# Preplanned Studies: Impact of Regions with COVID-19 Cases on COVID-Zero Regions by Population Mobility — Worldwide, 2021

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• ### Summary

Reducing population mobility and increasing the vaccination rate for severe acute respiratory syndrome coronavirus 2 can decrease the transmission of coronavirus disease 2019 (COVID-19).

What is added by this report?

In order to reduce the incidence of COVID-19 to the levels of influenza after restoring normal mobility, the efficacy against infection needs to be increased to 40% and the efficacy against symptomatic disease needs to be increased to 90%. The efficacy against infection has a more important impact compared to efficacy against symptomatic disease or death on the transmission of COVID-19 at the population level.

What are the implications for public health practice?

The population should continue maintaining non-pharmaceutical interventions and minimize international movement to prevent transmission of COVID-19. Furthermore, developing new vaccines or promoting booster vaccinations should be considered to increase efficacy.

•  [1] Elliott P, Haw D, Wang HW, Eales O, Walters CE, Ainslie KEC, et al. Exponential growth, high prevalence of SARS-CoV-2, and vaccine effectiveness associated with the Delta variant. Science 2021;374(6574):eabl9551. http://dx.doi.org/10.1126/science.abl9551CrossRef [2] Jara A, Undurraga EA, González C, Paredes F, Fontecilla T, Jara G, et al. Effectiveness of an inactivated SARS-CoV-2 vaccine in Chile. N Engl J Med 2021;385(10):875-84. http://dx.doi.org/10.1056/NEJMoa2107715CrossRef [3] National Immigration Administration of China. Key data on immigration management in the first half of 2021. 2021. https://www.nia.gov.cn/n897453/c1431039/content.html. [2021-10-10]. (In Chinese). [4] National Immigration Administration of China. In 2019, the number of inbound and outbound visits reached 670 million. 2021. http://www.gov.cn/xinwen/2020-02/15/content_5479396.htm. [2021-10-10]. (In Chinese). [5] U.S. Center for Disease Control and Prevention. COVID-19 pandemic planning scenarios. 2021. https://www.cdc.gov/coronavirus/2019-ncov/hcp/planning-scenarios.html#table-1. [2021-10-10]. [6] WHO. Report of the WHO-China joint mission on coronavirus disease 2019 (COVID-19). Beijing. 2020. https://www.who.int/publications/i/item/report-of-the-who-china-joint-mission-on-coronavirus-disease-2019-(covid-19). [2021-10-10].
• FIGURE 1.  Diagrams of the COVID-19 epidemic model. (A) the diagram of transregional mobility of population, (B) epidemic dynamics model compartments.

Note: S, Iasym, Isym, R, and D desperately denoted the susceptible, asymptomatic infective, symptomatic infective, removal and dead. And the v subscript indicated COVID-19 vaccination. Abbreviation: COVID-19=coronavirus disease 2019.

FIGURE 2.  Prediction of COVID-19 epidemic after restoring population mobility without NPIs. (A) The number of daily infections, (B) The number of cumulative infections with time..

Note: Blue denoted the asymptomatic cases and red denoted the symptomatic cases. This result was based on the efficacy against infections, symptomatic disease and death was separately 30%, 68.3%, and 86%, and the vaccination rate was 95%.

FIGURE 3.  The number of COVID-19 cases in response to different vaccine protection at (A) low rate of population mobility, (B) normal rate of population mobility.

Note: The size of the circle denoted the number of COVID-19 cases and the green shadow denoted the annual incidence was lower than influenza with the corresponding efficacy against infection and symptomatic disease and a vaccination rate of 95%, indicated that efficacy against death has a less impact on virus transmission so we only show the prediction of 90% efficacy against death.

TABLE 1.  Estimation of COVID-19 transregional model parameters and data sources.

