Simulation of future COVID-19 epidemic by vaccination coverage scenarios in Japan

Background COVID-19 continues to impose significant morbidity and mortality in Japan even after implementing the vaccination program. It would remain elusive if restrictions for its mitigation were to be lifted or relaxed in the future. Methods A simulation study that explored possible vaccination coverage scenarios and changes in the intensity of nonpharmaceutical intervention restrictions was performed to assess the impact of COVID-19 based on death count. Results Assuming the basic reproduction number of circulating viruses was 5.0, vaccines could prevent 90% of infections and 95% of deaths, and the vaccination coverage rate was high (75%, 80%, and 90% in people aged 12-39 years, 40-59 years, ≥60 years, respectively), approximately 50 000 deaths would occur over 150 days in Japan if all restrictions were lifted. Most deaths would occur among older adults, even if their vaccination coverage was assumed to be especially high. A low vaccination coverage scenario (45%, 60%, and 80% in people aged 12-39 years, 40-59 years, ≥60 years, respectively) would require periodic implementation of strict measures even if the modified lifestyle observed in 2020 was sustained and vaccines were very effective. Some restrictions could be relaxed under high vaccination coverage. However, in the worst-case scenario where vaccines had decreased efficacy, as we have observed for the Delta variant, and people lived a relaxed lifestyle, our simulation suggests that even high vaccination coverage would occasionally require strict measures. Conclusions We should carefully explore a manageable degree of restrictions and their relaxation. We will have to keep bracing for occasional surges of COVID-19 infection, which could lead to strict measures, such as those under a state of emergency. Such strategies are essential even after a wide rollout of vaccination.