On 11 March 2020, the National Institute of Allergy and Infectious Diseases (NIAID) told the U.S. Congress House Oversight and Reform Committee that the novel coronavirus — that causes Covid-19 — has a mortality rate of 10-times higher than the seasonal flu.
Specifically, at the congressional hearing, the NIAID director Dr. Anthony Fauci said, “The flu has a mortality rate of 0.1 percent. This [Covid-19 or SARS-CoV-2] has a mortality rate of 10 times that. That’s the reason I want to emphasize we have to stay ahead of the game in preventing this.”
10 x 0.1% = 1%.
In other words, Dr. Fauci reported that Covid-19 has a mortality rate of 1%, which he said had fallen from 2–3% after taking into account asymptomatic (i.e., symptomless) infections.
But what did Dr. Fauci mean by mortality rate?
In an editorial published on 29 February in the New England Journal of Medicine (NEJM) — co-authored by Dr. Robert R. Redfield, director of the CDC, and Dr. H. Clifford Lane, deputy director of NIAID — Dr. Fauci stated that influenza “has a case fatality rate of approximately 0.1%.”
Therefore, Dr. Fauci refers to the case fatality rate (CFR) when he said, “The flu has a mortality rate of 0.1 percent.”
But he should have said the infection fatality rate (IFR). Why?
The CFR is the total number of deaths divided by the total number of people that have the disease’s symptoms. In contrast, the IFR is the total number of deaths divided by the total number of people that carry the infection.
The confusion between ‘case’ and ‘infection’ fatality rates (CFR vs. IFR)
In contrast, the World Health Organization (WHO) stated that the flu has an infection fatality rate (IFR) of 0.1% or lower.
Who is right?
- The WHO who said influenza has an IFR of 0.1% or lower, or
- The NIAID and CDC directors who said influenza has a CFR of 0.1%?
In an article published August in the journal Disaster Medicine and Public Health Preparedness, titled “Public health lessons learned from biases in coronavirus mortality overestimation,” Dr. Ronald B. Brown at the University of Waterloo in Canada shed light on this confusion.
“A case fatality rate (CFR) is defined as the proportion of deaths among confirmed cases of the disease,” Dr. Brown said, “…while an infection fatality rate (IFR) is defined as the proportion of deaths relative to the prevalence of infections within a population.”
The catch is that cases refer only to those with clinical symptoms of the disease. Hence, asymptomatic (i.e., symptomless) people are not considered as a case.
Mathematically speaking, the CFR is the total number of deaths divided by the total number of people that have the disease’s symptoms. In contrast, the IFR is the total number of deaths divided by the total number of people that carry the infection, regardless of them having clinical symptoms or not.
“IFRs from samples across the population include undiagnosed, asymptomatic, and mild infections, and are often lower compared to CFRs, which are based exclusively on relatively smaller groups of moderately to severely ill diagnosed cases at the beginning of an outbreak,” Dr. Brown continued.
(Note that the mortality rate has its own definition: The total number of deaths per population of 100,000 persons. But people typically refer to the CFR or IFR when talking about mortality rate.)
The WHO got it right, Dr. Brown said, in that influenza has an IFR of 0.1% or lower, not a CFR of 0.1%.
Undercurrents 