Background
・It is important to forecast the rate of severe illness and the case fatality rate during the sixth wave in considering the response to it from the perspective of “balancing measures against infections and socioeconomic activities.”
・One guideline is to “continue social, economic, cultural and educational activities as much as possible while avoiding overwhelming the medical system.”
・Accurate forecasts of the rate of severe illness and the case fatality rate lead to the accurate prediction of when the medical system is likely to be overwhelmed.
・At the beginning of the sixth wave, we presented forecasts of the rate of severe illness and the case fatality rate and established a monitoring site.
・January 10, 2022 “The rate of severe illness and the case fatality rate during the sixth wave” (Taisuke Nakata and Wataru Okamoto)
・January 10, 2022 “Monitoring of the rate of severe illness and the case fatality rate during the sixth wave” (Taisuke Nakata and Wataru Okamoto)
・January 13, 2022 “Trends in the estimated values of the rate of severe illness and the case fatality rate during the sixth wave” (Taisuke Nakata and Wataru Okamoto)
・This document verifies the forecasts presented in January ex post facto based on the actual data taken during the sixth wave.
Important points
・Although the forecasts presented in January indicated that the rate of severe illness and the case fatality rate during the sixth wave would be considerably lower than those during the fifth wave, the actual rates of severe illness and fatality during the sixth wave were significantly lower.
・The actual rate of severe illness was consistent with the optimistic scenario and the actual case fatality rate was consistent with the basic scenario.
・It is highly possible that the low rate of severe illness was due to the essential nature of the Omicron variant.
・Real-time estimates of the rate of severe illness converged virtually with the final value more than two months before the wave was brought under control.
・The rate of severe illness converged virtually with the final value on January 24 (40 days after the start of the wave).
・The convergence of the case fatality rate was in early March (80 days after the start of the wave).
・The same trend was found not only during the sixth wave, but also in past waves.
Important points
・At the beginning of the sixth wave, the dominant opinion was that it would be difficult to grasp the rate of severe illness and the case fatality rate until the wave had been brought under control completely, but it was possible to establish a rough forecast from various information even in the initial stage of the wave.
・Partly on the basis of such uncertainty, a strong policy on the restriction of action was adopted during the sixth wave.
・It is recognized that there is academic value in waiting for the wave to come under control before estimating the rate of severe illness and case fatality rate accurately.
・On the other hand, policymakers do not have the leeway to wait for a wave to be brought under control completely.
Comparison of forecasts of the rate of severe illness in Tokyo and the actual rate
Factor analysis of the rate of severe illness
・Basic scenario: 0.15%
・Consistent with reality, the proportion of people vaccinated twice was 75% 63%: 0.166% (+13%)
・Consistent with reality, the proportion of elderly people was 10% 9.8%: 0.164% (-1.2%)
・Estimated rate of severe illness during the sixth wave overall: 0.037% (-77%)
・Reasons why the rate of severe illness during the sixth wave overall was much lower than the basic scenario
・Hypothesis [1]: The rate of severe illness (relative to the D variant) in unvaccinated people with the O variant is lower than that in the basic scenario
・If this parameter is changed from 30% 7%, it is consistent with the rate of severe illness during the sixth wave overall.
・This hypothesis implies that the low rate of sever illness was due to the essential nature of the Omicron variant.
・Hypothesis [2]: The rate of severe illness with the O variant after being vaccinated twice (relative to unvaccinated people infected with the O variant) is lower than that in the basic scenario.
・Even if this parameter is changed from 40% 0%, the rate of severe illness only decreases from 0.164% 0.098%.
・Consequently, it is difficult to explain the low rate of severe illness with hypothesis [2] alone.
・Hypothesis [3]: Both factors [1] and [2] contribute.
Comparison of forecasts of the case fatality rate in Tokyo and the actual rate
Factor analysis of the case fatality rate
・Basic scenario: 0.094%
・Consistent with reality, the proportion of people vaccinated twice was 75% 63%: 0.107% (+14%)
・Consistent with reality, the proportion of elderly people was 10% 9.8%: 0.105% (-1.9%)
・Estimated case fatality rate during the sixth wave overall: 0.109% (+3.8%)
・If the actual proportion of positive cases is taken into account, the basic scenario and the actual case fatality rate are almost the same.
・However, the two remaining parameters in the basic scenario are not necessarily correct.
Even if the parameters of the basic scenario are changed as follows,
case fatality rate (relative to the D variant) in unvaccinated people with the O variant: 30% 40%
case fatality rate (relative to the case fatality rate in unvaccinated people with the O variant) after two vaccinations with the O variant: 40% 15%,
the same case fatality rate is obtained.
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Estimated rate of severe illness during the sixth wave
Estimated case fatality rate during the sixth wave
Proportion of elderly people (60 years old or older) among newly positive people during the sixth wave
Proportion of people vaccinated twice among newly positive people during the sixth wave
