Coverage of the novel coronavirus pandemic teems with monstrous and sometimes contradictory statistics. Among the most vexing figures flitting across our screens, and spreading via text and tweet, is the case fatality rate (CFR)—the proportion of known infections that result in death. Early in the Covid-19 pandemic, World Health Organization officials announced an average CFR of 2 percent. Later on, they revised it up to 3.4 percent. In contrast, numerous epidemiologists have argued that the global case fatality rate is closer to 1 percent. These might seem like small differences, but when multiplied across large populations they translate to significant discrepancies in overall deaths.

Some experts have emphasized the difficulty of calculating the fatality rate of an emerging pandemic, explaining that current estimates are biased by a deficit of testing and by the lag time between onset of illness and death. Despite this counsel, news coverage and social media discourse has obsessed over CFRs and how they compare across pandemics throughout history. A popular refrain is that the new coronavirus has a frighteningly high fatality rate of at least 2 percent, which is supposedly comparable to that of the 1918 influenza pandemic, also known as the Spanish flu—one of the deadliest viral outbreaks in history. The truth is that this comparison is severely flawed and that the numbers it relies on are almost certainly wrong.

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Both newspapers and scientific journals frequently state three facts about the Spanish flu: It infected 500 million people (nearly one-third of the world population at the time); it killed between 50 and 100 million people; and it had a case fatality rate of 2.5 percent. This is not mathematically possible. Once a pandemic is over and all the numbers are tallied, its case fatality rate is simply the total number of deaths divided by the total number of recorded cases. Each country and city will have its own CFR, but it’s also common to calculate a global average. If the Spanish flu infected 500 million and killed 50 to 100 million, the global CFR was 10 to 20 percent. If the fatality rate was in fact 2.5 percent, and if 500 million were infected, then the death toll was 12.5 million. There were 1.8 billion people in 1918. To make 50 million deaths compatible with a 2.5 percent CFR would require at least two billion infections—more than the number of people that existed at the time.

Puzzled by this discrepancy, I started to investigate its possible origins. No one knows precisely how many people the Spanish flu infected and killed; estimates have generally increased over time and researchers still debate them. When describing the global toll of the 1918 pandemic, most people reference an influential 2006 study published in Emerging Infectious Diseases. The US Centers for Disease Control and Prevention, which publishes that journal, prominently displays the study on its website; and the article is one of the first Google search results for "Spanish flu fatality." In its opening paragraph, and with essentially no context, this study lists the three incongruent figures that have been so widely repeated: 500 million infections; 50 to 100 million deaths; 2.5 percent CFR. To be fair, the authors write that “case fatality rates” (plural) were “> 2.5%,” perhaps implying some variation from region to region. Because that figure is juxtaposed with worldwide infections and deaths, however, most people have interpreted it as a global average.

It’s not clear how the authors settled on 2.5 percent. The two sources they cite for this figure do not offer much support. One of them, a 1980 edition of a public health compendium, indicates a global CFR of 4 percent for the Spanish flu, nearly twice as high. The other, a 1976 book coauthored by a medical writer and a medical librarian, suggests that the virus had an overall infection rate of 28 percent and killed more than 22 million, which works out to a global CFR of at least 4.3 percent. I reached out to the authors of the 2006 paper to clarify. One never responded. The other said, "the figures you refer to are not our figures but widely cited figures of other scientists” and that he didn't "have any opinion about how accurate they might be." He suggested I contact the different scientists who came up with those numbers.