The strengths and limitations of the two approaches to infection risk assessment as well as to outbreak investigations are compared. This article reviews the fundamental theories, formulations, model developments, and modifications of these two approaches. Some newer studies have proposed to use dose-response models for assessing the infection risk of airborne-transmissible pathogens (e.g., Armstrong and Haas, 2007a Nicas, 1996 Sze To et al., 2008). They were then developed for assessing the infection risk of foodborne and waterborne pathogens ( Haas, 1983). Originally, dose-response models were mainly used for risk assessment of hazardous chemicals. Risk assessment models based on the dose-response relationship are called dose-response models. The dose-response relationship is used to describe the effect on organisms from the exposure to different doses of chemicals, drugs, radiation, bio-agents, or other stressors. The Wells–Riley model has been extensively used in analyzing ventilation strategy and its association to airborne infections in clinical environments (e.g., Escombe et al., 2007 Fennelly and Nardell, 1998 Nardell et al., 1991). The equation is based on the concept of ‘quantum of infection’ as proposed by Wells (1955) and is therefore termed the Wells–Riley equation. The Wells–Riley equation was developed by Riley and colleagues in an epidemiological study on a measles outbreak ( Riley et al., 1978). Currently, there are two approaches to quantitative infection risk assessment of respiratory diseases, which can be transmitted via the airborne route: the Wells –Riley model and the dose-response model. Quantitative infection risk assessment can also be used in epidemiological studies such as outbreak investigations. By comparing infection risks, the influence of different environmental factors on disease transmission and the effectiveness of different infection control measures can be evaluated. Infection risk is expressed by a probability of infection between 0 and 1. It describes the infection risk of an individual or a population to an infectious disease quantitatively.
Quantitative infection risk assessment can serve as a useful tool in epidemic modeling, parametric studies on disease transmission and evaluating the effectiveness of infection control measures. Risk assessors can select the approach that is suitable to their particular conditions to perform risk assessment. The Wells–Riley model and dose-response model offer specific advantages. This review serves as a reference for further development of infection risk assessment models of respiratory diseases. Even with many efforts by various investigators to develop and modify the risk assessment models, some limitations still persist. This review article summarizes the strengths and limitations of the Wells–Riley and the dose-response models for risk assessment of respiratory diseases. Some suggestions for further development of the risk assessment models are proposed. Respiratory deposition of aerosol induces heterogeneous infectivity of intake pathogens and randomness on the intake dose, which are not being well accounted for in current risk models. Spatial distribution of airborne pathogens is one of the most important factors in infection risk assessment of respiratory disease. Dose-response models can consider other disease transmission routes in addition to airborne route and can calculate the infectious source strength of an outbreak in terms of the quantity of the pathogen rather than a hypothetical unit. The Wells–Riley model allows quick assessment and does not require interspecies extrapolation of infectivity. This study reviews and compares these two approaches to infection risk assessment of respiratory infectious diseases. Some newer studies have also proposed the use of dose-response models for such purpose. The Wells–Riley model has been extensively used for quantitative infection risk assessment of respiratory infectious diseases in indoor premises. Quantitative infection risk assessment can provide quantitative analysis of disease transmission and the effectiveness of infection control measures.
Abstract Infection risk assessment is very useful in understanding the transmission dynamics of infectious diseases and in predicting the risk of these diseases to the public.
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