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Explained: Why COVID-19 case fatality rate should be read with caution

CFR may be used to determine the severity and prognosis of a disease. But in case of the current pandemic, CFR may not be a completely reliable method as the total number of COVID-19 cases counted depends on how aggressively countries are testing.

By: Explained Desk | New Delhi |
Updated: April 8, 2020 3:11:47 pm
coroanvirus, case fatality rate, what is case fatality rate, covid 19 cfr, corona case fatality rate, indian express, explained news A member of the health staff team wearing a protective suit to protect against coronavirus, transfers a patient into an ambulance at the Severo Ochoa hospital in Leganes, outskirts of Madrid, Spain. (AP Photo)

As countries deal with the coronavirus pandemic, the case fatality rate (CFR) figures put out are making it difficult to ascertain exactly how deadly COVID-19 is. For instance, Italy, which is the epicentre of the outbreak in Europe, has a CFR of 12.6 per cent, whereas Germany has a CFR of roughly two per cent. Italy has seen over 17,000 deaths, while Germany has reported over 2,000. Bahamas, with only 29 cases and five deaths has a CFR of 17.24 per cent.

While the global number of infected cases are recorded and updated daily, in order to figure out the prognosis of the disease, experts are not convinced whether CFR is the accurate way of doing so, as many people with coronavirus show mild symptoms or are asymptomatic, making it difficult to determine the total number of infected cases in a country.

What is Case Fatality Rate and how is it calculated?

CFR is calculated by taking the ratio of the total number of deaths occurring due to a particular cause to the total number of cases due to the same cause. For instance, Italy’s CFR can be calculated by taking the total number of deaths, over 16,000 divided by the total number of COVID-19 cases in the country, which is more than 132,000.

What does CFR tell us?

CFR may be used to determine the severity and prognosis of a disease. But in case of the current pandemic, CFR may not be a completely reliable method as the total number of COVID-19 cases counted depends on how aggressively countries are testing. According to Nina Schwalbe, in a piece for the World Economic Forum, in many places because of a lack of adequate testing supplies and resources, only those people who report themselves are being tested. This means testing in most countries is limited to the hospitals and presumably does not take into account those with mild symptoms who are not sick enough to report themselves and those who are asymptomatic and show no symptoms at all. This leads to an inflated CFR since the denominator will be smaller.

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CFR data as on April 8/Johns Hopkins University

Schwalbe further points out that depending on the type of test used, countries may only be counting people who are actively infected, excluding those who had the disease but were cured. This will lead to an underestimation of the denominator as well, and hence the CFR will be overestimated. “In other words, by not counting the people who don’t need hospital care, we are massively over projecting the percent of infected people who die of COVID-19. It’s a dangerous message that is causing fear all driven by a false denominator,” Schwalbe says.

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On the other hand, authors from institutes in France, Switzerland and China suggested in the journal Lancet that CFR is underestimated and they suggest that the denominator of the mortality rate be the total number of patients infected at the same time as those who died. However, this suggestion was criticised by Marc Lipsitch of the Harvard School of Public Health in the same journal. Referring to their suggestion he says, “The authors make the situation worse: correcting for delay (with an invalid method) without correcting for ascertainment of mild cases inflates the estimates, bringing them further from what most experts believe are the true numbers, around the 1–2 per cent range for symptomatic cases.”

Here’s a quick Coronavirus guide from Express Explained to keep you updated: What can cause a COVID-19 patient to relapse after recovery? | COVID-19 lockdown has cleaned up the air, but this may not be good news. Here’s why | Can alternative medicine work against the coronavirus? | A five-minute test for COVID-19 has been readied, India may get it too | How India is building up defence during lockdown | Why only a fraction of those with coronavirus suffer acutely | How do healthcare workers protect themselves from getting infected? | What does it take to set up isolation wards?

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