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BMC Medical Research Methodology
Published in: BMC Medical Research Methodology 1/2021

Open Access 01-12-2021 | Human Immunodeficiency Virus | Research article

Flexible modelling of risk factors on the incidence of pneumonia in young children in South Africa using piece-wise exponential additive mixed modelling

Authors: Jordache Ramjith, Kit C.B. Roes, Heather J. Zar, Marianne A. Jonker

Published in: BMC Medical Research Methodology | Issue 1/2021

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Abstract

Introduction

Recurrent episodes of pneumonia are frequently modeled using extensions of the Cox proportional hazards model with the underlying assumption of time-constant relative risks measured by the hazard ratio. We aim to relax this assumption in a study on the effect of factors on the evolution of pneumonia incidence over time based on data from a South African birth cohort study, the Drakenstein child health study.

Methods

We describe and apply two models: a time-constant and a time-varying relative effects model in a piece-wise exponential additive mixed model’s framework for recurrent events. A more complex model that fits in the same framework is applied to study the continuously measured seasonal effects.

Results

We find that several risk factors (male sex, preterm birth, low birthweight, lower socioeconomic status, lower maternal education and maternal cigarette smoking) have strong relative effects that are persistent across time. When time-varying effects are allowed in the model, HIV exposure status (HIV exposed & uninfected versus HIV unexposed) shows a strong relative effect for younger children, but this effect weakens as children grow older, with a null effect reached from about 15 months. Weight-for-length at birth shows a time increasing relative effect. We also find that children born in the summer have a much higher risk of pneumonia in the 3-to-8-month age period compared with children born in winter.

Conclusion

This work highlights the usefulness of flexible modelling tools in recurrent events models. It avoids stringent assumptions and allows estimation and visualization of absolute and relative risks over time of key factors associated with incidence of pneumonia in young children, providing new perspectives on the role of risk factors such HIV exposure.
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Metadata
Title
Flexible modelling of risk factors on the incidence of pneumonia in young children in South Africa using piece-wise exponential additive mixed modelling
Authors
Jordache Ramjith
Kit C.B. Roes
Heather J. Zar
Marianne A. Jonker
Publication date
01-12-2021
Publisher
BioMed Central
Published in
BMC Medical Research Methodology / Issue 1/2021
Electronic ISSN: 1471-2288
DOI
https://doi.org/10.1186/s12874-020-01194-6

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