| Objectives:1.To master the epidemiological characteristics of COVID-19 cases imported overseas by land in Yunnan Province,and provide scientific basis for adjusting the prevention and control strategy of COVID-19 imported overseas.2.To master the genome evolution and variation of SARS-Co V-2 strains imported from overseas by land in Yunnan Province.To provide theoretical support for preventing the transmission of imported cases from abroad,and also provide scientific basis for tracing the source of local epidemic transmission caused by imported cases from abroad.Methods:1.Based on existing circumstances research,this study collected information about COVID-19 cases imported by overseas land routes reported by Yunnan Province from January 2021 to December 2021.Descriptive statistical analysis methods were used to analyze the time,space,population and other epidemiological characteristics of the cases.The cases data were sorted out with Excel software,and the data were statistically analyzed with statistical software SPSS 21.0.The cases data was sourced from YNCDC and the infectious disease report information management system of the China Disease Prevention and Control Information System.Information about the age,gender,occupation,importing country or region,confirmed date,discharge date,case type,vaccination and other information of the COVID-19infected cases(confirmed cases and asymptomatic infected persons)were collected.2.The throat swab samples of COVID-19 cases imported by land from January2021 to December 2021 reported in Yunnan Province were reviewed by RT-PCR method.The case samples were from the CDC of 8 border cities in Yunnan Province.Using the Mini Seq sequencer of Illumina to sequence 387 positive samples meeting the sequencing requirements,and analyzing the genotyping and variation characteristics.The SARS-Co V-2 Wuhan-Hu-1 genome in GISAID website was taken as the reference sequence.Nexttrade online analysis platform(https://classes.nextstrain.org/)was used for sequence alignment,and Pangolin online typing platform(https://Pangolin.cg-uk.io/)was used for genome typing;Selecting the gene sequences of epidemic strains from different countries and regions in the same period in the GISAID database,using the MEGA7.0 software to construct the genetic phylogenetic tree,and analyze the genotyping and variation characteristics of the virus strains.Results:1.From January 1,2021 to December 31,2021,Yunnan Province reported 1527cases of COVID-19 infection imported from overseas by land,including 1190confirmed cases and 337 asymptomatic infections.The minimum age was 3 months,the maximum age was 87 years,and the average age M(IQR)was 28(11)years;Cases in the age group of 20 to 39 years accounted for 77.67%of the total number of infected cases(1186/1527);There were 1245 males and 282 females,with a sex ratio of 4.41:1;The top three occupations were 44.01%(672/1527)in business services,16.04%(245/1527)in catering and food industry,and 19.91%(304/1527)in farmers/migrant workers/workers.There was no statistically significant difference in age distribution between confirmed cases and asymptomatic infected persons(Z=-1.564,P=0.118);There was no statistical difference between sexes(χ~2=1.324,P=0.250);The difference in occupational distribution was statistically significant(χ~2=229.328,P<0.001).2.Overall,the number of newly infected cases in 2021 was increasing first and then slow down.From January to December,there were imported cases from overseas by land.From January to May,the number of imported cases was mainly sporadic.From June,the number of imported cases began to rise,and reached a peak from July to September,and decreased month by month from October to December.At the peak of the year,the cumulative land import cases from July to September accounted for62.93%(961/1527)of the total land import cases of the year.3.A total of 1315 cases were imported from Myanmar,accounting for 86.12%.The top three states and cities in the number of imported case reports were Dehong61.69%(942/1527),Baoshan 14.54%(222/1527)and Xishuangbanna 13.75%(210/1527).The total incidence rate of land entry throughout the year was 0.8%.Dehong Prefecture had the highest incidence rate of 6.21%(942/15165).Ruili City,a total of 935 cases,accounting for 61.19%of the province.Fujian had the largest number of inbound infected persons,with 276 cases,accounting for 18.07%.4.According to clinical classification,the cases were divided into asymptomatic infection,mild infection,moderate infection and severe infection,accounting for 22.07%(337/1527),48.66%(743/1527),28.42%(434/1527)and0.85%(13/1527)respectively.5.1524 