
The development of vaccines has made a great contribution to the fight against coronavirus disease 2019 (COVID19), but it should be clearly recognized that the elimination of severe acute respiratory syndrome coronavirus 2 (SARSCoV2) requires the combined efforts of humanity. In order to determine how the population mobility among different regions might affect the COVID19 epidemic, we constructed a transregional mobility dynamics model to assess the relationship between population mobility and the COVID19 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 COVID19 in COVIDzero regions to influenzalike 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 COVID19 in COVIDzero regions, i.e., regions with strong national commitments to suppressing COVID19 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 COVID19 at the global level.In order to group the transmission risk of COVID19 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 highrisk regions, mediumrisk regions, and lowrisk regions. COVIDzero 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 COVID19, 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 COVIDzero regions and other areas was constant. All parameters used in this model and COVID19 epidemic data sources were shown in Table 1, and more details of methods were shown in Supplementary Materials.
Figure 1.Diagrams of the COVID19 epidemic model. (A) the diagram of transregional mobility of population, (B) epidemic dynamics model compartments.
Note: S, I_{asym}, I_{sym}, R, and D desperately denoted the susceptible, asymptomatic infective, symptomatic infective, removal and dead. And the v subscript indicated COVID19 vaccination. Abbreviation: COVID19=coronavirus disease 2019.Label Value Reference ${R}_{trans}$ 0.0002, 0.0013* (34) ${R }_{0}$ 2.5 (5) ${{R} }_{sym}$ 70% (5) ${R}_{dea}$ 2.0% Calculated ^{†} ${R}_{rem}$ 1/5.4^{§} (6) $ \mathrm{\beta } $ 0.179 Calculated by ${R}_{0}$ ${R}_{as}$ 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 COVID19 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 COVIDzero regions without nonpharmaceutical interventions (NPIs). Among COVIDzero regions, a total of 234.2 million SARSCoV2 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 COVIDzero regions.Figure 2.Prediction of COVID19 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 COVID19 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 COVID19 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 COVID19 was decreased to 0.81/100,000 population.Figure 3.The number of COVID19 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 COVID19 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 COVID19 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 COVID19 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: COVID19=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 COVID19 epidemic by currently vaccine effectiveness and combinations of threshold efficacies that can reduce the number of infections to influenza.
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