 Label Value Reference ${R}_{trans}$ 0.0002, 0.0013* (3-4) ${R }_{0}$ 2.5 (5) ${{R} }_{sym}$ 70% (5) ${R}_{dea}$ 2.0% Calculated † ${R}_{rem}$ 1/5.4§ (6) $\mathrm{\beta }$ 0.179 Calculated by ${R}_{0}$ ${R}_{a-s}$ 75% (6) ${\mathrm{E}}_{inf}$ 30% Based on (1) ¶ ${\mathrm{E}}_{sym}$ 65.3% (2) ${\mathrm{E}}_{dea}$ 86% (2) * 0.0002 was the number of arrivals and departures of January to June 2021 and 0.0013 was from the whole year of 2019. They represented the current population mobility rate and normal mobility rate.† ${R}_{dea}$ is the sum of deaths divided by the sum of confirmed cases in all countries.§ ${R}_{rem}$ denoted that patients presented to the doctor within 2 days since the onset of symptoms (average 3.4 days) and were no longer transmissible.¶ More details about the estimation of ${\mathrm{R}}_{inf}$ could be found in Supplementary Materials, available in http://weekly.chinacdc.cn/.

TABLE 2.  Prediction of COVID-19 epidemic by currently vaccine effectiveness and combinations of threshold efficacies that can reduce the number of infections to influenza.

 Vaccination rate (%) Mobility rate (%) Vaccine effectiveness Symptomatic cases Deaths Infection (%) Symptom (%) Death (%)* 95 0.02 30.0 65.3 86.0 64,123,171 2,039,690 95 0.02 40.0 60.0 90.0 1,054,347 28,517 95 0.02 50.0 0.0 90.0 623,887 12,906 95 0.13 40.0 90.0 90.0 706,184 40,565 95 0.13 50.0 40.0 90.0 1,105,650 31,237 95 0.13 60.0 0.0 90.0 627,634 17,808 Abbreviation: COVID-19=coronavirus disease 2019.* Indicated that efficacy against death has a less impact on virus transmission so we only showed the prediction of 90% efficacy against death.

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###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

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Article Contents

## Impact of Regions with COVID-19 Cases on COVID-Zero Regions by Population Mobility — Worldwide, 2021

View author affiliations

### Summary

Reducing population mobility and increasing the vaccination rate for severe acute respiratory syndrome coronavirus 2 can decrease the transmission of coronavirus disease 2019 (COVID-19).

What is added by this report?

In order to reduce the incidence of COVID-19 to the levels of influenza after restoring normal mobility, the efficacy against infection needs to be increased to 40% and the efficacy against symptomatic disease needs to be increased to 90%. The efficacy against infection has a more important impact compared to efficacy against symptomatic disease or death on the transmission of COVID-19 at the population level.

What are the implications for public health practice?

The population should continue maintaining non-pharmaceutical interventions and minimize international movement to prevent transmission of COVID-19. Furthermore, developing new vaccines or promoting booster vaccinations should be considered to increase efficacy.

• 1. School of Public Health, Peking University, Beijing, China
• 2. China Medical Science Press, China Health Media Group, Beijing, China
• 3. Center for Intelligent Public Health, Institute for Artificial Intelligence, Peking University, Beijing, China
• 4. Center for Drug Abuse Control and Prevention, National Institute of Health Data Science, Peking University, Beijing, China
###### & Joint first authors.
• The development of vaccines has made a great contribution to the fight against coronavirus disease 2019 (COVID-19), but it should be clearly recognized that the elimination of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) requires the combined efforts of humanity. In order to determine how the population mobility among different regions might affect the COVID-19 epidemic, we constructed a transregional mobility dynamics model to assess the relationship between population mobility and the COVID-19 pandemic. Based on a previous study on vaccine efficacy against infection in the United Kingdom (1) and a study in Chile on the vaccine protective effects of CoronaVac (2), the baseline efficacy, indicated by efficacy against infection (${\mathrm{E}}_{inf}$), efficacy against symptomatic disease (${\mathrm{E}}_{sym}$), and efficacy against death $({\mathrm{E}}_{dea}$) was respectively 30%, 68.3%, and 86%. If the vaccination rate reaches 95%, allowing for transregional movement would result in 234.2 million infections (64.1 million symptomatic cases) and 2.0 million deaths (case fatality rate of 0.85%) within a year in unaffected regions. Increasing ${\mathrm{E}}_{inf}$ and ${\mathrm{E}}_{sym}$ to no less than 40% and 90%, respectively, could reduce the incidence of COVID-19 in COVID-zero regions to influenza-like levels. When the ${\mathrm{E}}_{inf}$ of the vaccine was higher, the ${\mathrm{E}}_{sym}$ could be reduced accordingly. No matter how effective the vaccine was, it could not eliminate COVID-19 in COVID-zero regions, i.e., regions with strong national commitments to suppressing COVID-19 transmission such as China. The human race should continue to develop vaccines and explore new ways to improve vaccine protection against infection in order to eliminate COVID-19 at the global level.

