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

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, I_asym, I_sym, 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.

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).