Question: You may have already answered this, but I haven’t been able to find it. What have you found to be the current fatality rate? I heard a really high number of 5.9% which I thought was much higher than reality. Could you also explain how it’s calculated?
Answer: I have talked about the case fatality rate (CFR, also called case fatality ratio) many times before, but I don’t believe I’ve ever described how it’s calculated and why it fluctuates. Let me remedy that gap! As you’ll see, that high CFR you heard was not off base (unfortunately). What I hope you’ll take away from this description is that COVID-19 is serious and far more serious than other conditions that it has frequently been compared with (e.g. flu).
CFR is a measure of disease severity. In essence, it is the proportion of people with a given condition who die from that condition. To calculate CFR, the numerator is deaths from the condition and the denominator is all people with the condition (deaths / cases) (Figure 1). Put to practice, Figure 2 shows the current CFR calculation and trends for the United States based on data from covidtracking.com. As of 6/9, the COVID-19 CFR for the United States is 5.4%. Now, if we were to use data from the Johns Hopkins Covid Dashboard, the current CFR in the US would be slightly different — 5.7%. This difference (5.4% vs. 5.7%) is due to slightly different numbers of cases and deaths compiled by these two aggregation services. As you can see in the line chart presented in Figure 2, CFR is not stagnant. And it’s important to recognize that CFR will continue to change based on multiple factors, including:
Data Completeness:
- Testing: As more mild cases are identified through widespread testing, we should expect CFR to decrease.
- Death Reporting and data completeness: For more on this, see Q&A from 5/5, but in essence, as we do a better job at capturing COVID deaths in hospital, long-term care, and home-based settings, we may see an increase in CFR. And for more on issues of data completeness, see Q&A of 5/22.
Health Systems and Medical Interventions:
- Health System Capacity/Readiness: In settings where hospitals are overwhelmed, as we saw in Italy, the CFR increases because health workers are less equipped to provide high quality care for their patients.
- Effective Treatment: As more treatments become available and as health professionals become more adept at caring for COVID patients (discussed in Q&A of 5/31), we should expect to see the CFR decrease.
- Vaccination: As vaccinations become available and used widely, we should expect to see fewer cases and fewer deaths. Among those vaccinated who still become infected (perhaps because the vaccine is not fully effective) we should expect to see fewer severe outcomes, which would reduce CFR.
Population Dynamics:
- Age structure: Because COVID severity varies dramatically by age, in general, the older the population, the higher the CFR. For more on age-specific COVID mortality, see Q&A of 5/23.
- Underlying Conditions: We know that risk of more severe outcomes, including death, is associated with underlying conditions, including hypertension, obesity, diabetes, and more. For more on underlying conditions, see Q&A of 4/14 and Q&A of 6/8. For populations that have more underlying conditions, we’d expect to see higher CFR.
Social/Structural Inequities:
- Racism: In populations that are marginalized, we see higher CFR. For a discussion of racism and health, see Q&A of 6/7 and Q&A of 6/1.
- Poverty: In populations that are more impoverished, we see higher CFR (see this recent research, for example).
Non-medical Interventions:
- Social Distancing, Mask Wearing, etc.: The more widespread social distancing, mask wearing, hand washing, and the like, the lower we’d expect the CFR to be. This is because populations more vulnerable to severe outcomes are better protected from exposure and because health systems are also better protected from becoming overwhelmed.
Note: This list is not exhaustive. I’m sure in writing this morning that I’ve unintentionally omitted some other factors. But as you can see, there are lots of factors that influence CFR. No doubt, CFR is an imperfect measure, but it does give us insight into disease severity, and that’s exactly what it’s meant to measure!
Figure 1. CFR Calculation (from CDC website)
Figure 2. COVID-19 Case Fatality Rate of 6/9
Calculation:
105,981 deaths / 1,970,596 cases = 0.05378
0.05378 x 100 = 5.4% (rounded to nearest tenth)
