The Coronavirus Is NOT The Spanish Flu

From Wired:

'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. 

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. 

'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. 

'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.' 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. Unfortunately, the two purported sources for the 2.5 percent fatality rate were published more than 40 years ago, and their authors are no longer with us. 

I was able to reach public health expert Niall Johnson, however, primary author of a 2002 study that produced the oft-quoted estimate of 50 to 100 million deaths during the 1918 pandemic. He confirmed that “the case fatality rate must be higher than is often given." 

Historian John Barry, who wrote the comprehensive 2004 book The Great Influenza, agreed that 2.5 percent is much too low. The CFR was possibly around 2 percent in the US and some other parts of the developed world, he said, but fatality rates were much higher elsewhere. 

Johns Hopkins University epidemiologist Jennifer Leigh recently told The Los Angeles Times that the overall fatality rate for Spanish flu may have been closer to 10 percent. We can calculate a range of plausible global fatality rates for the Spanish flu by varying the number of infections from 25 to 75 percent of the world population in 1918 and the number of deaths from 25 to 100 million. If we do so, we find that a reasonable estimate for the global case fatality rate of the Spanish flu is 6 to 8 percent. To be clear, this means that 6 to 8 percent of those who were infected died. 

Global mortality of the Spanish flu—which is to say, the proportion of all people everywhere (infected and uninfected alike) who died from the disease—was probably between 2 and 4 percent. A conflation of mortality across the world and fatality among the infected may partly explain some of the pervasive statistical confusion surrounding the Spanish flu.