cases imported from abroad,the average cure time was 21(17)days.In confirmed cases,the shortest cure time was 2 days and the longest was 138 days;The shortest cure time for asymptomatic infected patients was 2 days and the longest was87 days.There was no statistical difference in the average cure time between the confirmed cases and asymptomatic infected patients(Z=-0.698,P=0.485).6.In 1476 cases,11.79%have been vaccinated once,22.63%have been vaccinated twice,and 0.81%have been vaccinated three times.There was a negative correlation between infection degree and vaccination dose(r=-0.176,P<0.001).7.The 171 SARS-Co V-2 complete genome sequences with coverage of more than 99%can be divided into 9 branches according to Pangolin typing method:119sequences belong to branch B.1.617.2,26 sequences belong to branch B.1.36.16,8sequences belong to branch B.1.1.7,8 sequences belong to branch AY.30,4 sequences belong to branch AY.85,3 sequences belong to branch AY.31,and 1 sequence belongs to branch AY.23,branch AY.24,branch AY.57 respectively.8.171 cases were diagnosed from March to December.From March to June,B.1.36.16 branch was the main branch,and from April to June,B.1.1.7 branch(alpha variant)was detected.The branch of B.1.617.2(Delta variant)began to appear in June,and then increased month by month,reaching the peak in July and August.After September,the evolutionary branches of AY.23 and AY.24,which were based on B.1.617.2,appeared successively.9.In the phylogenetic tree jointly constructed by 171 whole-genome sequences and the epidemic sequences downloaded from the GISAID database in various countries of origin,a total of 207 sequences formed many different evolutionary branches.The 171 virus genomes determined this time were distributed in different evolutionary branches of the evolutionary tree.Compared with Wu Han-Hu-1reference strain,171 SARS-Co V-2 sequences were different,and the sequences in each branch were homologous.10.Compared with Wuhan reference strain,171 whole genome sequences belonged to five clusters:21A(Delta),21I(Delta),21J(Delta),20I(Alpha,V1)and20A.Compared with the branching clusters(21A,21I,21J)belonging to the Delta variant,the alpha variant had 29 molecular marker sites.There were 55 molecular marker sites in Delta mutant.There were 16 molecular markers in cluster 21A,cluster21I and cluster 21J in Delta mutant.Conclusions:In 2021,COVID-19 cases imported from overseas by land in Yunnan Province are mainly young men aged 20-39 who are engaged in business and work.The peak of case report is from July to September.The cases mainly come from Myanmar.The incidence rate of people entering Ruili,Dehong Prefecture is the highest.We should strengthen the standardized management of economic and trade exchanges between port cities in border areas,and formulate targeted epidemic prevention and control policies based on regional geographical characteristics and cross-border economic and trade characteristics.In 2021,COVID-19 cases imported by land in Yunnan Province will still be mild and moderate.Vaccination is still an effective way to prevent COVID-19 infection and reduce the severity of disease after infection.We should do a good job in the future to strengthen needle immunization and speed up the construction of population immunity barrier.The whole genome sequence of the virus obtained in this study is homologous with the Alpha strain,Delta strain and progeny that have been prevalent in Myanmar,Thailand,Laos,Vietnam and Cambodia in the same period,with B.1.617.2 as the main branch.The time distribution of variation of SARS-Co V-2 mutants imported by land in Yunnan Province in 2021 is consistent with the iteration time node of SARS-Co V-2 mutants in the world.Various types of COVID-19 variants prevalent in surrounding countries at the same time cause a high risk of imported epidemic in Yunnan Province.We should strictly do a good job in border epidemic prevention and control,and reduce the risk of outbreaks in local communities caused by imported variants.In 2021,among the SARS-Co V-2 mutants imported by land from Yunnan Province,the nucleotide mutation sites of Delta mutants are significantly more than those of Alpha mutants,and the nucleotide mutation of each branch subtype of Delta mutants is highly random and active.In view of the characteristics of constant renewal and rapid transmission of the variant strains,regular and fixed monitoring sentinels should be set up throughout the province to monitor the variation trend of each variant of COVID-19. |
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