## Background

■Infection with what seems to be the Omicron variant has been spreading rapidly in Okinawa, Yamaguchi, and Hiroshima since the end of December. Infection is also expanded in Tokyo and Osaka.

■Opportunities have increased for eating and drinking in groups during the year-end, New Year holidays, and the three consecutive holidays including Coming of Age day, as well as long-distance travel due to homecoming trips.

■Pre-emergency measures were implemented in Okinawa, Yamaguchi, and Hiroshima on January 9.

■Estimates were reported of vaccine efficacy, infectivity, and the rate of serious illness from studies in South Africa, the United Kingdom, etc.

■Even though the rate of serious illness is low, an explosion of infections could put a strain on medical care due to a rapid increase in the number of patients needing hospitalization.

■Although there are still many unknowns of what would happen in Japan, it is necessary to prepare a variety of possible scenarios that should be assumed through simulations.

## Simulation

■The number of agents is 1 million. Look at the average and standard deviation of 128 trials.

■Assuming that some degree of behavioral restraint will appear after January 11.

■Assuming that in Tokyo, the final vaccination rate is assumed to be 75% of the total population.

■Assuming that the 3rd vaccination is administered 7 months after the 2nd vaccination.

■Based on the speed of the spread of infections and replacement of the variant from early to mid-January, the proliferation power is assumed to be 1.8 times that of the Delta variant.

■The effects of pre-existing immunity and medicines are assumed to be 50%.

■The speed of post-onset aggravation of symptom associated with serious illness will be examined for 1/2, 1/4, and 1/8 of the Delta variant.

■It will also be examined in the case where a 5-week behavioral restriction is implemented from January 20 and where contact tracing is discontinued from January 20.

## Simulation Result #1-1 (without specific measures: change in number of positive cases)

■A rapid spread of infections is expected.

■The number of positive cases peaks around February 12, at 1.14 ±0.20%. (approx. 159,000 cases)

## Simulation Result #1-2 (without specific measures: change in number of seriously ill patients)

■If the toxicity is 1/2, a rapid spread is expected to occur.

■If the toxicity is 1/4, the peak is around March 18, a little less than 3 times more than the 5th wave.

■If the toxicity is 1/8, the peak is about 0.0002%. (approx. 27 cases)

## Simulation Result #1-3 (change in number of infected people with a variant virus)

■Change in distribution of the number of people infected with a variant virus in the simulator since December of last year.

■The 2nd peak for the Delta variant is around February 4, at 0.009 ±0.01%. The Omicron variant at this time is 2.37 ±1.38%.

■The peak for the Omicron variant is around February 21, at 6.11 ±0.90%. (at this time, a total of about 853,000 people are infected.)

## Simulation Result #2 (when behavioral restrictions are implemented: change in number of positive cases)

■A case where restriction equivalent to pre-emergency measures is implemented for 35 days from January 20.

■It is not expected to be apparently effective to reduce a rapid spread of infections.

■The number of positive cases peaks around February 16, at 0.88 ±0.18%. (about 123,000 cases), about 77% of when not implementing the restrictions.

## Simulation Results #3 (when contact tracing is discontinued: change in number of positive cases)

■A case where no testing is performed on contacts after January 20 in addition to the same behavioral restrictions mentioned at the previous page.

■The number of positive cases peaks around February 19, at 0.82%. (about 114,000 cases) There is no difference from the case of continuing contact tracing. The standard deviation between 128 trials is about 0.2, and the difference of 0.06% is within the range of fluctuation.

## Insights from the simulation

■Based on the speed of replacement of the Omicron variant, the proliferation power of the Omicron variant is assumed to be 1.8 times stronger than that of the Delta variant, and the frequency of gatherings is adjusted.

■Considering the increase in risk behaviors during the year-end and New Year holidays, preventive measures may have been implemented more effectively than last year.

■The number of positive cases peaks in mid-February, at about 0.88 ±0.18 % (about 123,000 cases) if behavioral restrictions are implemented, whereas 1.14 ±0.20 % (about 159,000 cases) if not implemented.

■In the case of implementing behavioral restrictions, there is almost no increase in the number of infected people even if contact tracing is discontinued. <- Because of the short incubation period of Omicron variant, it is less effective to find and isolate a spreader without any symptoms that can infect many people.

## Subjects

■As the nature of the Omicron variant becomes clearer, the simulation should be updated from time to time taking into consideration such information.

■Forecasting a strain on hospitals is needed adapting to the rate of hospitalization rather than the rate of serious illness.

■It is necessary to reconsider the estimation of parameters such as the effect of acquired immunity, referring to the situation in countries where an outbreak of the Omicron variant has preceded. <- In the UK, the average weekly number of new positive cases peaked out on January 5 at around 0.27% of the population. As of January 16, 0.45% in Israel and 0.43% in France. There are signs of peaking out.

## 補足