Covid-19 Is Not the Spanish Flu

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Despite the mathematical impossibility of the Spanish flu killing at least 50 million with a 2.5 percent fatality rate, this phantom statistic has drifted far and wide, materializing everywhere from blogs, Twitter, and The New York Times to the most prestigious medical journals. The New England Journal of Medicine recently published some commentaries repeating the incongruent figures. I contacted the journal’s editors and statistical consultants, pointing out the mistake and explaining what I’d discovered about its possible origins. A few days later I got a reply from Jennifer Zeis, director of media and communications: “Authors relied on different sources of information, which would yield discrepant values. There are published sources for each number, even if they are inconsistent.”

Of course estimates differ; the pandemic in question happened more than a century ago, and we don’t have anything close to complete or accurate records of its casualties. But that does not explain away a glaring mathematical incongruity, nor does it justify an abdication of scholarly responsibility. When errors slip past the safeguards of peer-reviewed research literature, they should be promptly corrected, especially when they have potential to breed misunderstanding and panic. Spanish flu has become synonymous with a viral apocalypse and, now, with the Covid-19 pandemic. This false equivalence depends largely on a spurious statistic that should never have been published. It is certainly possible, perhaps even inevitable, that a pandemic on the scale of the Spanish flu will happen again. But the latest estimates of Covid-19’s fatality rate, infectiousness, and its response to public health measures indicate that in relative terms it will not match the devastation of 1918. The Spanish flu’s global mortality rate of 3 percent would translate to more than more than 230 million deaths today.

There are many additional reasons not to make blithe comparisons between the current crisis and the 1918 pandemic: stark differences in health care infrastructure and medical technology; the ravages of the first world war; the unusual tendency of the Spanish flu to kill young adults; and the fact that many, if not most, people infected with influenza in 1918 died from secondary bacterial infections (as mass-produced antibiotics did not yet exist). The global fatality rate is just an average, and the CFR of any pandemic varies immensely by age, population, and geography. During the Spanish flu, for instance, it ranged from less than 1 percent in some areas to 90 percent in one Alaskan village. What gets lost in superficial analogies is that, despite some valid and instructive parallels between the two pandemics, there are many more differences. We can’t use half-contrived statistics about a century-old pandemic to predict what will happen today.

When WHO director-general Tedros Adhanom Ghebreyesus announced on March 3 that the novel coronavirus had a global case fatality rate of 3.4 percent, he was simply reporting known deaths divided by known cases, not an intelligent estimate or a definitive number. Infectious disease mathematician Adam Kucharski and his colleagues recently calculated that the true case fatality rate in China is between 0.3 and 2.4 percent; other researchers have concluded that the global CFR is likely similar. These estimates will continue to change with time and increased testing. Some experts anticipate that, if widespread testing were deployed, the global fatality rate would remain at or below 2 percent. There’s also a possibility, however, that the final global fatality rate will be higher than current data indicate. Near the start of the 2009 H1N1 pandemic, CFR estimates were 10 times too large. During the 2002-04 SARS outbreak, however, early CFR estimates were nearly three times too small.

The novel coronavirus pandemic is a major threat that demands a swift and robust response. Even a fatality rate between 0.5 and 1 percent is extremely alarming in a world as populous and interconnected as ours. Another crucial consideration is the virus’s potential to induce severe illness that may not be fatal but lasts for weeks, straining hospital resources and potentially leaving some people with lifelong health issues. If the multiplying outbreaks around the world are not curtailed, we could see staggering numbers of illnesses and deaths, especially among the elderly and those with underlying medical conditions. Recently, some infectious disease experts have suggested that Covid-19 could reach the scale of the 1957 avian influenza pandemic, which killed an estimated 1 million to 4 million people worldwide. But that is only one possible trajectory. The outcome of the current pandemic will not be shaped by any single statistic, but by a constellation of social, economic, and environmental factors-including the vulnerability of infected populations, the speed and scale of public health interventions, and the transparency of governments.

Numbers and charts convey a reassuring sense of certainty. But in the midst of an evolving crisis, that certainty is too often an illusion. A single, imprecise statistic generated more than a decade ago can suddenly proliferate, inciting panic and senseless hoarding that diverts resources from those who need them most. When experts and journalists uncritically pluck numbers from careless studies and clutch at fluctuating figures, hastily offering them up as beacons, they may do more to confuse than illuminate.

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