The role of fomites in disease spread and the transmission characteristics

Abstract

People inevitably touch surfaces every day, and they can potentially become infected by touching contaminated surfaces, also called fomites, and subsequent contacting their facial mucous membranes. The fomite route was once thought to be negligible in disease transmission, but more evidence has been provided in recent years to support its importance. Further studies of the relative importance of this route in virus transmission are necessary to aid in the selection of more appropriate intervention methods. Due to the complicated mechanisms of virus transmission, the available knowledge on the fomite route remains at an empirical level, and information on how such transmission occurs on multiple surfaces is very limited. A better understanding of the transmission characteristics of the fomite route is also essential to improve the effectiveness of surface decontamination strategies in indoor environments. To investigate the role of fomite routes, four representative outbreaks were analysed. A multi-agent modelling framework was developed to estimate the infection risks due to the long-range, close contact and fomite routes. Least-squares fitting was conducted to compare the distribution of the predicted infection risk in the various scenarios with that of the reported attack rates and to identify the probable transmission modes with the best fit. In the Middle East respiratory syndrome (MERS) outbreak, the virus probably spread via the long-range airborne route alone, while the fomite route might have played little or no role. In the severe acute respiratory syndrome (SARS) and influenza outbreaks, the two viruses were probably transmitted via combined long-range airborne and fomite routes, with the former predominant. Nevertheless, the contribution of the fomite route was non-negligible in the SARS transmission but was very small in the influenza transmission. In the norovirus outbreak, the virus might have been transmitted via the fomite route. To investigate the propagation characteristics of the number of contaminated surfaces, a novel and effective benchtop experiment was designed. An inflight norovirus outbreak was chosen as the scenario for the experiment. Hundreds of representative environmental surfaces in the plane were scaled down, and fluorescent particles were used as surrogate indicators of virus-laden aerosols. Fluorescence imaging techniques were used to quantify the fluorescent particles on each surface. The temporal diffusion of the contaminated surfaces followed an S-shaped logistic curve, and the aisle seats were more contaminated than the non-aisle seats. To investigate the temporal and spatial patterns of virus distributions on surfaces, a new Markov chain model was developed. A case of virus transmission in a general hospital ward was used as the test setting, and four common source conditions were considered. Theoretical inference and surface touch network analyses were conducted to explain the virus distribution patterns, and multi-agent simulations were performed to verify the prediction of the new model. With a sufficient number of touches, the virus concentrations on surfaces for a point source approached zero, whereas those for a continuous source exhibited a daily cycle. In addition, spatial distributions of the virus were influenced by the topological structures of surface networks and the transfer properties of the surfaces.