Influenza Transmission in a Community during a Seasonal Influenza A(H3N2) Outbreak (2010–2011) in Mongolia: A Community-Based Prospective Cohort Study

BACKGROUND: Knowledge of how influenza viruses spread in a community is important for planning and implementation of effective interventions, including social distancing measures. Households and schools are implicated as the major sites for influenza virus transmission. However, the overall picture of community transmission is not well defined during actual outbreaks. We conducted a community-based prospective cohort study to describe the transmission characteristics of influenza in Mongolia. METHODS AND FINDINGS: A total of 5,655 residents in 1,343 households were included in this cohort study. An active search for cases of influenza-like illness (ILI) was performed between October 2010 and April 2011. Data collected during a community outbreak of influenza A(H3N2) were analyzed. Total 282 ILI cases occurred during this period, and 73% of the subjects were aged <15 years. The highest attack rate (20.4%) was in those aged 1-4 years, whereas the attack rate in those aged 5-9 years was 10.8%. Fifty-one secondary cases occurred among 900 household contacts from 43 households (43 index cases), giving an overall crude household secondary attack rate (SAR) of 5.7%. SAR was significantly higher in younger household contacts (relative risk for those aged <1 year: 9.90, 1-4 years: 5.59, and 5-9 years: 6.43). We analyzed the transmission patterns among households and a community and repeated transmissions were detected between households, preschools, and schools. Children aged 1-4 years played an important role in influenza transmission in households and in the community at large. Working-age adults were also a source of influenza in households, whereas elderly cases (aged ≥ 65 years) had no link with household transmission. CONCLUSIONS: Repeated transmissions between households, preschools, and schools were observed during an influenza A(H3N2) outbreak period in Mongolia, where subjects aged 1-4 years played an important role in influenza transmission

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

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

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

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