We presented data on the circulation of influenza in children who were attending preschool, elementary school, or junior high school in Toda or Warabi, Japan, during five consecutive influenza seasons from 2014 to 2018. We also successfully established seasonal, high, and alert thresholds based on surveillance data from five consecutive seasons of influenza using the WHO method.
Number of children with influenza and their distribution by influenza type
In Japan, the governmental agency (Ministry of Health, Labour, and Welfare) in collaboration with the National Institute of Infectious Diseases (NIID) provides a weekly influenza outbreak report [19]. This report is based on a school surveillance in which the absence of children and temporary closure of schools are recorded; therefore, the date of onset of influenza cannot be assessed. Nonetheless, the total number of temporary school closures peaked in 2017, which supports our findings that the greatest number of influenza cases occurred in 2017. In junior high school children, our survey was different from the national report [19], i.e., the greatest prevalence of influenza cases were reported in 2016 (23.4%; Table 2). In our survey, influenza type B was responsible for a substantial share of all influenza cases, approximately > 40% in seasons where influenza type A circulated with low incidence in 2015 and 2017 (Table 1). The results of our survey are essentially similar to the outbreak trend summary of influenza for each season, as reported by NIID of Japan [19], whereas the prevalence of influenza type B among children in junior high school was approximately > 50% in 2015 (Table 2). Outbreaks differed by region, even within a single country, and surveillance in a local area of the present study was demanded.
Week of the influenza epidemic peak
In the national report [19], each peak of temporary suspension of facilities occurred in weeks 4, 7, 4, 5, and 4 in the 2014, 2015, 2016, 2017, and 2018 seasons, respectively. The week of the influenza epidemic peak in our survey occurred consistently earlier than that in the national report, although the gross tendency was similar. Our survey was different from the national report [19], which was based on the dates of absence from school in children with influenza, and it is likely that the week with the highest number of detected influenza cases proceeded the week of temporary school closures. Regional characteristics such as metropolitan area versus nationwide data [19] might affect the peak shift; however, relevant report has been lacking and we cannot say much about this difference.
The peaks of influenza type B occurred later than those of type A (Tables 1 and 2). The epidemic order is in accord with that observed in the northern hemisphere [20]. Understanding the geographical and temporal patterns of seasonal influenza could help strengthen influenza surveillance for the early detection of epidemics [21]. As Mosnier and his colleagues reported [22], timely data on the circulation of influenza collected within influenza surveillance systems is essential for optimizing influenza prevention and control strategies [21, 22].
Establishment and assessment of seasonal, high, and alert thresholds
We used WHO method-based thresholds and epidemic curves to assess the influenza seasons from 2014 to 2019; these data represent the onset and end of the influenza season, as well as the severity of influenza. Epidemic peaks occurred at week 51 or later, in particular at ≥ 2 weeks among preschool children (Tables 1 and 2). We did not find any apparent seasonal pattern on the weekly fluctuations or on the week of influenza epidemic peaks. Meanwhile, the seasonal, high, and alert thresholds were explored without being affected by the diverse seasonal patterns. The NIID provides another warning outbreak system. When the number of influenza cases that occur at regional sentinel sites is more than 30 per week per site, the warning is issued to a responsible public health center [23]. We provided three threshold levels (seasonal, high, and alert thresholds) for children and each school group in Toda and Warabi based on the surveillance data that were captured in the same region. Our community-based survey in children can be used for pandemic influenza assessment by accounting for a potential increase, with high sensitivity. It should be noted that we should alter these threshold values according to region and country.
Previous studies have proposed that the analysis of surveillance data could provide information about seasonal thresholds and epidemic curves that might help healthcare personnel in clinical management [16]. Although there is room for debate on the definition of the period of influenza activity [24], future surveillance data would allow for an assessment of early warnings with a revision of the nominated threshold levels [9].