Graduate School of Business Sciences, University of Tsukuba
Summary
・We compared the number of new positive cases against each of the following criteria: infectivity of the Omicron variant in Tokyo, start date of the 3rd vaccination, and restrictions based on vaccination certification.
・When the infectivity of the Omicron variant was 1.32 times (reported in South Africa), it was estimated that the number of new positive cases could be controlled to 1,800 per day by administering a 3rd vaccination within 210 days to all vaccinated persons.
・It was estimated that 2.0-fold increase in the infectivity of the Omicron variant (reported in the UK) could result in 9,100 new positive cases/day, even if a 3rd vaccination was administered within 210 days to all vaccinated individuals and with 80% of restaurants and event venues utilizing the vaccination/negative certification package. In addition, it was suggested that the number of new positive cases could be controlled to 4,500/day if the 3rd vaccination rate (rate of people receiving a 3rd vaccination among those who received a 2nd vaccination) could be fully implemented at 100%.
・In the case of a 2.0-fold increase in infectivity, the effect of setting the start of the third vaccination to 180 days is significant, however, if the start date is delayed to February, the spread of infection cannot be contained. However, we assumed that the speed of the 3rd vaccination was set at the same level as this year.
・The following 3 points are considered effective control measures:
・100% complete implementation of a 3rd vaccination within 210 days for all ages among those who received the 2nd vaccination
・At least 80% implementation of vaccination/negative proof package at restaurants and event venues
・3rd vaccination after 180 days, starting mid-January
Comparison of the Number of New Positive Cases
Comparison of 3rd vaccination and vaccination certificate restrictions
Infectivity relative to Delta 1.32 times
If the Omicron variant is 1.32 times more infectious than the Delta variant (as reported in South Africa), the number of new positive cases could be reduced to 1,800 per day by administering a 3rd vaccination within 210 days to all vaccinated persons.
Infectivity relative to Delta 1.32 times
A 3rd vaccination for all vaccinated persons within 210 days, as well as implementation of vaccination/negative proof package at restaurants and event venues, will further reduce the number of new positive cases.
Infectivity relative to Delta 1.5 times
If the Omicron variant is 1.5 times more infectious than the Delta variant, the number of new positive cases is 3,700/day when the 3rd vaccination is administered within 210 days to all vaccinated persons.
Infectivity relative to Delta 1.5 times
Even if the Omicron variant is 1.5 times more infectious than the Delta variant, if the vaccination/negative certification package can be implemented at the 80% level at restaurants and event venues, the number of new infections during the peak can be reduced to about 2,400 people/day.
Infectivity relative to Delta 2.0 times
If the Omicron variant is 2.0 times more infectious than the Delta variant (as reported in the UK), the number of new positive cases is expected to increase quite substantially compared to the summer of 2021.
Infectivity relative to Delta 2.0 times
If the Omicron variant is 2.0 times more infectious than the Delta variant (as reported in the UK), it is not sufficient to implement the vaccination/negative proof package to 80% of the population, and furthermore, it is estimated that the number of new infections during the peak period would be controlled to about 4,500 by increasing the 3rd vaccination rate.
Infectivity relative to Delta 2.0 times
It is important to start the 3rd vaccination as soon as possible.
Model Settings
1.Infection model by SEIR mathematical model and AI optimization method
The SEIR model, which takes into account population flow and AI technology (evolutionary optimization + quasi-Newton method), were used to optimize infection model estimation within and between three age groups (0-39 years, 40-59 years, and 60 years or older). The number of positive patient inflows from outside the prefecture was estimated from mobile spatial statistics data (NTT Docomo) and LocationMind xPop*1 and incorporated into the model, and the model was trained from data from March 1 to October 30, 2021.
2.Effects of vaccine and behavior change
・Vaccine efficacy was 33% for the 2nd and 75% for the 3rd dose against the Omicron variant. The Omicron variant was assumed to have started to infect 5 people via community spread on December 10.
・Measured values are used for changes in the effective reproduction number and the number of populations flows from March 1 to December 18. The effective reproductions number and risk of inflow of infected persons were estimated assuming that the population remaining in downtown areas (13:00, 19:00, and 21:00) and the population coming from outside Tokyo were at the same levels as in November after December 19.
・Vaccine decline effect
Assumed were a 66% decline at 180 days, taking the simple average of these reports that the vaccine was 64% effective in controlling infection in June 2021 when the Delta variant occurred (Israel Ministry of Health), 78% at 90 days for the Alpha variant (Oxford, Nature), 65.5% at 90 days (NEJM), and declined to 82% at a median of 143 days (US CDC). Booster vaccination is assumed to start on December 1 from those who have passed 180 and 240 days after the 2nd vaccination, and is assumed to return to the level of vaccine effect immediately after the 2nd dose.
・https://www.gov.il/en/departments/news/05072021-03,https://www.gov.il/en/departments/news/06072021-04
・Resurgence of SARS-CoV-2 Infection in a Highly Vaccinated Health System Workforce, DOI: 10.1056/NEJMc2112981, The new England journal of medicine
・COVID vaccines protect against Delta, but their effectiveness wanes, doi: https://doi.org/10.1038/d41586-021-02261-8, Nature
・Comparative Effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) Vaccines in Preventing COVID-19 Hospitalizations Among Adults Without Immunocompromising Conditions — United States, March–August 2021, CDC vol.70, 17, Sep. 2021
3.Tokyo suburbs agent base model
We constructed a model represented by 1,348 agents in 2 cities based on the household structure of a Tokyo suburb, and restricted workplaces, restaurants, and event venues by vaccination proof, and measured the rate of change in Rt from 500 trials each. Companions to eat and drink together were assumed to be chosen randomly from a network of friends (the number of friends was a power-law distributed Galton-Watson network).
*1 "LocationMind xPop" data is data that NTT DOCOMO collectively and statistically processes from mobile phone location information sent with permission from users of applications* provided by NTT DOCOMO. Location information is GPS data (latitude and longitude information) that is measured at a minimum of every five minutes, and does not contain information that identifies an individual. DOCOMO Map Navi Service (map application, local guide) and some other applications
Change in population remaining in downtown Tokyo at 21:00 and the human flow into the city
Antibody possession rate by vaccination
The time series of the effectiveness of vaccination in preventing the onset of illness is expressed as the amount of antibodies possessed (the number of people who have vaccine effectiveness).
Since vaccine effectiveness is expected to decline, the overall preventive effect of the vaccine is shown by the bold lines in the figures.
