Role of Preschool and Primary School Children in Epidemics of Influenza A in a Local Community in Japan during Two Consecutive Seasons with A(H3N2) as a Predominant Subtype

<div><p>Enhanced influenza surveillance was implemented to analyze transmission dynamics particularly driving force of influenza transmission in a community during 2011/12 and 2012/13 seasons in Odate City, Japan. In these two consecutive seasons, influenza A(H3N2) was the predominant influenza A subtype. Suspected influenza cases were tested by commercial rapid test kits. Demographic and epidemiological information of influenza positive cases were recorded using a standardized questionnaire, which included age or age group, date of visit, date of fever onset, and the result of rapid test kit. Epidemiological parameters including epidemic midpoint (EM) and growth rate (GR) were analyzed. In 2012/13 season, numbers of influenza A positive cases were significantly lower among preschool (212 cases) and primary school (224 cases) children than in 2011/12 season (461 and 538 cases, respectively). Simultaneously, total influenza A cases were also reduced from 2,092 in 2011/12 season to 1,846 in 2012/13 season. The EMs in preschool and primary school children were earlier than EMs for adult and all age group in both 2011/12 and 2012/13 seasons. The GR in 2012/13 season was significantly lower than that in 2011/12 season (0.11 and 0.18, respectively, p = 0.003). Multiple linear regression analysis by school districts revealed that GRs in both seasons were significantly correlated with the incidence of school age children. Our findings suggest that preschool and primary school children played an important role as a driving force of epidemics in the community in both 2011/12 and 2012/13 seasons. The reduction of total influenza A cases in 2012/13 season can be explained by decreased susceptible population in these age groups due to immunity acquired by infections in 2011/12 season. Further investigations are needed to investigate the effect of pre-existing immunity on influenza transmission in the community.</p></div

The distributions of Epidemic Midpoint (EM) by age groups in 2011/2012 and 2012/2013 seasons.

<p>*Day 0 was set as February 8.</p

Daily number of Influenza A positive cases by age group in 2011/12 (1A) and 2012/13 (1B) seasons in Odate City, Akita, Japan.

<p>* The day of epidemic midpoint was set as Day 0. Those dates were February 8, 2012 and February 2, 2013, respectively.</p

Risk factor analysis results when comparing influenza A positive cases and non-ILI among pregnant women.

<p>Co-variates were chosen based on initial univariate analyses. Estimates in bold indicate statistically significant co-variates.</p

Incidence rates (IRs) of ILI and sARI cases detected for pregnant women, stratified by trimester stage and for the overall cohort.

<p>Incidence rates (IRs) of ILI and sARI cases detected for pregnant women, stratified by trimester stage and for the overall cohort.</p

Risk factor analysis results when comparing sARI cases and non-sARI among infants under 6 months.

<p>Co-variates were chosen based on initial univariate analyses (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148421#pone.0148421.s007" target="_blank">S4 Table</a>). Estimates in bold indicate statistically significant co-variates.</p

Total positive case numbers of influenza A, influenza B and RSV and also those hospitalized for sARI, stratified by age groups and for the overall cohort.

<p>Total positive case numbers of influenza A, influenza B and RSV and also those hospitalized for sARI, stratified by age groups and for the overall cohort.</p

The weekly number of virus positives detected from ILI and sARI cases during the 2013/14 (left) and 2014/15 (right) seasons, for pregnant women (a) and infants under 6 months (b).

<p>The bars show the number of positive cases for influenza A (red), influenza B (blue) and RSV (green). The black lines indicate periods of limited point-of-care test kit supply, thus testing was compromised in those weeks. The overall testing rate for ILI and sARI cases are 94.8% and 100%, respectively, for pregnant women and 77.6% and 30.6%, respectively, for infants under 6 months.</p

The weekly number of influenza-like illness (ILI) [orange] and severe acute respiratory infections (sARI) [purple] cases during the 2013/14 (left) and 2014/15 (right) seasons, for pregnant women (a) and infants under 6 months (b).

<p>The black lines indicate periods of limited point-of-care test kit supply, thus testing was compromised in those weeks.</p

Risk factor analysis results when comparing ILI cases and non-ILI among pregnant women.

<p>Co-variates were chosen based on initial univariate analyses (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0148421#pone.0148421.s006" target="_blank">S3 Table</a>). Estimates in bold indicate statistically significant co-variates.</p