Estimation of influenza A/H3N2 attack rates.

(A) Inference of attack rates using simulated data for 69 participants, with same strains as tested in the Ha Nam data. Main plot: Blue lines show the estimated attack rate with binomial confidence interval; red lines show the attack rates in years when samples were taken; black circles show the true attack rate in the original simulation. Inset: year of circulation for the 57 test strains used, which included repeats in some years. (B) Main: accuracy of attack rate estimates in (A) was high for years in which serological samples were collected (shown as red dots). Hollow black points show the attack rate based on a 2-fold rise in titre against strain in that year (points shown for years 2008–2011, which had sufficient test strains or samples to perform this calculation); solid points show the attack rate based on a 4-fold rise. Inset: distribution of differences between estimated and actual attack rates in same years across 12 simulation studies. Red line indicates estimates from model; dashed black line shows estimates based on a 2-fold rise in titre; solid black line shows estimates based on a 4-fold rise. (C) Proportion of the Vietnam study population estimated to have been infected in each year, based on real data. Blue lines show the estimated attack rate with binomial confidence interval; red lines show attack rates in years when samples were taken. (D) Accuracy of attack rate estimates using different methods. Plot shows model estimates of attack rates in 2008–2012 (red points in (C)) and number of positive H3 isolates reported in Vietnam during the same intervals as the samples were taken. Hollow black points show the attack rate based on a 2-fold rise in titre against strain in that year (point not shown for 2012, as no test strains for this year were available, so a rise could not be calculated); solid points show the attack rate based on a 4-fold rise.