| The novel coronavirus pneumonia(COVID-19)epidemic has become a public health topic of greatest concern to researchers in recent years.Compared with many pandemics in history,COVID-19 has a greater ability for sustained transmission.And under the background of large-scale inhibition measures and vaccination,its development trend is still very changeable and uncertain.However,China not only took the lead in the successful control of the epidemic in the early stages of the outbreak,but also achieved good results in the following regular prevention and control phase.Therefore,through a retrospective analysis of the initial epidemiological characteristics of COVID-19 cases,studying the early transmission characteristics of COVID-19,exploring the impact of prevention and control measures on virus transmission,as well as exploring the prevention and control strategies of COVID-19 by establishing mathematical models combined with statistical analysis methods and making prevention and control recommendations based on the actual transmission of the epidemic in China during the normalized epidemic prevention and control work,all is of great significance both now and in the future.The main work of this dissertation is as follows:The first part analyzed the epidemiological characteristics of the initial COVID-19 epidemic in Chongqing in 2020,mainly including descriptive statistical analysis and estimation of important epidemiological parameters.Firstly,based on the retrospective and complete data of COVID-19 in Chongqing,the descriptive statistical analysis was applied to explore the epidemiological characteristics of the initial phase of COVID-19 outbreak in Chongqing,and to obtain the distribution of confirmed cases in time,space and population,so as to understand the characteristics of cases in the initial stage of the outbreak.Then,based on the time-series data of the cases,the likelihood functions were constructed to estimate the probability distribution of incubation period,from symptom onset to hospitalization and from symptom onset to diagnosis.Assuming that the probability distribution was gamma distribution,Weibull distribution or lognormal distribution,and then according to the Akaike information criterion(AIC criterion),the model with the lowest AIC value was selected as the optimal model.Furthermore,divided the cases into two parts by gender,before and after the "city closure" in Wuhan as well as the time point of activation of Level-1 response to major public health emergencies in Chongqing,and a comparative analysis of these transmission parameters was conducted to explore the impact of public awareness and prevention and control measures on these parameters.Finally,the basic reproduction number and effective reproduction number were estimated based on the maximum likelihood idea and Bayesian framework,and the effect of prevention and control strategy to deal with the epidemic of COVID-19 in Chongqing was evaluated and analyzed from a combination of quantitative and qualitative analysis.The results showed that the optimal fitting distribution of incubation period of early COVID-19 patients in Chongqing was gamma distribution,with an average of 7.22 days.The estimation of female patients was 7.36 days,slightly larger than that of male patients,and there was no significant difference between male and female patients.The optimal distribution of the interval from onset to hospitalization was gamma distribution,with an average period of 4.78 days.For the patients with the disease before January 23,the optimal fitting probability distribution was Weibull distribution,with an average of 5.94 days,which was about 1.5 times that of the patients after January 23.It showed that the "city closure" policy adopted in Wuhan and the Level-1 response measures implemented in Chongqing have played a good role in the timely isolation of patients.The optimal distribution of the interval from onset to diagnosis was gamma distribution,with an average of 5.47 days,and the interval before January 23 was 1.5 times higher than that of patients after onset.At the same time,it was calculated that the basic reproduction number of Chongqing was 2.38(95%CI,1.85 to 3.01).Through the above results,it can be found that the COVID-19 had a strong transmission ability in the early stages of Chongqing,2020.But Chongqing took timely prevention and control measures and the effect was obvious,and finally completed the goal of zero-COVID.The second part provided a preliminary study of the exit strategy of dynamic zeroCOVID policy in the normal prevention and control stage.Dynamic zero-COVID policy is the basic strategy for prevention and control of COVID-19 pandemic in China which depends on the citywide test-trace-isolation(CTTI).It is essential to estimate the number of CTTI rounds required and project the number of infected during implementation for the optimal allocation of medical resources for the policy.Based on the idea of infinitesimal method,the properties of binomial distribution and single-objective optimization model,two estimation models of the minimum rounds of CTTI and the expectation of detected infected numbers were constructed and evaluated in comparative analysis and empirical simulations under two scenarios of whether there was infection transmission during CTTI.Comparative analysis showed that ignoring the infection during the CTTI,the minimum rounds of CTTI would be underestimated.The minimum rounds of CTTI and the expectation of detected infected numbers were independent of the total population.We can completely adjust the CTTI strategy to key areas test-trace-isolation(KTTI)strategy,which coincided with the requirements of the testing scope in Notice on issuing the implementation Guide of Regional novel coronavirus Nucleic Acid testing Organization(third Edition)issued by China’s National Health Commission on March 22,2022.Meanwhile,the minimum rounds were negatively correlated with the sensitivity of nucleic acid detection and the intensity of isolation control during CTTI,and the formulas were obtained for the critical sensitivity of nucleic acid detection and the critical intensity of isolation control during CTTI,which was the threshold to achieve the goal of dynamic zero-COVID.The empirical evaluation and simulation of the model were consistent with three cases,including Delta mutant in Xi’an in December 2021,Omicron mutant in Chongqing in March 2022 and Shenzhen in January 2022.The simulation results showed that the intensity of isolation control during CTTI had the greatest impact on the estimated value of the minimum rounds of CTTI,and the time of the first local infection to be found during REPC was exponentially correlated with the total number of infections detected.In particular,the concepts of the critical value of nucleic acid detection sensitivity,the herd isolation threshold during CTTI or KTTI and the critical intensity of isolation control proposed in this study have certain practical significance for public health and epidemic prevention,especially the herd isolation threshold corresponding to the concept of immunization threshold in "herd immunity" is not the final critical value to achieve the prevention goal of dynamic zero-COVID.Compared with it,the smaller critical intensity of isolation control is the minimum isolation control intensity to ensure that the goal of zero-COVID strategy is achieved,which is worthy of further validation and promotion. |
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