Using simulation technology to analyze the COVID-19 epidemic in Changsha,

This study mainly uses simulation technology to simulate the COVID-19 epidemic in Changsha, Hunan Province, China, and analyze the impact of different prevention and control measures on the epidemic. we Collect the information of all COVID-19 patients in Changsha from January 21, 2020 to March 14, 2020 and relevant policies during the COVID-19 epidemic in Changsha. Established the SEIAR infectious disease dynamics model under natural conditions, and added isolation measures on this basis. Using Anylogic8.5, the COVID-19 epidemic in Changsha City was simulated under various conditions based on the established model.In this study we find that There were 242 COVID-19 patients in Changsha. including 121 males (50%) and 121 females (50%).Most cases occurred between February 6 and February 16. Through the calculation of the R t during the epidemic in Changsha, it is found that it is reasonable to resume work on February 8, because the R t value of Changsha dropped below 1 at this time.The simulation results show that reducing the contact rate of residents and reducing the success rate of virus transmission (wearing masks, disinfection, etc.) can effectively prevent the spread of COVID-19 and significantly reduce the number of peak patients.We believe that the disease is mainly spread by the respiratory tract. Therefore, the simulation results show that whether in the early or mid-stage of the epidemic, quarantining the names of residents or reducing the contact rate of residents is very effective in controlling the COVID-19 epidemic.


COVID-19(Coronavirus disease 2019)is an emerging respiratory infectious disease
caused by the severe acute respiratory syndrome coronavirus 2(SARS-CoV-2).The disease was discovered in Wuhan in December 2019. As of June 26, 2020, COVID-19 has caused a pandemic worldwide. It has affected most countries and regions, and has a very wide range [1].As of June 23, 2020 Beijing time, there were a total of 9081678 COVID-19 cases diagnosed worldwide, and 472649 deaths, affecting more than 200 countries and regions [2]. At present, most studies report the clinical characteristics of patients [3][4][5][6].
During the development of the epidemic in China, more studies have reported the expected end time and peak value of the epidemic. Evaluation of specific measures for epidemic prevention and control is still relatively rare.
The epidemic situation in China is gradually becoming clear, but the global epidemic situation cannot be ignored. Therefore, more research is needed to explain the specific prevention and control effects of various measures during the development of the epidemic situation, so as to provide specific reference for the policies of other countries and regions.
Changsha is the capital of Hunan Province, China, with a population of more than 8 million, and is the closest capital city to Wuhan (Figure 1). The first COVID-19 case in Changsha City was diagnosed on January 21, 2020. On January 24, the first-stage response to major public health emergencies was initiated and a series of preventive and control measures were taken, such as isolating patients. Screening of close contacts, Changsha achieved zero clearance of COVID-19 patients on March 14, and from March 14, 2020 to June 26, 2020, Changsha achieved the goal of zero to zero, including Media promotion and health education, public places and traffic are closed, large gatherings and visits are prohibited, personal protection, etc. COVID-19 patients grow for 3 months, and strictly prevent and control foreign imports. This study describes the epidemiological characteristics of COVID-19 patients in Changsha City as of March 14, 2020, and compares the effective regeneration numbers in different periods based on key policies and interventions to evaluate various public health measures and epidemics The relationship between development.

Data source
The reported cases of COVID-19 in Changsha from January 21 to March 14, 2020 were collected from the Changsha City Health Commission [7]. The patient information collected includes: whether it is the input medical record, severity, date of onset (date of self-report of symptoms such as fever, cough or other respiratory symptoms), date of diagnosis (laboratory confirmation of SARS-COV-2 infection in biological samples Date), gender, residential area. All identifiable personal information is deleted to protect privacy.

Classification of 4 Time Periods
In order to better reflect the dynamics of the COVID-19 epidemic and the corresponding intervention measures, this study divided the entire epidemic in Changsha, Hunan Province into four periods. It is considered to be the first phase before January 24, 2020. At present, Changsha City has not implemented obvious preventive measures against COVID-19. This is the Spring Festival period in China. There is a large number of population movements and population aggregation. At this time, patients imported from Wuhan and incubation patients can infect a large number of Changsha residents.
The second stage is from January 24, 2020 to February 8, 2020. During this period, Changsha City is believed to have taken obvious preventive measures against COVID-19 and has not resumed work. From January 24, 2020, Changsha City began to take strict prevention and control measures, strictly inspect entry vehicles, began to build the city's public health treatment center facilities, and strengthen the deployment of emergency medical supplies. Hubei immigrants were also subjected to unified medical observation and isolation, and 56 tourist attractions and all public places were closed. And on February 1st, a comprehensive treatment of COVID-19 patients began to ensure that each patient was treated correctly. The third phase is from February 8, 2020 to March 7, 2020. At this time, Changsha began to resume work gradually, and the flow of people gradually began to rise. At this time, Changsha gradually opened up various transportation hubs according to the number of daily patients, and gradually restored social order. While restoring production, conduct strict medical observations for migrant workers, implement necessary nucleic acid testing, strengthen volunteer services, and strictly grasp the diagnosis, symptoms, and discharge standards. And on February 29, 20 COVID-19 nucleic acid detection point hospitals and third-party detection institutions were identified. From March 7, 2020 to March 14, 2020 is the fourth stage. At this time, the pressure of overseas imports began to appear, and Changsha began to establish a working mechanism for the strict prevention of overseas epidemic imports, epidemic spread, and epidemic rebound, and strict inspection and quarantine and transshipment of foreign entry personnel. (Figure 2).

