Analysis and forecast of COVID-19 spread from the end of 2021 to the beginning of 2022

Advisory board of Ministry of Health, Labour and Welfare (MHLW) (Dr. Takayama: Okinawa). P4 (The proportion of unknown people is large)
・https://www.mhlw.go.jp/content/10900000/000877245.pdf
○If the vaccination rate of the population aged 60 or older is simplified into 90%, the infection-prevention effect on omicron will be 82%. Considering the Waning effect in Japan, it is equivalent to the effect on the delta variant.
○The vaccination rate of the population aged 20 to 60 is simplified into 75%. The infection-prevention effect on omicron is 66%. The Waning effect on delta is about 70% (assuming four months or longer). Keep about 80–90%
Effectiveness of BNT162b2 Vaccine against Omicron Variant in South Africa
Almost equivalent to the model of P1 (reported by this group on December 9)About 70 to 80%
https://www.nejm.org/doi/full/10.1056/NEJMc2119270?query=recirc_mostViewed_railB_article

Model of the vaccination effect on the delta variant (Japanese version) (material of the Cabinet Secretariat on December 9)
The vaccination effect is assumed to keep its peak for 14 days after vaccination and then attenuate. Back-calculated the prevention effect based on the numbers reported at the advisory board of MHLW. The waning effect on the elderly in the real environment is not strong. It is stronger than in other countries. Possible causes include wearing of a mask, differences in activities and communities, etc.

Model of the vaccination effect on the omicron variant (refer to 12/14): No need for a change
According to the reference, it is assumed that the prevention effects of the second and third vaccination on the delta variant are 65% and 75%, respectively. It is assumed that the decrease in the vaccination effect reaches its peak 14 days after vaccination as with the delta variant and then attenuates by 25% in half a year (linearly decreases to 40% and 50%).（Reference：https://khub.net/documents/135939561/430986542/Effectiveness+of+COVID-19+vaccines+against+Omicron+variant+of+concern.pdf/f423c9f4-91cb-0274-c8c5-70e8fad50074 ）

Since the vaccination rate of people aged 65 or older is high, the infection-prevention effect per capita is strong. As a result, the individual-level trend is similar (difference in communities)

Assume the timing at which to start the third vaccination is December 1, and assume the number of vaccinated people on the corresponding date before it will increase immediately after the start. (Start date of the second vaccination: Tokyo from 2021/3/10 and onward.)

The estimated value in the figure is the proportion of the omicron variant announced by Tokyo (per week) interpolated and smoothed and then multiplied by the number of new positive cases.
Though the proportion of the delta variant decreased, the total number did not decrease.
・The increase in the number of positive cases at the beginning of the year also contributed to rediffusion of the delta variant (strong vaccination effect)
・Increase in the number of infected people from January 4 to January 8 in Tokyo (simple multiple; high statistical instability because it is the beginning of the year)
　Delta variant: Twice the number of positive cases, Omicron variant: Four times the number of positive cases
The increased speed of the delta variant is equivalent to a rapid rise in July (four consecutive holidays) in the 5th wave. (The (approximate) difference in the effective reproduction number in appearance is 1.5 to 1.7)
* 12/28 MHLW ADB (66th) Reports by Dr. Nishiura and Dr. Suzuki: Differences among generations)
・Unlike July, rose at a high speed regardless of the infection-prevention effect of vaccination. Mainly related to “behaviors”
https://www.bousai.metro.tokyo.lg.jp/_res/projects/default_project/_page_/001/020/811/67/20220107.pdf

Report on 11/16: 370. Report on 12/20: 140 (corrected the vaccination effect from the data of MHLW ADB)
Reproduced using the actual data of 1/11: 206 (associated with behaviors)

Made a calculation assuming cases where having a meal together with other people and flows of people are controlled from January 11 (reddish brown) and recover (blue), respectively.
It was assumed that the third vaccination would be started on February 1 and the infectability of the omicron variant was 1.5 times stronger than that of the delta variant.
The vaccination rate of the third vaccination is the same as that of the second vaccination. If it is 80% or higher, the difference in the result will be minor.
*The inflow of infected people during the year-end and new-year holidays is assumed to be approximate (linear and constant: 5–10)

https://www.nejm.org/doi/full/10.1056/NEJMc2119270?query=recirc_mostViewed_railB_article

The proportion of seriously/moderately ill patients is estimated to be about 2/3 to 2/5 among hospitalized patients.

