Fine-scale family structure shapes influenza transmission risk in households: insights from a study of primary school students in Matsumoto city, 2014/15

Abstract

11Background: Households are important settings for the transmission of seasonal 12 influenza. Previous studies found that the per-person risk of within-household 13 transmission decreases with household size. However, more detailed heterogeneities 14 driven by household composition and contact patterns have not been studied. 15 Methods:We employed a mathematical model which accounts for infections both from 16 outside and within the household. The model was applied to citywide primary school 17 * Correspondence to: akira.endo@lshtm.ac.uk Keppel St., Bloomsbury, London, WC1E 7HT, UK. surveillance data of seasonal influenza in 2014/15 season in Matsumoto city, Japan. We 18 compared a range of models to estimate the structure of household transmission. 19Results: Familial relationship and household composition strongly influenced the 20 transmission patterns of seasonal influenza in households. Children had substantially 21 high risk of infection from outside the household (up to 20%) compared with adults (1-22 3%). Intense transmission was observed within-generation (between 23 children/parents/grandparents) and also between mother and child, with transmission 24 risks typically ranging around 5-20% depending on the pair and household composition. 25 Conclusions:We characterised heterogeneity in household transmission patterns of 26 influenza. Children were identified as the largest source of secondary transmission, with 27 family structure influencing infection risk. This suggests that vaccinating children 28 would have stronger secondary effects on transmission than would be assumed without 29 taking into account transmission patterns within the household. 30 31 Abbreviations: CPI, community probability of infection; RDK, rapid diagnostic kit; 32 SITP, susceptible-infectious transmission probability; MCMC, Markov-chain Monte 33Carlo; WBIC, widely-applicable Bayesian information criterion; CrI, credible interval. 34Respiratory infectious diseases transmitted by droplets, exemplified by 37 influenza, are known to spread over social contact networks (1,2). Social settings which 38 involve frequent contacts play important roles in the transmission dynamics (3,4). 39Households are considered as one of the main layers of transmission, as individuals 40 come in close contact with each other both conversationally and physically on a daily 41 basis (5-7). Many epidemiological studies have used household data to investigate the 42 transmission dynamics of influenza within households (8,9), particularly in terms of the 43 secondary attack rate (the number of household secondary cases divided by the number 44 of household members at risk). However, this assumes that an index case (the first case 45 in a household, who is considered to be infected outside the household) is responsible 46 for all subsequent household cases, and that all the other household members are 47 outside the household (community probability of infection; CPI) and the within-53 household transmission risk (10). Many household studies have employed the ...