Within and between classroom transmission patterns of seasonal influenza among primary school students in Matsumoto city, Japan

Abstract

Schools play a central role in the transmission of many respiratory infections. Heterogeneous social contact patterns associated with the social structures of schools (i.e., classes/grades) are likely to influence the within-school transmission dynamics, but data-driven evidence on fine-scale transmission patterns between students has been limited. Using a mathematical model, we analyzed a large-scale dataset of seasonal influenza outbreaks in Matsumoto city, Japan, to infer social interactions within and between classes/grades from observed transmission patterns. While the relative contribution of within-class and within-grade transmissions to the reproduction number varied with the number of classes per grade, the overall within-school reproduction number, which determines the initial growth of cases and the risk of sustained transmission, was only minimally associated with class sizes and the number of classes per grade. This finding suggests that interventions that change the size and number of classes, e.g., splitting classes and staggered attendance, may have a limited effect on the control of school outbreaks. We also found that vaccination and mask-wearing of students were associated with reduced susceptibility (vaccination and mask-wearing) and infectiousness (mask-wearing), and hand washing was associated with increased susceptibility. Our results show how analysis of fine-grained transmission patterns between students can improve understanding of within-school disease dynamics and provide insights into the relative impact of different approaches to outbreak control.

Keywords: class size; influenza; mathematical model; school; social network.

Fig. 1.

Transmission dynamics of seasonal influenza in primary schools in Matsumoto city, Japan, and estimated effects of interventions for SARS-CoV-2. (A) Epidemic curve of seasonal influenza by illness onset in primary schools in Matsumoto city, 2014/15. Colors represent different schools. Month names denote the first day of the month. (B) Scatterplot of the class sizes and the number of classes per grade in the dataset. Each dot represents a class in the dataset. Dots are jittered along the x-axis. Three schools had classes of fewer than 15 students (denoted by dotted horizontal line) and were excluded from the model fitting. (C) The scatterplots of the school attack ratio (%) against the mean class size and the mean number of classes per grade. The correlation indices (r) and 95% CIs are also shown. (D) Temporal clustering patterns of students’ onset dates with different levels of groupings reproduced from the school transmission model. The distributions of the deviance of each student’s onset from the group mean are displayed at overall, school, grade, and class levels. The SD of each distribution is also shown.

Fig. 2.

The estimated within-school transmission patterns of seasonal influenza among primary school students in Matsumoto city, Japan. (A) The overall school reproduction number (R_S) under different class structures. Whiskers represent the 95% credible intervals. (B) The breakdown of R_S corresponding to each type of within-school relationships. Whiskers represent the 95% credible intervals. (C) Stacked graphs of R_S based on the median estimates.

Fig. 3.

Reconstruction of students’ source of infection. (A) Epidemic curve stratified by the reconstructed source of infection. The conditional probability of infection from different sources was computed for each student and aggregated by date of illness onset. (B) Breakdown of the reconstructed source of infection. For each student, the source of infection was sampled based on the conditional probability to provide the proportion of students infected from each source. Bars denote posterior median and whiskers 95% credible intervals. (C) Expected relative changes in the school reproduction number under school-based interventions changing the structure of classes. Dots represent medians and whiskers 95% credible intervals. Reduced outside-class transmissions (i.e., from grademates or schoolmates) were also considered (50% reduction: blue; 90% reduction: green).