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Outbreak Reports: Response and Assessment of the Effectiveness of the Countermeasures for a COVID-19 Outbreak — Guizhou Province, China, March 2022

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  • Summary

    What is already known about this topic?

    Many regions in China have recently reported outbreaks of the coronavirus disease 2019 (COVID-19) caused by the Omicron variant.

    What is added by this report?

    Wuchuan County, Guizhou Province reacted quickly and implemented accurate intervention measures to effectively control the outbreak. The susceptible-exposed-infectious-asymptomatic-removed (SEIAR) model was applied to evaluate the effectiveness of intervention measures.

    What are the implications for public health practice?

    Fast response measures should be taken to prevent the spread of outbreaks caused by the Omicron variant.

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  • Funding: Supported by Qian Ke He Support Plan (2021)-Normal 027
  • [1] Tan ZL, Chen ZX, Yu AP, Li XY, Feng YN, Zhao X, et al. The first two imported cases of SARS-CoV-2 Omicron Variant - Tianjin Municipality, China, December 13, 2021. China CDC Wkly 2022;4(4):76 − 7. http://dx.doi.org/10.46234/ccdcw2021.266CrossRef
    [2] Guo QF, A RH, Liang LJ, Zhao X, Deng AP, Hu Y, et al. An imported case of BA.2 lineage of omicron variant COVID-19 - Guangdong Province, China, December 28, 2021. China CDC Wkly 2022;4(5):98 − 9. http://dx.doi.org/10.46234/CCDCW2022.001.http://dx.doi.org/10.46234/CCDCW2022.001
    [3] Backer JA, Eggink D, Andeweg SP, Veldhuijzen IK, van Maarseveen N, Vermaas K, et al. Shorter serial intervals in SARS-CoV-2 cases with Omicron BA.1 variant compared with Delta variant, the Netherlands, 13 to 26 December 2021. Euro Surveill 2022;27(6):2200042. http://dx.doi.org/10.2807/1560-7917.ES.2022.27.6.2200042.http://dx.doi.org/10.2807/1560-7917.ES.2022.27.6.2200042
    [4] Boucau J, Marino C, Regan J, Uddin R, Choudhary MC, Flynn JP, et al. Duration of viable virus shedding in SARS-CoV-2 omicron variant infection. medRxiv 2022. http://dx.doi.org/10.1101/2022.03.01.22271582.http://dx.doi.org/10.1101/2022.03.01.22271582
    [5] Chen TM, Rui J, Wang QP, Zhao ZY, Cui JA, Yin L. A mathematical model for simulating the phase-based transmissibility of a novel coronavirus. Infect Dis Poverty 2020;9(1):24. http://dx.doi.org/10.1186/s40249-020-00640-3CrossRef
    [6] Lao XY, Luo L, Lei Z, Fang T, Chen Y, Liu YH, et al. The epidemiological characteristics and effectiveness of countermeasures to contain coronavirus disease 2019 in Ningbo City, Zhejiang Province, China. Sci Rep 2021;11(1):9545. http://dx.doi.org/10.1038/s41598-021-88473-4CrossRef
    [7] Liu ZR, Zhao ZY, Wu JB, Rui J, Gong L, Lin SN, et al. Evaluation of transmission dynamics and control measures of COVID-19 in Anhui Province in 2020. Anhui J Prev Med 2021;27(1):1 − 5,19. http://dx.doi.org/10.19837/j.cnki.ahyf.2021.01.001 (In Chinese). CrossRef
    [8] Liu WK, Ye WJ, Zhao ZY, Liu C, Deng B, Luo L, et al. Modelling the emerging COVID-19 epidemic and estimating intervention effectiveness - Taiwan, China, 2021. China CDC Wkly 2021;3(34):716 − 9. http://dx.doi.org/10.46234/ccdcw2021.177CrossRef
    [9] Xue L, Jing SL, Zhang K, Milne R, Wang H. Infectivity versus fatality of SARS-CoV-2 mutations and influenza. Int J Infect Dis 2022;121:195 − 202. http://dx.doi.org/10.1016/j.ijid.2022.05.031CrossRef
  • FIGURE 1.  The fitting results between SEIAR model and the actual data of COVID-19 cases in Wuchuan County. (A) The result in the first stage without interventions; (B) The result in the second stage with interventions taken in the first stage but not in the second stage.

    Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed; COVID-19=coronavirus disease 2019; CI=confidence interval.

    TABLE 1.  Definition and values of parameters in SEIAR model of COVID-19 in Wuchuan County.

