Analysis of Omicron variant: cost of booster inoculation and contact isolation

Summary of December 2021 Publication
　・The effect of booster vaccination 6 months after the 2nd vaccination is outstanding
　・The spread of infections can be prevented if booster vaccination is implemented 8 months after the 2nd vaccination, vaccine passport is issue to those who received a booster vaccination, and the number of people who have not received a booster vaccination going out decreased by 50%

・The level of the number of infected and severely ill people varies greatly depending on parameters such as infectivity and severity of illness.

＜Possibility that the December 2021 publication may have estimated infectivity to be higher than it actually is＞
・Failed to account for the generation time which is half of the delta variant
・The test failed to account for the fact that the intense contacts of the positive person also received medical examinations.

＜Possibility of high estimated rate of severe illness＞
・The probability of moderate and severe disease was set equal to that of the delta variant.

＜Update of Simulation and Consideration of Social Impact＞
・In light of the above considerations, downward revision of infectivity and risk of serious illness of Omicron variant; shortened generation time
・Scheduled booster vaccination (starting in February, average of 85,000 doses/day in order of completion of second vaccination)
・Consideration of the degree of social activity slowdown brought about by quarantine of contactees

・Qualitative results were maintained after changing parameters
　・Effect of shortening the time to booster vaccination to 6 months
　・Effect of reducing the number of outings by 50% using a vaccine passport system for those who have not been vaccinated with a booster

・Low infectiousness and severity rates indicate that if booster vaccinations from February proceed as planned, it may be possible to respond without strong behavioral restrictions, including passports.

・“Quarantine of persons in close contact with a positive person” has a strong effect of preventing the infection spread, but is likely to result in a huge number of quarantined persons, and there are concerns about its impact on the economy.

・Model for Tokyo based on “Relaxing behavioral restrictions following the spread of vaccination” (released as COVID-19 AI & Simulation Project on August 24)
・Individual attributes: age, gender, industry, occupation, frequency of eating out
・Individual probability of infection and serious illness determined by age (in 10-year increments) and vaccination status
・Situations of in-person contact: homes, schools, workplaces, facilities for the elderly, restaurants, and other contacts with unspecified number of persons

・Chiba, Asako. 2021. "The effectiveness of mobility control, shortening of restaurants' opening hours, and working from home on control of COVID-19 spread in Japan" Health & Place 70: 102622.
・Chiba, Asako. 2021. "Modeling the effects of contact-tracing apps on the spread of the coronavirus disease: Mechanisms, conditions, and efficiency" PLoS ONE 16(9): e0256151.
・Reference: Kerr et al. (2020)

※１　The period from the time of exposure to the onset of disease. ※２　It is assumed that 90% of all persons with whom a test-positive person had contact in the home, workplace, school, or elderly care facility 7 days prior to the date of positive exposure, and 10% of those in restaurants or other contacts, will be tested as a contactee. In accordance with this assumption, the basic infectivity of the virus has been revised downward from the data released on 25-Dec-2021.

・Daily and by age group (in 10-year increments) in Tokyo reflects actual vaccination results
・Booster vaccinations will be administered from early February 2022
・The average immunization pace of approximately 85,000 persons/day was conducted in the order of completion of the second round of vaccinations

→　Average number of days to receive booster vaccination after the second vaccination is 236 days.

※３　In the version submitted on 25 December, 2021, vaccination status and booster schedule are rounded to the nearest month. Booster vaccination was fixed at 6 or 8 months after the second dose.

Qualitative results from the December 2021 publication hold with different parameters:
・Shortening the booster vaccination to 6 months later will significantly reduce the number of people infected
・The spread of infections can be prevented with the passport if "half of those who have not received a booster vaccination are allowed to go out compared to the normal level".

・In the model, if the complementation rate of contactees increases to nearly 100%, the source of infection can be eliminated at an early stage, resulting in fewer people being quarantined.
・In reality, it is difficult to achieve a nearly 100% coverage rate, resulting in a huge number of quarantined individuals.
・Relative cost of quarantine increases when risk of severe illness is extremely low

・Qualitative results were maintained after changing parameters
・Effect of shortening the time to booster vaccination to 6 months
・Effect of reducing the number of outings by 50% using a vaccine passport system for those who have not been vaccinated with a booster

・Low infectiousness and severity rates indicate that if booster vaccinations from February proceed as planned, it may be possible to respond without strong behavioral restrictions, including passports.

・“Quarantine of persons in close contact with a positive person” has a strong effect of preventing the infection spread, but is likely to result in a huge number of quarantined persons, and there are concerns about its impact on the economy.