Figure 2.

A) Susceptible-infectious transmission probability for the 1918 influenza pandemic, by household size. In addition to results from the Baltimore, Maryland, study, which are shown in red (squares, nonparametric; line, parametric), results are also shown for a study of 2009 H1N1 pandemic influenza (22) (orange circles) and 2 further studies of seasonal influenza transmission (green diamonds, Epigrippe Study (21); blue triangles, Tecumseh Study (20)). The studies used different methods, so estimates are not exactly comparable; the comparison with the Tecumseh Study (20) may be the most valid, since we analyzed serologically confirmed infections in initially seronegative households. The 2 other studies were also comparable, since they were based on follow-up of symptomatic cases after an index case (21, 22). All of the studies show evidence of declining susceptible-infectious transmission probability in larger households, but this is less pronounced for the 1918 Frost and Sydenstricker study (19) than for the others, perhaps because of secular changes in the nature of the household (see Web Appendix). Bars, 95% confidence interval. B) Characterization of a measure of interperson variability in infectiousness by means of a plot of the proportion of transmission attributable to the X% most infectious individuals. The brown line corresponds to homogeneity (when all infected persons have identical infectiousness); the lower red line shows the best-fitting model. The upper red line corresponds to the estimate from a different but plausible model with misreporting (see Web Appendix), and thus the red shaded area corresponds to estimates with model uncertainty. The curves were compared with a previous analysis of several infectious diseases (34): The blue line shows the most variable of the infectious diseases studied (severe acute respiratory syndrome (SARS)) and the green line the least (pneumonic plague). C) Predicted number of secondary cases attributable to within-household transmission (blue bars) and between-household transmission (orange bars) for an index case living in a household of size 5, in the absence of prior immunity or public health interventions. An index case infects an average of 0.69 persons in his or her household and 1.22 persons outside of it. The distributions are highly overdispersed. D) Distribution of immune individuals within households of different sizes predicted by the best-fitting model, using the same color scheme as in Figure 1. The yellow diamonds show the average proportion of immune individuals.