Review on the optimization of medical supplies distribution under major public health emergencies

In recent years, frequent public health incidents have brought huge challenges to the economic development and emergency logistics systems of countries around the world. Facing the problems of unfair distribution of medical supplies and low distribution efficiency under major public health emergencies, this article systematically analyzes the literature on material distribution and distribution related to public health incidents in the past 20 years. The literature is reviewed from three aspects: the location and allocation of medical supplies distribution center, vehicle routing optimization, and location-routing optimization. Through the analysis of the existing research, several future research directions are proposed.


INTRODUCTION
Major public health emergencies refer to the sudden occurrence of major infectious diseases, mass diseases of unknown causes, major food and occupational poisoning, and other events that seriously affect public health, which cause or may cause serious damage to public health under certain conditions [1]. Major public health emergencies like the COVID-19, Ebola virus, SARS virus, H1N1 influenza, plague virus are sudden, public, severely harmful, comprehensive and systematic in response.
In the process of fighting the COVID-19 in 2020, a large number of medical supplies such as masks, protective clothing, goggles and medical equipment have flooded into the epidemic area. As the management department of medical supplies, the Red Cross has poor professionalism in the management of supplies, insufficient manpower allocation, and uneven distribution across regions, which ultimately caused the embarrassing situation of uneven distribution and oversaturation of regional resources. Medical supplies play a very important role in the emergency rescue process. How to quickly raise and deploy medical supplies when major public health emergencies occurs is a problem that needs to be solved urgently.
Looking at the operation of emergency logistics in the face of the prevention and control of the COVID-19 in my country, there are still certain shortcomings. My country's insufficient production capacity, insufficient material reserves, chaotic management, uneven distribution, and low deployment efficiency have been exposed one by one.
In order to better protect the public's life health and property loss, fast delivery of materials is very important in epidemic prevention and control. Therefore, a literature review on the optimization of the distribution of emergency medical supplies in the context of public health events in my country is conducted to analyze the problems and deficiencies in my country's emergency logistics, and to improve my country's rapid response and response capabilities in the face of public health emergencies. It also has certain reference significance for future research in this field. IOP Conf. Series: Earth and Environmental Science 820 (2021)

2.REVIEW OF THE LOCATION AND ALLOCATION OF MEDICAL SUPPLIES DISTRIBUTION CENTER
The Location and allocation problem (LAP) of medical materials distribution center is a hot issue in the optimization of material distribution. Distribution center, as the key node of logistics distribution, is the key node of the entire logistics distribution network. Therefore, the location of distribution center will be the key problem in the distribution network, and the LAP problem is to solve the problem of efficiency and fairness in the distribution process. The following documents are sorted out from the three aspects of medical supplies considering only the location of the distribution center, only issue of distribution, and the issue of location-allocation,as shown in Table 1. Based on the analysis of the above literature, the site selection considers factors such as natural conditions, medical capabilities, cost, and environmental impact, and establishes a P-center location model and a distribution center location model. The distribution of supplies took into account the age of the population, risk level, number of confirmed cases, time range and other factors, and applied the multi-objective mathematical planning model, hierarchical constraint method, PCA, Davies-Bouldin index and K-means clustering method to allocate the emergency supplies such as anti-virus drugs, ambulances, hospital beds and funds. The location allocation problem considers service waiting time, cost and other factors, establishes P-center model, mixed integer nonlinear programming model, emergency logistics network optimization model, and uses genetic algorithm and hybrid enumeration search to solve the model.
Future research can also be carried out from the following aspects:1)The site selection takes into account factors such as physical asset energy, reserve factors, population density, distribution center capacity, and optimal reserves of the reserve warehouse, making the site selection more reasonable and scientific.2)Considering people's psychological and physiological problems to meet the maximization of material distribution needs will be a major trend of future research.3)In the future, consider factors such as demand time tolerance and material allocation order to determine the priority of material distribution.

