Why are the death rates for COVID-19 so difficult to calculate?

Dr. Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization (WHO), announced a COVID-19 mortality rate of 3.4 per cent in a press conference on March 3, 2020. Yet how positive the figure is? We look at what British experts on the subject have to say.

The complexities of estimating an effective mortality rate for COVID-19 have been weighed in by various experts.
The complexities of estimating an effective mortality rate for COVID-19 have been weighed in by various experts.

Dr Ghebreyesus announced a total of 90,893 cases of COVID-19 worldwide in the media briefing, with 3,110 resulting in death.

In his declaration Dr. Ghebreyesus has made some comparisons to the common flu. These included the reality that COVID-19 “does not spread as effectively” as influenza, there are no COVID-19 vaccines and treatments, and containment methods could work for COVID-19 but would not be feasible for the flu.

An important and relevant observation Dr. Ghebreyesus made regarding the severity of the two diseases. No-one is immune to COVID-19 unlike flu. As a result, there are more people vulnerable to infection and many are susceptible to “severe illness.”

“Globally, about 3.4% of reported COVID-19 cases have died. By comparison, seasonal flu generally kills far fewer than 1% of those infected.”

– Dr. Tedros Adhanom Ghebreyesus

But how did they measure this mortality rate? And what are the unique challenges during an outbreak when determining the death rate?

Some of the top health experts in the world have been weighing in. Below, we’ll sum up their opinions.

Why calculating the death rate is so ‘tricky’

“It’s incredibly difficult to determine the ‘case fatality ratio,’ or death rate, during an epidemic,” says John Edmunds, a professor at the London School of Hygiene & Tropical Medicine’s Institute for Mathematical Modeling of Infectious Diseases in the UK.

Prof. Edmunds notes that this difficulty is due to the long time between the onset of the disease and the fatality.

For COVID-19, the time is 2–3 weeks or more, he says. Therefore, we will use the number of confirmed cases from a few weeks ago to calculate the event fatality rate, rather than at the present time.

The case fatality rate is characterized by experts as “the ratio of deaths from a particular cause to the total number of cases due to the same cause.”

However, continues Prof. Edmunds, in the case of a “rapidly expanding outbreak,” the number of cases from a few weeks ago will always be much smaller than the current one, so that “the true case fatality ratio will be higher.”

“We don’t record all of the cases,” Prof. Edmunds says. “We usually only record a small proportion of them, in fact. If in fact there are many more accidents, then the case-fatality ratio is lower.

In conclusion, estimating the true case fatality ratio is “tricky,” says the researcher.

“What you can safely say […] is that if you divide the number of reported deaths by the number of reported cases [to get the case fatality ratio], you will almost certainly get the wrong answer.”

– Prof. John Edmunds

Why 3.4% is likely an overestimate

Dr Toni Ho, an infectious disease specialist at the Medical Research Council (MRC)–Glasgow Center for Virus Research University, UK, shares similar sentiments.

She continues to say that the 3.4% figure is likely to be an underestimate, mainly due to the difficulties of estimating mortality rates described above.

“For all recorded cases, the stated mortality rate of 3.4 per cent is taken from confirmed deaths. It is undoubtedly an overestimate, as a number of countries have had limited testing, such as the United States (112 confirmed, 10 deaths) and Iran (2,336 cases, 77 deaths). Therefore, few of the mild cases have been picked up, and [the total number of cases] we are observing is the tip of the iceberg.”

Actually, the overestimation could be 10 times higher than the reality, notes Mark Woolhouse, a professor of infectious disease epidemiology at Edinburgh University, UK.

“I]f a significant number of mild cases is missing or not reported, then it percentage[ 3.4 percent] is too high.”

“Though there is disagreement about this, some studies have suggested that it is approximately 10 times too high. This would bring the death rate in line with some strains of influenza.”

– Prof. Mark Woolhouse

Importance of regionality and other factors

Regionality is another factor which confuses the calculations.”[ T]he number of reported cases and deaths are likely to vary depending on the population,” says Tom Wingfield, senior clinical lecturer and honorary physician consultant at the Liverpool School of Tropical Medicine, UK.

“For example, COVID-19 cases reported earlier in the outbreak and Hubei province deaths were mainly among people admitted to hospital who may not have identified less severe cases in the community.”

“[ T]he evidence suggests that[ case fatality ratios] were higher in the earlier stages of the epidemic than in the last few weeks and higher inside than outside of China.”

Paul Hunter, a professor of medicine at the University of East Anglia (UEA), UK, also gives his view, stating, “We[…] don’t know if the Chinese phenomenon would happen anywhere — in the UK, we hopefully won’t have such an intense outbreak in a small area.”

Additional factors that might influence the fatality ratio of accidents include “how injuries and fatalities are classified,” Wingfield says.

He gives the example of the “spike in cases in China when the case description has been expanded to include those clinically diagnosed rather than verified by testing.” However, the researcher continues, the case fatality ratio “the change over time during the outbreak.”

However, the researcher continues, the case-fatality ratio “may change over time during the outbreak.”

“The factors contributing to this may include: viral mutations[…]; host-related factors, such as immune response from different infected subpopulations; and epidemiological factors, such as exposure levels and repeated exposure.”

Finally, the actual number of deaths may be underreported, says Wingfield.

Death rate may only be known at the end

“Early estimates of fatality rates tend to be higher and then decrease as the epidemic progresses,” says the Royal Holloway, University of London, UK, biological anthropologist Jennifer Cole.

“This is primarily because early figures are based exclusively on the more severe cases— those seeking hospital treatment — and therefore do not reflect mild cases.”

“It is not until later in the epidemic that large numbers of people[…] like all passengers on quarantined ships have been in touch with everyone who has been contaminated, or that the entire population of a city has been screened for more accurate numbers.

The scientist goes on to stress that early intervention is important. “Early cases may also lead to death, since the symptoms of patients may be more severe until they seek treatment. The earlier people receive treatment, the better opportunities they may have to make a full recovery.”

“Scientists don’t always consider explaining why figures change as more information about a situation emerges, which can leave people feeling confused and not sure whether figures are reliable or not. Estimates and projections should always be put into context. If figures and estimates change, it’s important to clearly explain why this has happened.”

– Jennifer Cole

For information on how to prevent the spread of coronavirus, this Centers for Disease Control and Prevention (CDC) page provides advice.

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