In order to group the transmission risk of COVID-19 in various countries around the world, we used a proportion of existing cases in the overall population of countries to rank and group these countries and regions into three levels, including high-risk regions, medium-risk regions, and low-risk regions. COVID-zero regions were defined as countries or regions committed to reducing the number of domestic cases to zero, such as China. The countries covered by each region were shown in Figure 1A. Considering the disease characteristics of COVID-19, we added categories to include asymptomatic infected cases and divided each category into two according to vaccination status (Figure 1B). The model assumed that the mobility rate between COVID-zero regions and other areas was constant. All parameters used in this model and COVID-19 epidemic data sources were shown in Table 1, and more details of methods were shown in Supplementary Materials.

Figure 1.  Diagrams of the COVID-19 epidemic model. (A) the diagram of transregional mobility of population, (B) epidemic dynamics model compartments. Note: S, Iasym, Isym, R, and D desperately denoted the susceptible, asymptomatic infective, symptomatic infective, removal and dead. And the v subscript indicated COVID-19 vaccination. Abbreviation: COVID-19=coronavirus disease 2019.
 Label Value Reference ${R}_{trans}$ 0.0002, 0.0013* (3-4) ${R }_{0}$ 2.5 (5) ${{R} }_{sym}$ 70% (5) ${R}_{dea}$ 2.0% Calculated † ${R}_{rem}$ 1/5.4§ (6) $\mathrm{\beta }$ 0.179 Calculated by ${R}_{0}$ ${R}_{a-s}$ 75% (6) ${\mathrm{E}}_{inf}$ 30% Based on (1) ¶ ${\mathrm{E}}_{sym}$ 65.3% (2) ${\mathrm{E}}_{dea}$ 86% (2) * 0.0002 was the number of arrivals and departures of January to June 2021 and 0.0013 was from the whole year of 2019. They represented the current population mobility rate and normal mobility rate.† ${R}_{dea}$ is the sum of deaths divided by the sum of confirmed cases in all countries.§ ${R}_{rem}$ denoted that patients presented to the doctor within 2 days since the onset of symptoms (average 3.4 days) and were no longer transmissible.¶ More details about the estimation of ${\mathrm{R}}_{inf}$ could be found in Supplementary Materials.

Table 1.  Estimation of COVID-19 transregional model parameters and data sources.

Based on the study in Chile mentioned above, ${\mathrm{E}}_{sym}$and ${\mathrm{E}}_{dea}$of CoronaVac was 68.3% and 86%, respectively (2). And the ${\mathrm{E}}_{inf}$ was estimated to 55% by the study produced in the United Kingdom (1). Considering the declining immunity in the vaccinated population, ${\mathrm{E}}_{inf}$ was finally estimated to 30% (Supplementary Material). Based on the baseline efficacies of the vaccines, referred to ${\mathrm{E}}_{inf}$, ${\mathrm{E}}_{sym}$, and ${\mathrm{E}}_{dea}$ were 30%, 68.3%, and 86%, respectively, the global vaccination rate could reach 95% and then people in other areas could move within COVID-zero regions without non-pharmaceutical interventions (NPIs). Among COVID-zero regions, a total of 234.2 million SARS-CoV-2 infections were projected to occur in one year, of which 170.1 million (72.6%) were asymptomatic cases and 64.1 million (27.4%) are symptomatic cases (Figure 2A), with a total death count of 2.0 million (case fatality rate 0.85%). The highest number of new symptomatic cases per day was estimated 376,600, which would appear on the Day 262 after lifting the restrictions and the highest number of deaths per day was 11,400, which would appear on the Day 266 (Figure 2B) in COVID-zero regions.

