Projections of the incidence of COVID-19 in Japan and the potential impact of a Fall 2023 COVID-19 Vaccine

Abstract

Background: The study objective was to estimate the incidence of COVID-19 infection, hospitalization, and deaths in Japan from September 2023 to August 2024 and potential impact of a Fall 2023 COVID-19 vaccine for adults 18 years and older on these outcomes. Methods: A previously developed Susceptible Exposed Infected Recovered model for the United States (US) was adapted to Japan. The numbers of symptomatic infections, COVID-19 related hospitalizations, and deaths were calculated. Given differences in vaccination coverage, masking practices and social mixing patterns between the US and Japan, all inputs were updated to reflect the Japanese context. Vaccine effectiveness (VE) values are hypothetical, but predicted based on existing VE values of bivalent BA.4/BA.5 boosters against BA.4/BA.5 in Japan, from the VERSUS test-negative case-control study. Sensitivity analyses were performed. Results: The base case model predicts overall that there will be approximately 35.2 million symptomatic COVID-19 infections, 690,000 hospitalizations, and 62,000 deaths in Japan between September 2023 and August 2024. If an updated COVID-19 vaccine is offered to all adults aged 18 years and older in Fall 2023, the model predicts that 7.3 million infections, 275,000 hospitalizations and 26,000 deaths will be prevented. If vaccines are only given to those aged 65 years and older, only 2.9 million infections, 180,000 hospitalizations and 19,000 deaths will be prevented. Sensitivity analysis results suggest that hospitalizations and deaths prevented are most sensitive to initial vaccine effectiveness (VE) against infection and hospitalizations, and the waning rate associated with VE against infection. Symptomatic infections prevented was most sensitive to initial VE against infection and VE waning. Conclusions: Results suggest that a Fall 2023 COVID-19 vaccine would reduce total numbers of COVID-19 related infections, hospitalizations, and deaths.

The potential economic impact of the updated COVID-19 mRNA fall 2023 vaccines in Japan

“…Please see the previous clinical manuscript for further details. 28 Vaccine coverage was also reduced in scenarios to 50% and 75% of base case values. The initial VE of the Moderna vaccine and the monthly waning rates were varied as described in the vaccine section above.…”

“…The base case incidence projections and all of the incidence scenario analyses have been previously described. 28 In the current analysis, we use all of the same inputs from the previous Japanese analysis except for VE as described below. As described in the previous analysis, we assume uptake of the Fall 2023 vaccine by age is similar to the first COVID-19 booster in Japan.…”

“…As described in the previous analysis, we assume uptake of the Fall 2023 vaccine by age is similar to the first COVID-19 booster in Japan. 28 The VE against infection reduces the incidence of asymptomatic or symptomatic infection on each day of the simulation. The average incremental protection against hospitalization due to COVID-19 infection is also calculated on a monthly basis within the simulation and used within the infection consequences model.…”

“…The age-dependent probabilities of hospitalization in the unvaccinated were derived previously. 28 The percent of cases treated in the ICU care were calculated based on an insurance claims analysis of the DeSC database. 36 Hospitalization probabilities are reduced according to VE for those who are vaccinated; VE is assumed to vary by vaccination strata and is calculated within the SEIR model.…”

“…estimating the corresponding clinical impact provides the uptake over time. 28 The inputs to the SEIR model were varied in several scenarios. The cost-effectiveness of vaccination was tested using the six different incidence strategies that were developed in Kohli et al 28 These scenarios were created by changing the calibration process (Adjusted Tokyo data 2.5X or 1.5X; Adjusted Tokyo data 2.0x with Revised Waning), a scenario assuming change in contact patterns during the year (Seasonality: phi=0.2), and two further scenarios assuming that a new variant that reduced vaccine VE and natural immunity evolved (immune escape assumed to happen either April 2024 or June 2024).…”