SEIR-type Models And Data Analysis Of Non-pharmaceutical Interventions And Vaccination Strategies In COVID-19

COVID-19 has caused heavy losses to human life and property.Mathematical models play a vital role in critical issues like analyzing the risk of epidemic transmission,evaluating the effectiveness and timeliness of various non-pharmaceutical interventions(NPIs),forecasting and early warning of epidemic situation,and reallocating public health resources.Currently,vaccine's ready make it possible to reopen or alleviate NPIs,however,removing the intervention too early or too late may bring us adverse effects,such as re-outbreak,economic recession and so on.Therefore,it's of great research value to analyze and design NPIs exit strategies and vaccination program,and to study how vaccination can make up for the risk of secondary outbreak caused by intervention alleviation.Firstly,based on the classical SEIR compartment model and actual data,with considering the dynamic changes of contact rate and detection rate caused by interventions,a non-autonomous SEIAR transmission model with NPIs was established in the second chapter.Through sensitivity analyses we evaluated the impact of contact limitation,detection and diagnosis,and individual behavior change due to media coverage on the epidemic.According to the estimated parameter values,we calculated the basic reproduction number to be 4.32 before the national lockdown in Italy.The mainly conclusions of chapter 2 are that Italy was in a severe epidemic status and the control measures were not sufficiently timely and effective in the beginning.Non-pharmaceutical interventions,including contact restrictions and improvement of case recognition,play a big part in containing the COVID-19 epidemic.To simulate stepwise NPIs exit strategies and vaccination program at present,in the third chapter,we designed a dynamic balance control strategy that combined multiple stepwise non-pharmaceutical intervention relaxation with vaccination(called EVS mechanism).By means of parameter estimation,sensitivity analysis and threshold analysis,we studied the strength of non-pharmaceutical interventions in six typical countries such as China and the US,the dynamic balance relationship between vaccine coverage and the intervention relaxation intensity,and the influence of EVS mechanism start time and vaccination rate on the COVID-19 trend.China needs the highest vaccine coverage to balance the NPIs exit,which fully reveals the importance of strong interventions in early China.Different vaccination time and vaccination rate have a huge impact on the EVS mechanism,for example:under the given exit strategy 1(ES1),the number of newly cases in stage 1 can be reduced is 232 times as much as that in stage 3 by adding 1%vaccine rate;and under the given ES2,a secondary epidemic with a peak of 17 520 occurred when the vaccine coverage rate changed to 0.2 times of the original set,this peak value was much larger than that the original scenario(2613).The difference of vaccine coverage between Indonesia,Russia and the UK for balancing the two NPIs exit strategies is less than 0.1,the cumulative vaccine coverage required by the UK and the US to balance the second wave of outbreaks is far lower than that of the first wave.For countries in descending period at present,if they keep the intervention like before,there will be no secondary epidemic.The minimum vaccine coverage required for Brazil and the US to avoid a secondary outbreak under ES1' was 0.92 and 0.18,respectively;while the minimum vaccine coverage rates required for China,Brazil and US to avoid a re-outbreak under the more relaxed NPIs exit strategie(ES2')were 0.41,0.95 and 0.95.The results in Chapter 3 reveals the normalized prevention and control strategies(the EVS mechanism)can well replace the complete non-pharmaceutical measures.Especially,if a country can't control the first epidemic wave in a short time,it is more likely that a new epidemic with a larger peak will break out again.When the vaccine is limited,it is necessary to maintain non-pharmaceutical intervention.For countries in ascending period at present,vaccination can reduce the peak number of new confirmed cases and make it arrive earlier;for countries countries in descending period,there is an effective cumulative vaccine coverage threshold,when the vaccine supply is greater than the threshold,there will be no re-outbreak,otherwise there will be a secondary outbreak.