The severity of Covid-19, illustrated by the mortality from the disease, remains undefined. Variable from one country to another, it also fluctuates according to death, healing and methodological choices. The term “mortality” alone can mean several different quantities. Why is it so difficult to come up with a figure that is apparently easy to calculate? The notion of mortality vaguely refers to people who die, of course, but in relation to what? Stricto sensu, the “death rate” is the number of Covid-19 deaths divided by the general population considered (that of a city or a country for example).
We can also divide the number of victims by the number of clinically confirmed cases: this is what is currently used in national reports and those of the World Health Organization (WHO) and which should logically be called ” case fatality rate “, except that the same term” mortality “is also used. In English, this is called “case fatality rate“(CFR).
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Confusion over death toll
So, should we review the vocabulary? “It is not so important,” said Antoine Flahault, director of the Institute of Global Health at the University of Geneva. The term “mortality” is more easily understood and remains precise, at least as long as it is clarified what it relates to: general population, confirmed cases (symptomatic) or even infected people (symptomatic and asymptomatic). “
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Important bias
These calculations allow us to approach the epidemic differently. The case fatality rate is undoubtedly the easiest and quickest to calculate in the midst of an epidemic. It provides a snapshot of the situation, with important limitations. Imagine 100 people diagnosed with coronavirus and two of whom die the next day. The CFR or case fatality rate is then 2%. But at the time of the calculation, nothing guarantees the future fate of the 98 survivors: will they cure definitively or will some of them finally die, which will weigh down the balance sheet? In other words, the case fatality rate often underestimates the actual severity of an epidemic disease.
Of all the figures, it is the excess mortality that we will retain
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A more interesting variant of the calculation consists of no longer including the only clinically confirmed cases, but all of the cases considered as infectious, the confirmed but also the asymptomatic, those not declared or even diagnosed without supporting tests, for example via epidemiological investigations. This is the “death rate from infection” or “fatality rate infection(IFR), but usage is more apt to use the term mortality, which is confusing. The IFR gives a more general idea of the severity of the disease, but suspected cases are also estimated with questionable accuracy, which is an important source of bias.
