Forecast of severe illness rate, case fatality rate and hospitalization rate during the 7th wave

Date: 2022.07.05
Researcher: Taisuke Nakata
Organization: Graduate School of Economics, Faculty of Economics, The University of Tokyo

Analysis

・In this document, we summarized quantitatively the determinants of the rate of severe illness during the 7th wave.

・We estimated how the following four factors contribute to the “average rate of severe illness.”
　・Proportion of elderly people
　・Effect in prevention of severe illness (after infection) of 2nd dose
　・Effect in prevention of severe illness (after infection) of 3rd dose
　・Effect in prevention of severe illness (after infection) of 4th dose

・We observed closely how far the rate of severe illness decrease during the 7th wave compared with the 6th wave.
　・We present three scenarios < optimistic, basic and pessimistic >

・We performed the same analysis for the hospitalization rate and the case fatality rate.

Important points

・The rate of severe illness, the case fatality rate and the hospitalization rate are likely to be slightly lower compared to the 6th wave, but it is unlikely they will be significantly lower.
　・In the basic scenario, it is about 0.7 to 0.8 times the level of the 6th wave, and in the optimistic scenario, it is about 0.5 times.

・The possibility that it will be higher than the 6th wave cannot be excluded.
　・In the pessimistic scenario, it is about 1.5 to 2 times.
　・Risk factors such as a high proportion of elderly people among positive cases, a rapid decline in vaccine efficacy, and the potential for a high rate of severe illness with the BA4 and BA5 variants.

Assumptions

・* The period of the 6th wave is defined as from December 15, 2021 to April 30, 2022.

・We assume the 7th wave epidemic will be from July to September this year.

・We assume that the prevalent strain is Omicron and that the rate of severe illness, the case fatality rate and hospitalization rate will be unchanged from the 6th wave.

・We assume that the final vaccination rate with a fourth dose among the elderly is 75%.

・We assume that the final vaccination rate with a fourth dose among the non-elderly is 20%.

・The protective effect of vaccination against infection, hospitalization, severe illness and death deteriorates with time. We estimated the speed of that decline from Japan data.

Vaccination rate assumptions

[Elderly] We assume that recipients of a 3rd vaccination will receive the 4th vaccination 180 days later. Ultimately, 75% overall complete the 4th vaccination.

[Non-elderly] We assume that recipients of a 3rd vaccination will receive the 4th vaccination 180 days later. Ultimately, 20% overall complete the 4th vaccination.

Vaccination rate assumptions

* In calculations, we use the average value during the wave period.

Infection prevention effect assumptions

* See page 21
Calculated by the authors based on “Vaccination status by age group in positive, severe, and fatal cases” presented by Dr. Suzuki, Advisory Board, Ministry of Health, Labour and Welfare

Rate of severe illness during the 7th wave

3 scenarios (rate of severe illness)

* p. 25 We estimated the risk ratio based on Ministry of Health, Labour and Welfare “Rate of severe illness and case fatality rate during the 6th wave (provisional version)”
(Figures in [ ] correspond to the 90% confidence interval)

Results (rate of severe illness, Tokyo standard)

Results (rate of severe illness, other standard)

For the rate of severe illness under the national standard, we used the final convergence values of “Rate of severe illness and case fatality rate during the 6th wave: Tokyo” (Taisuke Nakata and Wataru Okamoto) https://covid19outputjapan.github.io/JP/icudeathmonitoring.html の最終的な収束値を使用。

・The new Tokyo standard means “the number of severely ill patients based on the characteristics of the Omicron strain,” a new monitoring item introduced by the Tokyo metropolitan government in February 2022. We prepared new time series data on the number of severely ill patients based on Tokyo Metropolitan Government press releases and estimated the rate of severe illness during the 6th wave. For the estimation method, refer to “Monitoring of the rate of severe illness and the case fatality rate during the 6th wave” (Taisuke Nakata and Wataru Okamoto).