    ParameterDescriptionUnitValueSource
    βThe transmission ratePerson−1×day−1Model simulating
    κThe transmissibility of A to I10.5Reference (6)
    ωThe relative number of incubation periodDay−10.33Reference (4)
    ω’The relative number of latent periodDay−10.33Reference (4)
    pThe proportion of A10.33Report data
    γThe relative rate of infection period of IDay−10.2Reference (5)
    γ’The relative rate of infection period of ADay−10.2Reference (5)
    Note: “–” means not applicable.
    Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed; COVID-19=coronavirus disease 2019; A=asymptomatic; I=infectious.
    Download: CSV

    TABLE 2.  Comparation of simulated results of the SEIAR model with the reported cases.

    Onset dateFirst stage
    (No intervention)
    Second stage
    (No intervention)
    Report cases
    New casesAccumulated
    cases
    New casesAccumulated
    cases
    New casesAccumulated
    cases
    March 6000000
    March 7000000
    March 8111100
    March 9121200
    March 10131300
    March 11252511
    March 12383812
    March 1341241246
    March 1471941617
    March 15102931929
    March 16154432209
    March 172367324312
    March 1834101227012
    March 1952153229012
    March 2078231231012
    March 21117347233012
    March 22176523235012
    March 23264787237012
    March 243971,184239012
    March 255951,779241012
    March 268912,670243012
    March 271,3324,002245012
    March 281,9825,984246012
    March 292,9358,919248012
    March 304,30913,228250012
    March 316,25319,480251012
    April 18,91728,397253
    April 212,40740,804254
    April 316,68557,488156
    April 421,43778,925157
    April 525,986104,911158
    April 629,378134,289160
    April 730,734165,023161
    April 829,702194,725162
    April 926,658221,383164
    April 1022,465243,848165
    April 1118,018261,866166
    April 1213,938275,804167
    April 1310,515286,31900
    Note: “–” means not applicable.
    Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed.
    Download: CSV

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Response and Assessment of the Effectiveness of the Countermeasures for a COVID-19 Outbreak — Guizhou Province, China, March 2022

View author affiliations

Summary

What is already known about this topic?

Many regions in China have recently reported outbreaks of the coronavirus disease 2019 (COVID-19) caused by the Omicron variant.

What is added by this report?

Wuchuan County, Guizhou Province reacted quickly and implemented accurate intervention measures to effectively control the outbreak. The susceptible-exposed-infectious-asymptomatic-removed (SEIAR) model was applied to evaluate the effectiveness of intervention measures.

What are the implications for public health practice?

Fast response measures should be taken to prevent the spread of outbreaks caused by the Omicron variant.

  • 1. Guizhou Center for Disease Control and Prevention, Guiyang City, Guizhou Province, China
  • 2. National Institute of Parasitic Diseases, Chinese Center for Tropical Diseases Research, Chinese Center for Disease Control and Prevention; NHC Key Laboratory of Parasite and Vector Biology; WHO Collaborating Center for Tropical Diseases; National Center for International Research on Tropical Diseases, Shanghai Municipality, China
  • 3. Doctorate School of Chemical and Biological Sciences for Health (CBS2), University of Montpellier, Montpellier, France
  • Corresponding author:

    Yan Huang, cdchuangyan@163.com

  • Funding: Supported by Qian Ke He Support Plan (2021)-Normal 027
  • Online Date: July 27 2022
    Issue Date: July 29 2022
    doi: 10.46234/ccdcw2022.141
  • Coronavirus disease 2019 (COVID-19) is an acute respiratory infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In China, the Omicron variant was initially detected in the respiratory tract samples of 2 imported cases in Tianjin Municipality, on December 13, 2021 (1). The first local transmission case with the Omicron variant BA.2 in China was reported on December 29, 2021, in Guangdong Province (2). Subsequently, over 30 provincial-level administrative divisions (PLADs) reported outbreaks caused by the Omicron variant. The Omicron variant BA.2 became the preponderant strain in China in just a few months. An imported case who returned to Wuchuan County, Zunyi City, Guizhou Province from Zhejiang Province was reported on March 11, 2022. The next 11 cases were successively detected from March 12 to 17, 2022 after a series of emergency measures. Wuchuan’s outbreak was effectively controlled after the implementation of comprehensive countermeasures, including closing key areas, conducting region-wide and county-wide nucleic acid screening, restricting inbound and outbound traffic, and tracking and isolating close contacts. Once the outbreak was under control, this study used the susceptible-exposed-infectious-asymptomatic-removed (SEIAR) model to evaluate the effectiveness of Wuchuan’s prevention and control measures during its March, 2022 COVID-19 outbreak caused by Omicron variant BA.2.

    • A total of 12 cases were reported in Wuchuan from March 11 to 17, 2022. Of these cases, 9 were symptomatic and 3 were asymptomatic. Overall, the full course of vaccination was administered to 11 cases, of which 6 were administered with booster injections. The initial case was detected by an individual’s nucleic acid test on March 11, 2022, and the other 11 cases were detected through close contact tracking and nucleic acid screening on March 12–15 and 17.