Figure 2.  Prediction of COVID-19 epidemic after restoring population mobility without NPIs. (A) The number of daily infections, (B) The number of cumulative infections with time.. Note: Blue denoted the asymptomatic cases and red denoted the symptomatic cases. This result was based on the efficacy against infections, symptomatic disease and death was separately 30%, 68.3%, and 86%, and the vaccination rate was 95%.

On the basis of maintaining the current mobility rate, we explored what kind of combination of vaccine protective efficacy could lift the restrictions. Due to the COVID-19 vaccine having an ${\mathrm{E}}_{dea}$ of 86%, we presented results that assumed ${\mathrm{E}}_{dea}$ to be 90% (Figure 3A). Predictions of other combinations of vaccine protective efficacy could be found in the Supplementary Materials. The results suggested that in order to reduce the annual incidence of COVID-19 to influenza, the vaccine’s ${\mathrm{E}}_{inf}$ needed to be increased to more than 40% concurrently with an ${\mathrm{E}}_{sym}$ of at least 60%. If the ${\mathrm{E}}_{inf}$ was increased to 50%, the ${\mathrm{E}}_{sym}$ could decrease to 0%. In addition, when the 3 efficacies reached at least 90%, the annual incidence of COVID-19 was decreased to 0.81/100,000 population.

Figure 3.  The number of COVID-19 cases in response to different vaccine protection at (A) low rate of population mobility, (B) normal rate of population mobility. Note: The size of the circle denoted the number of COVID-19 cases and the green shadow denoted the annual incidence was lower than influenza with the corresponding efficacy against infection and symptomatic disease and a vaccination rate of 95%, indicated that efficacy against death has a less impact on virus transmission so we only show the prediction of 90% efficacy against death.

Finally, we assessed what protective efficacy would be required to restore the population mobility to ensure that the incidence of COVID-19 could be lower than that of influenza. After restoring the population mobility rate of 2019, a higher vaccine protective effect was required. In order to reduce the annual incidence of COVID-19 to that of influenza, the vaccine’s ${\mathrm{E}}_{inf}$ and ${\mathrm{E}}_{sym}$ needed to be increased to no less than 40% and 90%, respectively. With a higher ${\mathrm{E}}_{inf}$, the ${\mathrm{E}}_{sym}$ could be decreased (Figure 3B). When the ${\mathrm{E}}_{inf}$ was increased to 70%, the ${\mathrm{E}}_{sym}$ could be decreased to 0 (Table 2).

 Vaccination rate (%) Mobility rate (%) Vaccine effectiveness Symptomatic cases Deaths Infection (%) Symptom (%) Death (%)* 95 0.02 30.0 65.3 86.0 64,123,171 2,039,690 95 0.02 40.0 60.0 90.0 1,054,347 28,517 95 0.02 50.0 0.0 90.0 623,887 12,906 95 0.13 40.0 90.0 90.0 706,184 40,565 95 0.13 50.0 40.0 90.0 1,105,650 31,237 95 0.13 60.0 0.0 90.0 627,634 17,808 Abbreviation: COVID-19=coronavirus disease 2019.* Indicated that efficacy against death has a less impact on virus transmission so we only showed the prediction of 90% efficacy against death.

Table 2.  Prediction of COVID-19 epidemic by currently vaccine effectiveness and combinations of threshold efficacies that can reduce the number of infections to influenza.

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