１．Spreading rapidly due to factors such as the inflow from infected regions at the end of the year or behaviors of people waiting to enter or return to Japan.
２． The proportion of posts related to “year-end party” in SNS is three times larger than that of the previous year. That of the New Year party is also about twice that of the previous year (report by Dr. Toyoda of University of Tokyo; NHK-related site)
https://www3.nhk.or.jp/news/html/20211229/k10013409101000.html

The increase in flows of people and the reservation condition of transportation facilities can be confirmed by multiple methods.
Item 1 is a hypothesis because there were few people at first. Without this hypothesis, the effective reproduction number of the omicron variant is extremely high (10 or more; uncertain because the population is small).
Scenario 2 is related to behaviors, and this relationship with the number of new positive cases has already been reported (strong relationship with the nighttime retention population, etc.).
(Reference) 80% in Nagoya “returned home and had a meal together during the year-end and new-year holidays”
(80% of 196 infected people: 154) Announcement by Nagoya: Site related to Tokai Television
https://news.yahoo.co.jp/articles/1de6cd3a871609c502e7caa3c0db871e6a01a2c6

・There is a possibility that the decrease in the number of new positive cases appeared to be drastic due to (1) an end to the temporary increase in the effective reproduction number associated with consecutive holidays in the expansion period and (2) the infection-prevention effect of vaccination in addition to the decrease in contact opportunities.
・Used machine learning to virtually reproduce cases where (1) vaccination did not proceed from July 11 and onward, where (2) behaviors do not change, and where (3) there is no natural infection, and approximated the relationships based on the effective reproduction number between July 23 and September 20, 2021. Effect to reduce the effective reproduction number
(Approximation: July 23 to September 20, 2021)

Learning up to April 2021. After that, actual data is used as much as possible for reproduction.
* Equivalent flows of people to the average value between January 6 and February 6, 2020.
** Twitter data is virtually set based on past data. Habits before the coronavirus catastrophe such as having a meal together without a mask are not considered.
***The number of asymptomatic infected people is assumed to be four times larger than the number of positive cases (approximate).
The table shows the average values of one week with a big reduction effect.
Reduction effect = [-(R t(Prediction of the observed value)-Rt(Each condition))/Rt(Each condition)]
Rt: Effective reproduction number
*The impact of the long vacation is limited to August 1 to 4 (equivalent to July 22 to 25)
*Estimated based on the case of London (material of the AI periodic meeting of the Cabinet Secretariat/Hirata September 27)
***The weather conditions are the temperature and humidity only. The weather is not considered.
Modified the report material of the periodic meeting for the COVID-AI simulation of the Cabinet Secretariat (December 14, 2021)

Summary: Assuming that the countermeasures against infection will be continued, when the effects of new-year holidays disappear, the increase will be smaller, although it will continue to rise. The numbers of seriously and moderately ill patients (proportion) is smaller than that of the delta variant.
Excluding the differences among generations, the effective reproduction number (depending on flows of people and behaviors) of the omicron variant may be equivalent to that of the delta variant. In this case, considering the weakening of the infection-prevention effect of vaccination, it may become 1 or less with the lifestyle level of mid-November.
Contribution to shortage in society: Direct contacts of infected people > Number of hospitalized patients > Number of seriously ill patients
Delta variant
- The effective reproduction number from November to early December is 1 or less (the infection-prevention effect of vaccination is a little weaker).
- In the prediction of November (downwardly revised slightly in December), predicted the peak of the delta variant to be around January 10 assuming that no omicron would appear, and it was roughly reproduced. Peaking-out.
Omicron variant
- The infection-prevention effect of vaccination on the omicron variant may be about 70% on the delta variant. The data of Japan is limited and needs attention.
- Continue to pay attention to the generation time and effective reproduction number.