Unexpectedly Intriguing!
28 May 2020

Here's what we know today, or rather, here's what the U.S. Centers for Disease Control and Prevention (CDC) knows today about how deadly the COVID-19 coronavirus infections are:

Symptomatic Case Fatality Ratio (CFR) for COVID-19

These figures are the CDC's best estimates as of 20 May 2020, which specifically apply for those who have developed COVID-19 symptoms, which according to the CDC as of 13 May 2020, can include the following:

  • Fever or chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue
  • Muscle or body aches
  • Headache
  • New loss of taste or smell
  • Sore throat
  • Congestion or running nose
  • Nausea or vomiting
  • Diarrhea

After being exposed to a sufficient amount of the SARS-CoV-2 coronavirus to become infected, early studies have indicated COVID-19 symptoms can take anywhere from 4.5 to 5.8 days to develop, with a median incubation period of 5.1 days. According to the CDC, about 35% of those who become infected may never develop any of the listed symptoms associated with COVID-19. The coronavirus infection does not appear to be deadly for those who do not develop symptoms.

After the onset of symptoms, another early study found the typical period before death might occur could range between 15 and 22 days, with a median period of 18.5 days. Combined, that gives an estimated period from infectious exposure to death of anywhere from 20 to 28 days, with a median period of about 25 days. The incidence of death from COVID-19 increases with age, though most who have succumbed to it have also had underlying medical conditions, which may be more strongly correlated with an increased risk of death than age alone.

If the indicated percentages in the chart seem lower than what you may have expected, that is because the analyses on which the CDC is relying to produce these estimates is considering its spread among the whole population. Many of the deaths that have driven news headlines have occurred within especially vulnerable subsets of the population, such as among elderly Americans who reside in nursing homes or other long term care facilities, for whom coronavirus infections combined with the health conditions that led them to live in such facilities in the first place greatly increases their risk of death far above what would apply for the general population.

Compared to seasonal influenza, such as experienced during the 2018-2019 flu season, COVID-19 is about four times as deadly overall. While it is not anywhere as apparently deadly as previously thought, the available evidence indicates the SARS-CoV-2 coronavirus is still more deadly than a typical flu virus.


28 May 2020: All this assumes the CDC's figures are valid, but there are unanswered questions about how the CDC's analysts have determined them. Follow this space for developments in that story.

29 May 2020: Could the CDC's "best estimates" have come from mixing the results of viral and antibody tests in their assessments of how deadly the SARS-CoV-2 coronavirus is?

“You’ve got to be kidding me,” Ashish Jha, the K. T. Li Professor of Global Health at Harvard and the director of the Harvard Global Health Institute, told us when we described what the CDC was doing. “How could the CDC make that mistake? This is a mess.”

Viral tests, taken by nose swab or saliva sample, look for direct evidence of a coronavirus infection. They are considered the gold standard for diagnosing someone with COVID-19, the disease caused by the virus: State governments consider a positive viral test to be the only way to confirm a case of COVID-19. Antibody tests, by contrast, use blood samples to look for biological signals that a person has been exposed to the virus in the past.

A negative test result means something different for each test. If somebody tests negative on a viral test, a doctor can be relatively confident that they are not sick right now; if somebody tests negative on an antibody test, they have probably never been infected with or exposed to the coronavirus. (Or they may have been given a false result—antibody tests are notoriously less accurate on an individual level than viral tests.) The problem is that the CDC is clumping negative results from both tests together in its public reporting.

Mixing the two tests makes it much harder to understand the meaning of positive tests, and it clouds important information about the U.S. response to the pandemic, Jha said. “The viral testing is to understand how many people are getting infected, while antibody testing is like looking in the rearview mirror. The two tests are totally different signals,” he told us. By combining the two types of results, the CDC has made them both “uninterpretable,” he said.

Suddenly, we're having to go from covering apparently very good news to instead covering a new potential scandal for the CDC, on top of their previous self-inflicted black eye in failing to develop effective viral tests in the first place.

29 May 2020: According to this report, the current best estimate outside of the CDC for the Symptomatic Case Fatality Ratio of COVID-19 falls somewhere between 0.5% and 1.0%.

22 June 2020: More than a month after it set its "best estimate" of COVID-19's symptomatic fatality ratio at 0.4% for the general population, the CDC hasn't updated it. Originally, the CDC had estimated a value of 0.25% for this figure on 31 March 2020, which the agency increased to 0.5% on 14 April 2020, before revising it again to the present level of 0.4% on 20 May 2020.

23 June 2020: The American Council on Science and Health has posted the latest available COVID-19 death totals by age group from the CDC, and also provides additional estimates for the infection fatality ratio for COVID-19. The CDC's best estimate above falls within the ranges for the other estimates, though at their lower end.

28 July 2020: We're late in addressing it here, but the CDC has revised its estimate of the SARS-CoV-2 coronavirus' Infection Fatality Rate. On 10 July 2020, they indicated the rate was 0.6%, which would correspond to a Symptomatic Case Fatality Rate of just under 1.1%. You can find more of our previous coverage on this revision here.

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