Influenza Transmission in Households During the 1918 Pandemic

Abstract

Analysis of historical data has strongly shaped our understanding of the epidemiology of pandemic influenza and informs analysis of current and future epidemics. Here, the authors analyzed previously unpublished documents from a large household survey of the "Spanish" H1N1 influenza pandemic, conducted in 1918, for the first time quantifying influenza transmissibility at the person-to-person level during that most lethal of pandemics. The authors estimated a low probability of person-to-person transmission relative to comparable estimates from seasonal influenza and other directly transmitted infections but similar to recent estimates from the 2009 H1N1 pandemic. The authors estimated a very low probability of asymptomatic infection, a previously unknown parameter for this pandemic, consistent with an unusually virulent virus. The authors estimated a high frequency of prior immunity that they attributed to a largely unreported influenza epidemic in the spring of 1918 (or perhaps to cross-reactive immunity). Extrapolating from this finding, the authors hypothesize that prior immunity partially protected some populations from the worst of the fall pandemic and helps explain differences in attack rates between populations. Together, these analyses demonstrate that the 1918 influenza virus, though highly virulent, was only moderately transmissible and thus in a modern context would be considered controllable.

Disparities in influenza mortality and transmission related to sociodemographic factors within Chicago in the pandemic of 1918

“…The range of reproduction numbers across census tracts was 1.09-1.54, which is on the lower end of previous estimates for the 1918 pandemic (35)(36)(37)(38). However, basic reproduction numbers (R 0 ) greater than three have been reported on small spatial scales, such as military bases (39), prisons and ships (26), and even in some cities (25).…”

The influence of changing host immunity on 1918–19 pandemic dynamics

“…Despite pre-dating the isolation of the influenza virus, epidemiological records from the 1918 influenza pandemic have allowed much to be inferred about the transmission characteristics of the 1918 pandemic virus (henceforth pH1N11918) (e.g. Fraser et al, 2011;Valleron et al, 2010). However the ecological conditions which gave rise to the three distinct peaks of mortality over a period of just nine months in England and Wales (Smallman-Raynor et al, 2002;Pearce et al, 2011) remain intriguing.…”

How Modelling Can Enhance the Analysis of Imperfect Epidemic Data

“…Finally, even when asymptomatic or inapparent infections are not observed at all, the impact they have on the epidemic dynamics may be observed on apparently unrelated statistics, providing a pathway to characterize them. For example, Fraser et al used reporting of symptoms during the 1918 influenza outbreak in a large sample of households in Baltimore to estimate the proportion of asymptomatic infections [56]. The main idea behind their approach is that, if a large proportion of household members report symptoms, this indicates that the proportion of asymptomatic infections cannot be very high.…”