuppression
3. Diabetes
4. Chronic respiratory diseases such as COPD
5. Hypertension
1. It is a more accurate measurement than the CFR.
2. It only includes documented cases.
3. It overestimates the true death rate.
4. It is a constant with regard to age distribution, comorbidities, and access to medical care among individuals.
5. It is always higher than the case fatality rate.

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