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Modeling a Modern-Day Spanish Flu Pandemic

February 21, 2013
Editor’s Note: The 1918 influenza pandemic killed tens of millions of people around the world, making it one of the deadliest infectious disease outbreaks in modern history. In this article, Nita Madhav, a Principal Analyst in AIR’s Research and Modeling Group, characterizes the historic 1918 pandemic and estimates the effects of a similar pandemic occurring today using the AIR Pandemic Flu Model (scheduled for release in Summer 2013).

Why Was the 1918 Pandemic So Devastating?

Compared to other flu viruses, the virus that caused the 1918 pandemic was highly transmissible (as measured by the basic reproduction number, R0, the number of new infections generated by an infected person entering into a population with no immunity to the disease). For seasonal flu, the average R0 value is around 1.30, but the median R0 for the 1918 pandemic was much higher, ranging from 1.80 to 2.48. In some regions, the R0 of the pandemic pathogen even soared to 4.50. The notably high transmissibility of the 1918 pathogen may have been due, in part, to genetic factors that amplified the virus’s replication ability.

In addition to being highly transmissible, the 1918 virus was highly virulent, as measured by the case fatality rate (CFR), or the percentage of sick individuals that later die of the disease. During the 1918 pandemic, the CFR ranged between 2.0% and 5.0%, while seasonal flu today typically has a CFR of 0.2% in developed countries. Recent research has shown that certain mutations in multiple genes of the 1918 virus contributed to its marked virulence.


igure 3. Mortality rates caused by a modern-day recurrence of the 1918 Spanish Flu pandemic, as estimated by AIR (Source: AIR)

The stochastic “modern day Spanish flu” event generated for the AIR model produces an age CFR profile similar to that of the actual 1918 pandemic. Specifically, the stochastic pandemic event exhibits increased mortality rates among young- to middle-aged adults (25–34 years of age), yielding a “W”-shaped mortality profile very similar to the mortality profile of the actual 1918 pandemic. This “W”-shaped profile can be attributed to the ability of the simulated virus to cause a cytokine storm, demonstrating the model’s ability to capture the effects of this complex syndrome.

Table 1. Mortality and life insurance losses estimated by AIR for seven countries due to a modern-day recurrence of the 1918 influenza pandemic (Source: AIR)
Country
Number of Deaths
Industry Life Insurance Losses
(USD billions*)

Australia
15,000– 26,000
0.8–1.4

Canada
20,000–37,000
2.1–3.8

France
36,000–62,000
2.1–3.7

Germany
48,000–85,000
1.8–3.3

Japan
83,000–145,000
8.9–15.2

UK
36,000–64,000
3.1–5.5

U.S.
188,000–337,000
15.3–27.8

*All losses converted from local currency to USD using exchange rates as of 12/31/2012.