Department of Information Systems Science, Soka University
Background and summary
■On July 12, a declaration of a state of emergency was issued for the 4th time.
■If we were able to sustain Japanese domestic vaccinations in real terms on a scale of 1.5 million people per day, we would reach an inoculation rate of 70% by the latter half of August.
■However, in order to achieve an inoculation rate of at least 60%, it would appear that some kind of instructional measures will be necessary.
■An Israeli bulletin* stated that the Pfizer vaccine was 64% effective in preventing infection by the Delta variant.
■We will compare differing situations where the final vaccination rate is in the range of 70% to 100%.
■Assuming potential movement restrictions that could be enacted starting in September, we will refer to exit strategies.
Simulations
■We conducted 4 simulations where the final vaccination rates were 70%, 80%, 90%, and 100% of the population.
■We assumed that the frequency of gatherings following the declaration of a state of emergency would be around 137% of the previous time. (0.27% → 0.37%)
■The simulations were to last until January of next year.
■Assuming that the declaration of a state of emergency would take place by August 22, we conducted 3 simulations regarding movement restrictions after the end of the subsequent Paralympic Games. (Pages 3 and 4)
■We will conduct a simulation that assumes that movement restrictions will be lifted by the end of October, and that there will be an increased likelihood of coming into contact with other people at the end of the year, like there was last year. (Page 5)
■We ran the simulations 64 times for each setting, and calculated the mean and standard deviation.
■We compared the transitions in the number of positive patients and the number of infected individuals.
Simulation results: (1) Loosening of movement restrictions following the end of the Paralympic Games to around 80% of normal time
■After reaching the peak in the middle of October, the large number of infected individuals will persist. (Frequency of gatherings 4%)
■With an inoculation rate of 70%, there is a temporary drop from the peak, but it then goes immediately back up, and the elevation continues on in that manner.
■Higher inoculation rates can suppress the number of infected individuals after the peak, but even with an inoculation rate of 100%, a high level of infection persists.
Simulation results: (2) Movement restrictions ongoing following the end of the Paralympic Games
■A high level of infection persists with the restrictions based on the degree of implementation of priority measures for prevention of the spread of disease. (Frequency of gatherings 2%, upper graph)
■Even with enhanced restrictions in place, it is not bringing the infections to an end. (Frequency of gatherings 1%, lower graph)
Simulation results: (3) If restrictions are lifted at the end of October, and there is an increased likelihood of coming into contact with other people at the end of the year
■After conducting a simulation with movement restrictions (frequency of gatherings 2%) from the implementation of priority measures for prevention of the spread of disease by the end of October, we assumed that the restrictions would be lifted and things would return to normal.
■Assuming that there was an increased likelihood of coming into contact with other people at the end of the year, like there was at the end of the previous year.
■Even with an inoculation rate of 90%, we achieve a number of infected individuals similar to that in January of this year.
What the results of the simulations suggest
■A relaxation of the movement restrictions is linked to immediate spread, due to the Delta variant infection spreading with increasing speed and the drop in efficacy of the vaccine in preventing infection.
→ From September on, measures that accompany the movement restrictions will also be needed.
■The inhibitory effect from movement restrictions due to the implementation of priority measures for prevention of the spread of disease is not sufficient.
■If there is a high vaccination rate, the number of infected individuals can also be suppressed to a certain degree. However,
■even if every person was able to get vaccinated, the infections would persist, since they do not have a 100% efficacy in preventing infection.
■It would appear that we should aim for an exit strategy not by the total eradication of the novel coronavirus, but by building a framework for sustainable everyday prevention, testing, and treatment.
Supplemental information: Assumptions in the simulation and how to interpret the graph
■■ Assumptions in the simulations based on these materials
✓SimEpidemic model based on（ http://www.intlab.soka.ac.jp/~unemi/SimEpidemic1/info/simepidemic-model.html ）
✓In addition, with regard to the Delta variant, we assumed that the probability of daily infection when unvaccinated and uninfected individuals come into contact with infected individuals would be 84.375% with the Delta variant (assuming 50% with the conventional variant and 75% with the Alpha variant, and we assumed it would be almost replaced during the 20-day period after June 8.
✓So that we could conform to the transition of the weekly mean number of positive patients by July 16, we mainly adjusted the transition of the “frequency of gatherings” parameter, and established a subsequent scenario for its continuation.
✓Other details can be found at http://www.intlab.soka.ac.jp/~unemi/SimEpidemic1/info/simepidemic_sim_emg2104F.html
※1: A parameter of the individual base infection simulator used in this study, SimEpidemic （http://www.intlab.soka.ac.jp/~unemi/SimEpidemic1/info/simepidemic-model.html ）.
The modeled individuals reproducing the phenomenon of people gathering establish the frequency by which the force that draws people to the location of the gathering is generated in simulation time steps.