3.OPTIMIZATION OF MEDICAL MATERIAL DISTRIBUTION ROUTE
The traditional material distribution takes benefit as the main goal and takes a long time, while the distribution of emergency materials is characterized by timeliness, multi-objective and weak economy. Therefore, the optimization of the distribution path of emergency medical materials has far-reaching significance for scientific decision-making. Some scholars have conducted in-depth studies on the Vehicle Routing Problem (VRP) in public health emergencies, and the research results are shown in Table 2.
In summary and discussion from the above literature, the VRP problem under public health emergencies mainly starts from the types of materials, the number of models and the number of distribution centers, and takes into account the material delivery time, transportation cost, demand satisfaction rate, transportation distance and delay loss, and urgent demand Degree and other factors to build models. Models include vehicle routing model, distribution optimization model, multi-objective planning model. and use genetic algorithm, water drop algorithm, cluster analysis, optimization theory and other methods to solve the model, and finally plan the optimal solution of the vehicle path to achieve material delivery Requirements for fairness and efficiency.
The research on VRP issues has made great progress, and future research should be further expanded and deepened on this basis. Some studies hypothesize that different types of materials can be mixed and have no substitute for each other, while materials such as vaccines and blood need to be transported through the cold chain, so the optimal distribution of multiple types of medicines needs to be considered in the future. In the study of the vehicle routing problem, considering factors such as multiple material distribution centers, multiple emergency medical supplies, multiple models, road conditions, insufficient supply at demand points, and uncertain demand, the planning model for such multiple distribution centers and multiple specifications drugs will be the focus of future research.

4.OPTIMIZATION OF MEDICAL MATERIAL DISTRIBUTION LOCATION-ROUTING
After a disaster occurs, how to determine the number and location of emergency material distribution centers according to the situation in the disaster area, and how to choose the route is a hot research problem at present. Many scholars have carried out a series of studies on the Location and Routing Problem (LRP) of post-disaster medical supplies distribution, as shown in Table 3.
Analyze from the above literature, consider from a single goal, mainly with the shortest time or the best system as the goal, considering factors such as the priority of material demand, facility protection, road network damage, and the mutual influence of the distribution plan path to establish an LRP optimization model. Algorithms, ant colony algorithm and other intelligent algorithms are used to solve model problems. From the perspective of multi-objective considerations, scholars not only take the minimum time as the goal, but also consider the minimum cost, maximum demand site satisfaction, and minimum resource usage. Considering the risks of overdue vehicle operation, limited operating speed, and facility failure in the distribution process, a multi-objective LRP model, emergency logistics positioning-path robust optimization model is established, and hybrid algorithms and robust optimization methods are used to solve the problem. The research is developing towards multiobjective, multi-factor and hybrid algorithm. The research on LRP problem of medical supplies after disaster has been relatively mature. Wang Chun [29] conducted research on the location-path problem of medical supplies for public health emergencies, but the research is not deep enough. At present, relevant scholars are almost blank in the research on LRP of medical supplies under major public health emergencies. Therefore, from the perspective of fairness and efficiency, how to consider factors such as vehicle transportation time, cost, resources, infectious disease risk, poor road network, and satisfaction of demanders, and how to select the location-route problem of medical supplies under major public health emergencies, location-inventory-routing problem for further study of the future is an urgent need to solve a big problem. Table 3.
Research on LRP of post-disaster medical supplies

5.FUTURE RESEARCH
Through sorting out and discussing relevant literature on medical material distribution and distribution in major public health emergencies, future research can also be carried out from the following aspects.
(1)When selecting the location of the distribution center, consider factors such as property assets, population density, and distribution center capacity to ensure the scientific and rationality of the site selection.
(2)Under major public health emergencies, in order to prevent the spread of the epidemic and reduce the flow of people, the implementation of the village and city closure policies will have a certain impact on the road network. Long-term home quarantine, the psychological and physiological factors of residents in affected areas, and the policies issued by the government will all have a certain impact on people's physiology and psychology. Therefore, the above two factors should be included in the consideration of material distribution and distribution.
(3)The location-route problem of medical supplies after the disaster has achieved certain research results. However, under the major public health emergencies, the problem of location-route of medical supplies is still relatively shallow. In the future, we can deeply consider the situation of multiple roundtrip delivery of vehicles, the demand of demand points may exceed the capacity of vehicles, and the situation of multimodal transportation of emergency supplies. In the context of major public health emergencies, site selection-stock-path issues should also be studied in depth.
(4)Practicality and scientific of the model:the research model is relatively single, and in the future, it is possible to consider the cross-discipline to construct rich models. Although the model is verified by examples, it still needs further research on whether it is feasible in actual situations. The problem of parameter setting in the model is very subjective, and the setting basis should be given to ensure the objectivity and scientific of the research.

6.CONCLUSION
Under major public health emergencies, problems such as uneven distribution of medical supplies and low delivery efficiency have been exposed one by one. Material distribution optimization considers the three major issues of LAP, VRP, and LRP. Based on the perspective of fairness and efficiency, considering multiple factors, multiple goals and other factors, the model is optimized to make the model better guide practice. The resolution of these problems will enhance the ability to raise and deploy medical supplies in the event of major public health emergencies.