・We assumed that the "“relative rate of severe illness compared with the 6th wave " is the same as the Tokyo standard under all standards.

Case fatality rate during the 7th wave

3 scenarios (case fatality rate)

Result (case fatality rate)

・The case fatality rate in the 7th wave may be slightly lower compared to the 6th wave, but it is unlikely to decrease significantly. ・In the pessimistic scenario, it is 2.0 times the 6th wave, and the possibility of a higher case fatality rate than the 6th wave cannot be excluded.

Hospitalization rate during the 7th wave

3 scenarios (hospitalization rate)

Results (hospitalization rate)

・The hospitalization rate in the 7th wave may be slightly lower compared to the 6th wave, but it is unlikely to decrease significantly.
・In the pessimistic scenario, it is 1.6 times the 6th wave, and the possibility of a higher hospitalization rate than the 6th wave cannot be excluded.

・Note that the hospitalization rate may vary greatly depending on changes in admission standards.

Basic information on the 6th wave

Proportion of people vaccinated among positive cases

6th wave average

[Less than 65 years of age]
Unvaccinated / 1 dose: 41.8%
2 doses: 52.5%
3 doses: 5.8%

[65 years of age or older]
Unvaccinated / 1 dose: 10.3%
2 doses: 64.9%
3 doses: 24.8%

* Prepared by the authors based on “Vaccination status by age group among positive, severe and fatal cases” submitted by Dr. Suzuki, Advisory Board, Ministry of Health, Labour and Welfare https://www.mhlw.go.jp/content/10900000/000853141.pdf (p.59)

Proportion of elderly people among positive cases (6th wave)

Prepared by the authors based on the “details of announcements of people newly testing positive for COVID-19” in Tokyo
https://catalog.data.metro.tokyo.lg.jp/dataset/t000010d0000000068

Calculated by the authors (excluding cases of unknown vaccination history) based on “Vaccination status by
age group among positive, severe and fatal cases” submitted by Dr. Suzuki, Advisory Board,
Ministry of Health, Labour and Welfarehttps://www.mhlw.go.jp/content/10900000/000853141.pdf (p.59)

Proportion of elderly people among positive cases (most recent)

Prepared by the authors based on the “details of announcements of people newly testing positive for COVID-19” in Tokyo
https://catalog.data.metro.tokyo.lg.jp/dataset/t000010d0000000068

Infection prevention effect of vaccines during 6th wave (estimated values)

* Calculated by the authors based on “Vaccination status by age group among positive, severe and fatal cases” submitted by Dr. Suzuki, Advisory Board, Ministry of Health, Labour and Welfarehttps://www.mhlw.go.jp/content/10900000/000853141.pdf (p.59)
* Note that these are results based on simple estimates that take into account the ratio of the number of positive cases to the population of each group.

Severe illness prevention effect of vaccines during the 6th wave (estimated values)

Fatality prevention effect of vaccines during 6th wave (estimated values)

Trend in infection prevention effect of vaccines (estimated values)

Severe illness and fatality prevention effect of vaccines during the 6th wave (estimated values)

* Prepared by the authors based on Ministry of Health, Labour and Welfare “Rate of severe illness and case fatality rate during the 6th wave (provisional version)”https://www.mhlw.go.jp/content/10900000/000929082.pdf (p.59)
* Estimates of the rate of severe illness and the case fatality rate among 119,109 people infected with COVID-19 during the period from January 1 to February 28, 2022 in the three prefectures of Ishikawa, Ibaraki and Hiroshima, by age group and vaccination history

Taisuke Nakata is supported by JSPS Grant-in-Aid for Scientific Research (KAKENHI), Project Number 22H04927, the Research Institute of Science and Technology for Society at the Japan Science and Technology Agency, COVID-19 AI and Simulation Project (Cabinet Secretariat), the Center for Advanced Research in Finance at the University of Tokyo, and the Tokyo Center for Economic Research.