Transmissibility of 1918 pandemic influenza


The 1918 influenza pandemic killed 20–40 million people worldwide1, and is seen as a worst-case scenario for pandemic planning. Like other pandemic influenza strains, the 1918 A/H1N1 strain spread extremely rapidly. A measure of transmissibility and of the stringency of control measures required to stop an epidemic is the reproductive number, which is the number of secondary cases produced by each primary case2. Here we obtained an estimate of the reproductive number for 1918 influenza by fitting a deterministic SEIR (susceptible-exposed-infectious-recovered) model to pneumonia and influenza death epidemic curves from 45 US cities: the median value is less than three. The estimated proportion of the population with A/H1N1 immunity before September 1918 implies a median basic reproductive number of less than four. These results strongly suggest that the reproductive number for 1918 pandemic influenza is not large relative to many other infectious diseases2. In theory, a similar novel influenza subtype could be controlled. But because influenza is frequently transmitted before a specific diagnosis is possible and there is a dearth of global antiviral and vaccine stores, aggressive transmission reducing measures will probably be required.

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Figure 1: Graph of the logarithm of excess P&I deaths in 1918 for the ten most populous cities in the US.
Figure 2: Histogram of initial and extreme estimated R values for 45 cities during the 1918 influenza pandemic.
Figure 3: Graph of the relationship between the serial interval (ν) and the magnitude of R.


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We thank J. Wallinga and D. Olson for discussions and C. Raviola for data procurement. This work was supported in part by the National Defense Science and Engineering Graduate Fellowship (C.E.M.), the Medical Scientist Training Program Fellowship (C.E.M.), NIAID (J.M.R. and M.L.) and the Ellison Medical Foundation (M.L.).

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Correspondence to Christina E. Mills.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Notes

This file includes Supplementary Data, Methods, Discussion and Figure Legends. (DOC 100 kb)

Supplementary Figure 1

Histogram of the proportion of infected hosts assumed to be infectious between 1 and 8 days post infection in the model. (PDF 15 kb)

Supplementary Figure 2

Histogram of the proportion of all deaths occurring on each day in the five weeks following infection. (PDF 15 kb)

Supplementary Figure 3

Transmission rate ratio, the adult:child CFP ratio (m) and the number of serial intervals required for the exponential growth rate to reflect the magnitude of R. (PDF 213 kb)

Supplementary Table 1

Spreadsheet of the weekly excess pneumonia and influenza mortality data originally published in Collins, S. D., Frost, W. H., Gover, M. & Sydenstricker, E. Mortality from Influenza and Pneumonia in 50 Large Cities of the United States 1910-1929. Public Health Reports 45, 2277-2328 (1930). (XLS 23 kb)

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