Fine-scale family structure shapes influenza transmission risk in households: Insights from primary schools in Matsumoto city, 2014/15

Abstract

Households are important settings for the transmission of seasonal influenza. Previous studies found that the per-person risk of within-household transmission decreases with household size. However, more detailed heterogeneities driven by household composition and contact patterns have not been studied. We employed a mathematical model that accounts for infections both from outside and within the household. The model was applied to citywide primary school seasonal influenza surveillance and household surveys from 10,486 students during the 2014/15 season in Matsumoto city, Japan. We compared a range of models to estimate the structure of household transmission and found that familial relationship and household composition strongly influenced the transmission patterns of seasonal influenza in households. Children had a substantially high risk of infection from outside the household (up to 20%) compared with adults (1-3%). Intense transmission was observed within-generation (between children/parents/grandparents) and also between mother and child, with transmission risks typically ranging from 5-20% depending on the transmission route and household composition. Children were identified as the largest source of secondary transmission, with family structure influencing infection risk.

Fig 1. A schematic illustration of household chain-binomial model.

Nodes in different colours correspond to different types of individuals (e.g., father, sibling, etc.). Transmission patterns are illustrated taking household i as an example. Coloured dotted edges represent the risk of external infection ε to each individual. Solid grey edges denote person-to-person transmission risk (PTR) from one type of person to another. PTR from type l to k is given as ρ_kl, which refers to the risk of transmission given that the individual of type l is infectious. Households have different compositions and ρ_kl may also vary according to the composition. On the other hand, ε is the risk from outside the household and thus assumed to be identical across households.

Fig 2. Estimated risk of external infection and relative intensity of within-household contact.

(A) Estimated risk of external infection for each type of individual. (B) The estimated relative intensity of within-household contact. Values are scaled so that the median of contact intensity between adults is 1 (horizontal dotted line). Whiskers indicate 95% credible intervals (CrI).

Fig 3. Contact patterns and risk of infection in specific household compositions.

(A)-(C) Network graphs showing contact intensity between individuals for different household compositions: (A) “nuclear family”, (B) “many-siblings family”, (C) “three-generation family”. Node colours represent the type of individuals. Edges denote the relative intensity of contact (c_kl) between individuals. (D)-(F) Risk of infection in households of different compositions stratified by source. Light grey: risk of infection from outside the household; dark grey: risk of infection from within the household. Whiskers indicate the 95% CrI. (G)-(I) Unconditional risk of infection and conditional risk given an introduction of infection into a household. Light grey: overall risk of infection for each individual in the household; dark grey: risk of overall infection conditional that a student is infected outside and introduces infection into the household. Infection of the student is considered given, and thus the conditional risk for the student is not shown. Whiskers indicate the 95% CrI.

Fig 4

The effective amount of contacts experienced by individuals (η_kl) in different household compositions. (A) Child; (B) Father; (C) Mother; (D) Other. The coloured compartments denote the breakdown of effective contacts allocated to each individual in the household, which corresponds to SITP given that individual is infectious.

Fig 5. The risk of primary/secondary infection to individuals in different household compositions and its source.

(A) Child; (B) Father; (C) Mother; (D) Other. The coloured compartments denote the breakdown of sources. Household compositions are displayed in the same order as Fig 4. The risk of primary infection in children was set to be 16.4%, the average between those of “students” and “siblings”. Note that the scale of the y-axis in (A) is different from the other three panels.