Models and statistical analysis Estimated R t
This study describes the age and gender distribution of COVID-19 cases in Changsha City.
According to an article published in the New England Journal of Medicine by the CDC team, the inter-generational time of COVID-19 conforms to the gamma distribution, with mean and standard deviations of 7.5 and 3.4 [8], respectively, using R version 4.0.0. Based on the COVID-19 case data reported by Changsha City, calculate the change of Rt and its 95% confidence interval for the whole period of Changsha City.The effective reproduction number(R t ) is defined as the mean number of secondary cases generated by a typical primary case at time t in a population, calculated for the whole period over a 7-day moving average.
In the formula, ) (  is the gamma function with the parameter  ；the shape parameter  and the inverse scale parameter  have the following simple relationship with the mean  and Standard Deviation  of the gamma distribution,which can be used 8 for calculation  and  : Suppose we want to calculate the effective regeneration number at time t (unit: day), that is, to calculate the average number of infected people at time t. Considering the infected individual u, whose infection time t t u  , it is necessary to investigate how likely the infected individual is to be infected by the patient u within t after For each patient infected at time t, such an expected value can be calculated, so the effective regeneration number at time t can be approximated as: In the formula,  t R represents the average number of infected individuals on day t before T and T, which can be estimated directly by the Willinge-Teunis method. Therefore, this study mainly uses formula (5) and formula (6) to calculate Rt.

SEIAR model
A SEIAR model was established to simulate the transmission of SARS-CoV-2 in Changsha. Population in this model was divided into five categories according to the disease status: susceptible (S), exposed (E), infected (I), asymptomatic (A) and recovered (R). The model was developed based on the following facts or assumptions, which assumed that some individuals moved among categories because of infection or recovery: 1) Susceptible person (S) was assumed to have an equal infected rate (β s ) with the symptomatic infected person (I) and  β S with asymptomatic infected person (A); 2) After infected, the exposed person (E) would turn to I or A after a certain exposed period After the isolation measures in Changsha City, the main framework of the model remains unchanged, and some parameters in the model are changed (Figure 4 c) While once symptomatic infected person was diagnosed, they would be isolated, the 1/γ 2 represented the time from onset to diagnosis of symptomatic infected person.
From the actual data, we have calculated that the average time from onset to diagnosis was 3 days, so, the infectious period of I was γ 2 =1/3; While those asymptomatic infected person would not be easily found and isolated, the recovery day was equal to 14 days [9], so, the infectious period of A was γ 1 =1/14.

d)  is the isolation rate of residents in Hunan Province during the COVID-19
epidemic.This value cannot be accurately estimated, so use simulation to adjust. e)  is the number of daily contacts of COVID-19 patients.This value cannot be accurately estimated, so use simulation to adjust.
f)  is the efficiency of virus transmission.This value cannot be accurately estimated, so use simulation to adjust.

DISCUSSION
In this study, we found that the number of male patients was similar to female, which was different from the patients in Wuhan [5,6], but the same with the results of the whole country [10,11]. The possible reason is that Wuhan, as the birthplace of the pathogen, has extremely tight medical resources and a growing number of patients, which will inevitably lead to missed diagnosis and death omissions, so it may be different from other places , further studies are needed to find out whether there are differences between men and women. People of all ages were generally susceptible to SARS-CoV-2, and the age of patients in various time periods is mainly concentrated in 40-59 years old, which is consistent with the results of other provinces in China [10,11].
This means that the epidemic is a major threat and challenge for all mankind and must be strictly controlled,especially for those who suffer from basic diseases of the elderly is the most important epidemic prevention.Since the sealed off Wuhan on January 23, according to Baidu's migration map, a few people have returned to Changsha for Spring Festival reasons and some early patients may have come from or contact with Wuhan or other areas in Hubei, but the latter patients were mostly local transmission patients.
Since the first case of COVID-19 in Changsha was diagnosed on January 21, 2020, a series of preventive and control measures were taken to deal with it, most of the patients were found and diagnosed at an earlier time, which would reduce the proportion of severe patients and reduce the harm of this disease. conditions for the epidemic, the spread of the disease will proceed at a very fast rate, but after effective prevention and control measures are taken, the spread of the disease can be effectively blocked [12,13]. Take Changsha as an example，after the closure of the city on February 27, the spread of COVID-19 was effectively suppressed.
After simulating the COVID-19 epidemic in Changsha, it was found that reducing epidemic. Currently, many countries and regions are facing public health emergencies similar to those in Changsha from January to March. such as isolating cases, increasing social distance (such as less going to crowd gathering places,eliminating group activities, etc.), improving personal hygiene behavior (wearing masks, washing hands frequently)etc.

CONCLUSIONS
Through the analysis of related disease prevention measures, public health interventions in Changsha City, such as: blockade of cities, isolation of residents, closure of public places, etc., can effectively control the epidemic of COVID-19. These