      Upon the occurrence of the outbreak, Guizhou Province, Zunyi City, and Wuchuan County immediately established three levels of communications linkage to implement coordinated countermeasures. They started the contingency plan and traffic control within 2 hours after confirmation of the first case. Epidemiological investigation, the checking of close contacts and other relevant persons, and the isolation of cases and close contacts were conducted right away. The epidemiological investigation in the field revealed that the first case was a courier who had lived and worked in Yuhang District, Hangzhou City, Zhejiang Province until March 7, 2022. Some key areas were divided into temporary close control areas and management control areas based on the epidemiological investigation’s risk evaluation, and one close contact of the first case was detected nucleic acid positive on March 12. The region-wide nucleic acid screening was conducted on March 13, and 4 additional cases were detected. However, there was no epidemiological correlation among these cases according to the results of the investigation. Wuchuan then extended the scope of nucleic acid screenings on March 14, and 1 case was detected by isolating close contacts and nucleic acid screening. Wuchuan’s outbound and inbound traffic were restricted on March 15, and 2 cases were detected in isolation locations on the same day. A county-wide nucleic acid screening was conducted on March 16 because of the rising number of cases, no cases were detected. A second cycle of county-wide nucleic acid screening was then conducted on March 17, and 3 cases were detected. With these additional data points, Wuchuan’s response team was able to confirm the epidemiological correlation among the 12 cases by meticulous research. Furthermore, of the 12 cases, the viral genomes of 4 cases were sequenced and all belonged to the Omicron BA.2 branch, which was homologous with the results of a recent outbreak in Zhejiang Province.

      During the period of the outbreak, the secondary close contacts were monitored and their nucleic acid results were collected via door-to-door testing; the persons at risk who resided with cases in the same space at the same time were given yellow health codes and conducted their nucleic acid tests under supervision through phone calls; and the list of close contacts in need of testing was checked daily to ensure rigorous adherence to testing requirements. Efficient nucleic acid test administration training helped support this process so that the results of nucleic acid tests were consistently available same-day. In total, 7 cycles of regional nucleic acid screening, 6 cycles of the nucleic acid screening of streets or towns that reported cases, and 4 cycles of the nucleic acid screening of the whole county were completed. In addition, the total number of individual nucleic acid screenings exceeded 1,667,000: there were 628 close contacts tracked and isolated, and 3,180 secondary close contacts were checked and monitored. If no case was detected in 4–5 cycles of nucleic acid screening over the course of 1 week, or if no close contact was added, a close control area would be unlocked after risk evaluation. However, if infections were detected in a close control area, 1–2 cycles of nucleic acid screening would be added for the area. Later, the block of the area would be removed in accordance with the results of the risk evaluation.

      The SEIAR model was developed to analyze the transmission dynamics of COVID-19 and to evaluate the effectiveness of countermeasures taken during the COVID-19 outbreak. According to the transmission mechanisms of COVID-19, people who were entered into the SEIAR model were divided into five types: susceptible (S), exposed (E), infectious (I), asymptomatic (A), and removed (R). As presented in Table 1, the parameters of the SEIAR model included β, ω, ω’, p, γ, and γ’, among which β was obtained by simulating the reported data with the model data; p was calculated in accordance with the reported data of the outbreak and set to 0.33; 1/ω and 1/ω’ were set to 3 days on the basis of a previous study (3); 1/γ and 1/γ’ were set to 5 days in accordance with a previous study (4); and κ was set to 0.5 in accordance with a previous study (5). As illustrated in Figure 1, the SEIAR model was well aligned with the data of reported cases (R2=0.605, P<0.05). The SEIAR model divided the outbreak into the natural transmission stage (before March 13, 2022), the effective control stage (March 13–15, 2022), and the entire control stage (after March 15, 2022). The effective reproduction numbers (Reff) of the 3 stages were 6.32, 0.83, and 0. After interventions such as the isolation of close contacts or the quarantining of key areas, transmission decreased and the Reff in the second stage declined by 86.87% compared with that of the first stage. Subsequently, after multiple cycles of region-wide and county-wide nucleic acid screenings and traffic restrictions, the Reff declined to 0 by the end of the control stage. As described in Table 2, the simulation results indicated that if any intervening measures were not taken in the first stage, the total number of infected cases would have increased to 1,184 and 19,480 by March 24 and 31, respectively. In stark contrast, only 12 cases were reported during the outbreak. This result indicated that 98.99% and 99.94% of the population avoided COVID-19 infection after the implementation of prevention and control measures in the first stage. Furthermore, if effective measures had not been taken in the second stage, the number of cases would have increased to 39 and 51 by March 24 and 31. After countermeasures were adopted, the actual number of reported cases was reduced by 69.23% and 76.47% relative to those simulated for the second stage. Finally, if countermeasures were not taken during these two early periods of the outbreak, the outbreak is predicted to have continued until April 12. In reality, the outbreak ended on March 17, indicating that the epidemic time was significantly shortened by the series of countermeasures implemented.

