Clarifying the True Fatality Rate of Covid-19: Same as the Flu?
By: Shin Jie Yong
Tags: Health, Science, Ideas, Coronavirus, Life

Clarifying the True Fatality Rate of Covid-19: Same as the Flu?

[updated Oct] Comparing fatality rate jargons may lead to nowhere, but there are better metrics to grasp the health toll of Covid-19.

Author’s note: Please see an update at the end of the article too.

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%.

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 infections.

And Dr. Fauci probably meant to say that Covid-19 has an IFR of 1% (not CFR of 1%) after having considered asymptomatic infections.

The true fatality rate of Covid-19

To reiterate, Dr. Fauci said the Covid-19 mortality rate dropped from 2–3% to 1% after adjusting for asymptomatic infections. “The coronavirus mortality rate of 2–3% that was adjusted to 1% in Congressional testimony is consistent with the coronavirus CFR of 1.8–3.4% (median 2.6%) reported by the CDC,” Dr. Brown explained.

Thus, among those who became symptomatic, Covid-19 is not any deadlier than the flu. This had led to the notion that lockdowns were not necessary as Covid-19 is just like the seasonal flu.

“Furthermore, the WHO reported that the CFR of the H1N1 influenza virus is also 2–3%, similar to the unadjusted 2–3% CFR of the coronavirus reported in Congressional testimony, with no meaningful difference in mortality,” he added. “Evidence from the WHO confirmed that the approximate CFR of the coronavirus is generally no higher than that of seasonal influenza.”

As follows,

Covid-19 has an IFR of 1% and CFR of 2–3%. More recent data by the CDC and WHO, however, estimates that Covid-19 has an IFR of 0.65% and 0.5-1%, respectively.

The flu has an IFR of 0.1% or lower and CFR of 2–3%. (Please also see the update below about this.)

Thus, among those who became symptomatic (i.e., CFR), Covid-19 is not any deadlier than the flu. This had led to the notion that lockdowns were not necessary as Covid-19 is just like the seasonal flu.

Overall, a higher transmission rate leads to more infections and then more cases and, eventually, more deaths. Now, the CFR alone is not as important.

But such proponents seem to have been confused about the differences between CFR and IFR, as Dr. Fauci and others did. Among the total infection events (i.e., IFR), Covid-19 is still 6.5-times deadlier than the flu.

This means that a person is more likely to become a case (symptomatic) when infected with SARS-CoV-2 than with the influenza virus. The main reason is that SARS-CoV-2 is a novel virus in which little preexisting immunity exists, so a lot more infected people will get sick.

The true health burden of Covid-19

Even if the IFRs of SARS-CoV-2 and influenza might be the same, as some data may suggest, SARS-CoV-2 is more contagious with a basic reproductive number (R-naught) of 2.5. This means an infected person can pass the virus to two people or so, resulting in 2000+ infections after ten rounds of transmissions. The flu, by contrast, has an R-naught of 1.3, which equates to 56 infections after ten rounds.

Further, SARS-CoV-2 has a longer incubation period, the period between infection and symptom onset, than influenza (approx. 5–12 vs. 2 days). This means that asymptomatic people can stay infectious for a longer time and, thus, more widespread silent viral transmission.

The fact that Covid-19 kills more than flu is quickly evident by “the number of deaths that exceed what is seen in typical years.”

Overall, a higher transmission rate leads to more infections and then more cases and, eventually, more deaths. Now, the CFR alone is not as important. Not to mention also that SARS-CoV-2 infections are more likely to become cases than with influenza.

The fact that Covid-19 kills more than flu is quickly evident by “the number of deaths that exceed what is seen in typical years,” the academic journal Nature reported in September. In other words, deaths from Covid-19 are not deaths that ‘would happen anyway’.

Source: Nature

More info on excess mortality or deaths in 2020 vs. previous years:

Source: ourworldindata.org. Similar data on other countries (e.g., the USA, Sweden, Germany, Spain, Netherlands, etc.) can be found here.

14 September 2020 update/corrections on CFR

An email asked for the specific source of the IFR and CFR of the flu, which is <0.1% and 2–3%, respectively, in this article. I then realized that Dr. Brown cited the IFR of the seasonal flu, but CFR of the Spanish flu pandemic. For the seasonal flu, the CFR is 0.1%.

So, Dr. Fauci is right to say that the seasonal flu has a CFR of 0.1%, although he compared this CFR of flu with the IFR of Covid-19 (i.e., 1%). In this way, it is an underestimate when Dr. Fauci said that the mortality rate of Covid-19 is 10-times higher than the flu. Thus, the prior numbers I cited overestimate the lethality of the flu and discredited Dr. Fauci, which I sincerely apologize.

8 October 2020 updates/corrections on IFR

The CDC has changed the IFR values of Covid-19 into age-specific estimates, which are now very low at 0.003% for 0–19 years, 0.02% for 20–49 years, 0.5% for 50–69 years, and 5.4% for 70+ years.

But there seems to be no data on age-specific IFR of the seasonal flu. The WHO only stated that “the infection mortality rate (the number of reported deaths divided by the number of infections) will be lower. For seasonal influenza, mortality is usually well below 0.1%.” How much below 0.1% is unclear. Admittedly, comparing mortality rates between Covid-19 and the seasonal flu may lead to nowhere. Perhaps we should look at other metrics instead, such as R-naught and excess deaths.