Modelling the Effects of Population Structure on Childhood Disease: The Case of Varicella

Realistic, individual-based models based on detailed census data are increasingly used to study disease transmission. Whether the rich structure of such models improves predictions is debated. This is studied here for the spread of varicella, a childhood disease, in a realistic population of children where infection occurs in the household, at school, or in the community at large. A methodology is first presented for simulating households with births and aging. Transmission probabilities were fitted for schools and community, which reproduced the overall cumulative incidence of varicella over the age range of 0–11 years old. Moreover, the individual-based model structure allowed us to reproduce several observed features of VZV epidemiology which were not included as hypotheses in the model: the age at varicella in first-born children was older than in other children, in accordance with observation; the same was true for children residing in rural areas. Model predicted incidence was comparable to observed incidence over time. These results show that models based on detailed census data on a small scale provide valid small scale prediction. By simulating several scenarios, we evaluate how varicella epidemiology is shaped by policies, such as age at first school enrolment, and school eviction. This supports the use of such models for investigating outcomes of public health measures. Author Summary: Individual-based models of disease transmission have increasingly included detailed demographic data to more accurately describe places where population mix and infection occur. These models may help to understand in more detail heterogeneities in transmission and improve public health decisions. Here, the spread of varicella, a childhood disease, is studied in such a model where spatial and population structures are explicitly modelled. The model focuses on children, organized in households, schools and municipalities, in agreement with census data. The detailed structure of the population used in the model allows for reproducing several observed differences in the epidemiology of varicella, for example, variation in age according to birth rank and place of residence. These results support using detailed models with the eventual aim of improving decisions in public health.