      ParameterDescriptionUnitValueSource
      βThe transmission ratePerson−1×day−1Model simulating
      κThe transmissibility of A to I10.5Reference (6)
      ωThe relative number of incubation periodDay−10.33Reference (4)
      ω’The relative number of latent periodDay−10.33Reference (4)
      pThe proportion of A10.33Report data
      γThe relative rate of infection period of IDay−10.2Reference (5)
      γ’The relative rate of infection period of ADay−10.2Reference (5)
      Note: “–” means not applicable.
      Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed; COVID-19=coronavirus disease 2019; A=asymptomatic; I=infectious.

      Table 1.  Definition and values of parameters in SEIAR model of COVID-19 in Wuchuan County.

      Figure 1. 

      The fitting results between SEIAR model and the actual data of COVID-19 cases in Wuchuan County. (A) The result in the first stage without interventions; (B) The result in the second stage with interventions taken in the first stage but not in the second stage.

      Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed; COVID-19=coronavirus disease 2019; CI=confidence interval.
      Onset dateFirst stage
      (No intervention)
      Second stage
      (No intervention)
      Report cases
      New casesAccumulated
      cases
      New casesAccumulated
      cases
      New casesAccumulated
      cases
      March 6000000
      March 7000000
      March 8111100
      March 9121200
      March 10131300
      March 11252511
      March 12383812
      March 1341241246
      March 1471941617
      March 15102931929
      March 16154432209
      March 172367324312
      March 1834101227012
      March 1952153229012
      March 2078231231012
      March 21117347233012
      March 22176523235012
      March 23264787237012
      March 243971,184239012
      March 255951,779241012
      March 268912,670243012
      March 271,3324,002245012
      March 281,9825,984246012
      March 292,9358,919248012
      March 304,30913,228250012
      March 316,25319,480251012
      April 18,91728,397253
      April 212,40740,804254
      April 316,68557,488156
      April 421,43778,925157
      April 525,986104,911158
      April 629,378134,289160
      April 730,734165,023161
      April 829,702194,725162
      April 926,658221,383164
      April 1022,465243,848165
      April 1118,018261,866166
      April 1213,938275,804167
      April 1310,515286,31900
      Note: “–” means not applicable.
      Abbreviation: SEIAR=susceptible-exposed-infectious-asymptomatic-removed.

      Table 2.  Comparation of simulated results of the SEIAR model with the reported cases.

    • Previous studies predicted the transmission of COVID-19 and assessed the effect of prevention and control measures taken during the COVID-19 pandemic by using the SEIAR model (68). After implementing prevention and control measures, this study used the SEIAR model; after finding that the model fitted the reported data well, this study divided the outbreak into three stages. The P parameter in the SEIAR model is the proportion of asymptomatic infections. The P value in our study was similar to that obtained for Taiwan, China (8), higher than that reported for Anhui Province (7), and lower than that found for Zhejiang Province (6). The differences among the P values of PLADs may be associated with local demography and economics. In this study, the outbreak was transmitted through a breakthrough infection in the first stage. Because the Reff was 6.32, the data from Wuchuan indicates that one case could have infected an average of 6.32 cases even despite high vaccination coverage rates: a value that was similar to values found in outbreaks in South Africa, the United States, and Canada (9).

      After the occurrence of the outbreak, a series of countermeasures were taken, such as the dividing and closing of key areas, the isolation of close contacts, the living at home of secondary close contacts, the conduction of regional nucleic acid screening, the starting of county-wide nucleic acid screening, and the implementation of traffic controls. The Reff value of the COVID-19 outbreak decreased gradually until the outbreak was ultimately controlled. The outbreak involved 12 cases and this number of cases was notably lower than that predicted by the SEIAR model simulation in the absence of intervention, suggesting that the countermeasures taken here were remarkably effective. This is significant because Wuchuan is one of the most important counties in Zunyi; if the implementation of countermeasures was not prompt, the outbreak may have spilled over to the Zunyi, Tongten, and Guiyang cities of Guizhou Province; the Luzhou City of Sichuan Province; or Chongqing Municipality. However, no cases spilled out in this outbreak, further confirming that the response measures enacted were reasonable. Therefore, this study suggests the following measures: 1) dividing key areas as early as possible; 2) tracing and isolating close contacts; 3) conducting county-wide nucleic acid screening; and 4) comprehensive and detailed epidemiological investigation.

      This study was subject to some limitations. It had a relatively small sample size, and it did not consider natural births and deaths. This situation might lead to a slight bias between the reality of the Wuchuan outbreak and the results simulated by the SEIAR model.

    • No conflicts of interest.

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