Demographic characteristics of total study population and all ILI cases during the overall study period, ILI cases and influenza A(H3N2) positive cases during influenza A(H3N2) outbreak period, and household transmission index cases and household transmission secondary cases.

<p>Abbreviations: ILI, influenza-like illness; SD, standard deviation.</p

Secondary attack rates and relative risk of ILI by age and gender of household contacts.

<p>Abbreviations: ILI, influenza-like illness; SAR, secondary attack rate; CI, confidence interval; NA, not applicable.</p

Secondary attack rates and relative risk of ILI by household size.

<p>Abbreviations: ILI, influenza-like illness; SAR, secondary attack rate; CI confidence interval.</p

Secondary attack rates and relative risk of ILI by age group of index cases.

<p>Abbreviations: ILI, influenza-like illness; SAR, secondary attack rate; CI, confidence interval; NA, not applicable.</p

Attack rate during the influenza A(H3N2) outbreak period.

<p>Attack rate for (A) children aged <20 years and (B) adults aged ≥20 years.</p

Epidemic curves for each affiliation.

<p>Epidemic curves of household transmission index cases, household transmission secondary cases, and non-household transmission ILI cases in (A) school A, (B) school B, (C) preschool C, (D) preschool D, (E) preschool E, (F) preschool F, (G) children at home, and (H) adults. The letters attributed to the household transmission secondary cases are the affiliations of their index cases; A, school A; B, school B; C, preschool C; D, preschool D; E, preschool E; F, preschool F; G, children at home; H, adults.</p

Relationship between index cases and secondary cases with household transmission.

<p>Relationship of household transmission (A) among children and (B) among children and adults. Red circles represent index cases, blue triangles represent secondary cases, and arrows show the direction of transmission. The letters represent affiliations; A: school A; B: school B; C: preschool C; D: preschool D; E: preschool E; F: preschool F; G: children at home; H: adults.</p

Distribution of study population, ILI cases, secondary cases, and ILI attack rates for each affiliation.

<p>Abbreviations: ILI, influenza-like illness; NA, not applicable.</p

Epidemic curve of ILI cases and the proportion influenza-positive samples.

<p>Influenza A(H3N2) positive cases are shown in red, influenza A(H1N1)pdm09 positive cases are shown in blue, influenza negative ILI cases are shown in yellow, and ILI cases without sample collection are shown in gray. Influenza positive rate for each day was calculated as the number of influenza positive samples divided by the number of samples tested.</p

Density plot on the basis of age group and date of onset.

<p>Density plot of (A) household transmission index cases and non-household transmission ILI cases (n = 231) and (B) household transmission secondary cases (n = 51).</p