Optimal vaccine allocation strategy: Theory and application to the early stage of COVID-19 in Japan.

In this paper, we construct an age-structured epidemic model to analyze the optimal vaccine allocation strategy in an epidemic. We focus on two topics: the first one is the optimal vaccination interval between the first and second doses, and the second one is the optimal vaccine allocation ratio between young and elderly people. On the first topic, we show that the optimal interval tends to become longer as the relative efficacy of the first dose to the second dose (RE) increases. On the second topic, we show that the heterogeneity in the age-dependent susceptibility (HS) affects the optimal allocation ratio between young and elderly people, whereas the heterogeneity in the contact frequency among different age groups (HC) tends to affect the effectiveness of the vaccination campaign. A counterfactual simulation suggests that the epidemic wave in the summer of 2021 in Japan could have been greatly mitigated if the optimal vaccine allocation strategy had been taken.

Three years of COVID-19-related school restrictions and mental health of children and adolescents in Japan.

During the 3 years of the COVID-19 pandemic, Japanese children had to live with strict mitigation measures at school, such as eating school lunches silently and wearing masks during physical exercise classes, even after those mitigation measures have been relaxed worldwide. Excursions and other school events were frequently cancelled, especially in 2020 and 2021. This study conducts a retrospective survey on school experiences to understand how the strict mitigation measures were related to children's mental health and well-being. Results revealed school excursion cancellation to be associated with a higher risk of developing depressive symptoms {odds ratio [OR] 1.543 [95% confidence interval (CI) 1.109-2.148]}, and high cancellation rate of other school events to be associated with dissatisfaction in school experience [OR 1.650 (95% CI 1.222-2.228)]. In the subsample analysis, we found that girls and children with no extracurricular activities tended to exhibit depressive symptoms due to the cancellation of school excursions. Overall, the study demonstrated that persistent strict mitigation measures at schools might be a key factor in understanding children's mental health and psychological well-being during a long-lasting pandemic.

COVID-19 risk perceptions in Japan: a cross-sectional study.

We conducted a large-scale online survey in February 2023 to investigate the public's perceptions of COVID-19 infection and fatality risks in Japan. We identified two key findings. First, univariate analysis comparing perceived and actual risk suggested overestimation and nonnegligible underestimation of COVID-19 risk. Second, multivariate logistic regression analyses revealed that age, income, education levels, health status, information sources, and experiences related to COVID-19 were associated with risk perceptions. Given that risk perceptions are closely correlated with daily socioeconomic activities and well-being, it is important for policy-makers and public health experts to understand how to communicate COVID-19 risk to the public effectively.

Heterogeneous risk attitudes and waves of infection.

Many countries have experienced multiple waves of infection during the COVID-19 pandemic. We propose a novel but parsimonious extension of the SIR model, a CSIR model, that can endogenously generate waves. In the model, cautious individuals take appropriate prevention measures against the virus and are not exposed to infection risk. Incautious individuals do not take any measures and are susceptible to the risk of infection. Depending on the size of incautious and susceptible population, some cautious people lower their guard and become incautious-thus susceptible to the virus. When the virus spreads sufficiently, the population reaches "temporary" herd immunity and infection subsides thereafter. Yet, the inflow from the cautious to the susceptible eventually expands the susceptible population and leads to the next wave. We also show that the CSIR model is isomorphic to the SIR model with time-varying parameters.