Factors Influencing the Case Fatality Rate (CFR)

00:01 So let's go back to the CFR, the case fatality rate, which again, is the proportion of known cases that end up dying.

00:08 Several things affect the CFR.

00:11 Here are a few of them.

00:13 First is the clinical experience that doctors have in treating the disease.

00:19 So the more experience we have, the more likely we are to be able to prevent death.

00:24 The more complicated and high technology and well resourced the health care system, the better able we are to fend off death.

00:32 So it's not unreasonable to assume that wealthier countries with better healthcare systems might have lower CFRs for the same disease than poor countries with lesser resourced healthcare systems.

00:46 And as an epidemic unfolds throughout time, then we get more clinical experience and the CFR probably will come down as well.

00:55 Now that might change if the epidemic is so severe that the health care system becomes overwhelmed, doctors become exhausted, the resources get used up.

01:07 In such a case, even though more time has passed and more clinical experience has been gained, the CFR might go up because the healthcare system is unable to respond as well.

01:18 Of course, the patient always provides the most variants in these analyses.

01:24 So comorbidities affect CFR.

01:27 In the case of COVID-19, we know that people with diabetes or hypertension or obesity are more likely to die than people without these comorbidities.

01:36 So population with more people with comorbidities is going to have a higher CFR than populations without those comorbidities.

01:46 One could also consider age to be something of a comorbidity.

01:51 So, a population that skews older might have a higher CFR than one that skews younger for diseases like COVID-19.

02:02 And lastly, new treatments emerge as an outbreak unfolds and more resources are put to solving the problem.

02:11 So, in the case of COVID-19, the advents in using monoclonal antibodies or some of the new antiviral drugs came months after the first cases.

02:22 As a result, the probability of dying if you become infected comes down somewhat if you have access to some of these new medications.

02:32 So the CFR is not a biological constant.

02:35 It's entirely dependent on the context of infection.

02:40 And by context, I mean where you got infected, when you got infected, who you are and what you have access to in terms of treatment.

02:51 Other issues with the CFR include that it really does depend on how we define cause of death.

02:59 It isn't always clear whether the disease was the likely cause of death because most deaths are the result of many factors.

03:08 So some clinical determination, a qualitative determination and opinion must be garnered by a clinician to determine whether or not a certain death was likely caused by the disease.

03:24 As well, prolonged sickness that does not lead to death might complicate matters.

03:31 So what do I mean by that? At best, we hope that when computing the CFR, the people who got infected and the people who died did so within somewhat of the same timeframe.

03:46 But individuals who gets sick and linger for a very long period of time, weeks, months and sometimes years and then die complicate the data somewhat because their infection was recorded an earlier time period and their death recorded in a later time period.

04:02 So that might skew the CFR computation to be less severe-looking if an individual has not died yet, but will die later on.

04:18 Of course, measuring the denominator is a big issue.

04:21 The denominator again is the number of people with the disease.

04:25 So consider an overburdened health care system or the early days of the COVID-19 epidemic in well-resourced countries in Western Europe or North America.

04:37 In general, there are so many cases and so few tests and the system was being overwhelmed that only the most symptomatic cases were presenting themselves to be tested.

04:48 As a result, you find cases that are more serious that are more likely to die.

04:53 So it makes sense in such a scenario for the CFR to be higher in the earlier days.

04:59 And as more testing becomes available, and we test more asymptomatic or lesser symptomatic people, you get lesser severe disease and individuals who are less likely to die.

05:11 So the CFR should come down.

05:13 As a result, CFR will change over time, even in the same population in the same country, depending on the nature of testing.