Ask any child, a world without Walt Disney would be unimaginable. Born in December 1901, Walt was sixteen years old when he caught the 1918 pandemic influenza — and survived. A century has passed since the great 1918 pandemic, in which tens of millions died, the deadliest in history. When an anniversary of a major event comes round, we can ask what if the event were to occur today.
Catastrophe modelers can also reimagine the event being different from what it actually was. This counterfactual perspective leads to important insights into pandemic risk, which have only recently emerged from virological research.
Influenza ward at Camp Funston, Kansas. Image taken in 1918 by a U.S. Army photographer [Public domain], via Wikimedia Commons
A key feature of the 1918 pandemic was the high mortality rate among young adults, between the ages of about 20 and 40. In New York and elsewhere, there was a notable peak at age 28. This anomalously high death rate among young adults has been ascribed to the overreaction of their immune response systems. But the immune response system of a 28 year-old is not significantly different from that of an 18 year-old, so there has to be another explanation.
Some simple arithmetic provides a clue. Subtract 28 from 1918 and you get 1890. This was the year of the Russian Flu pandemic. In contrast with the H1N1 strain of 1918, the Russian Flu strain was H3N8. Virologists have discovered that childhood exposure to a previous influenza pandemic is imprinted on the immune system, and affects future response to a different pandemic strain. Those born around 1890 would have been exposed to the H3N8 Russian Flu, and would have been relatively more vulnerable to the 1918 H1N1 pandemic.
Had the Russian Flu pandemic occurred a decade later, then the age range of heightened vulnerability to the 1918 pandemic would have shifted downwards to 10 to 30. With an immune system imprinted by H3N8, the young teenager Walt Disney might then not have made it. Conversely, had the Russian Flu pandemic occurred a decade earlier, then the age range of heightened vulnerability would have shifted upwards to 40 to 60, and the 49 year-old Mahatma Gandhi might not have survived his bout of pandemic influenza in India.
This year, the Aussie flu circulating has been H3N2. Middle-aged adults who were exposed to the H3N2 Hong Kong pandemic flu of 1968 have been at far less risk from this than a future pandemic swine flu H1N1 (as in 1918), or avian flu (H5N1). Indeed, if the 1918 H1N1 virus were to be unleashed on the world again, middle-aged rather than young adults would be particularly vulnerable. Life and health insurers should take note.
To download a new whitepaper entitled “Reimagining the 1918 Pandemic”, click here.