This study used a simplified SEIR model to evaluate reproduction numbers. The basic reproduction number obtained from the model simulations was about 0.6 in both Tokyo and Osaka during the third wave convergence period, whereas it was 1.17 in Tokyo and 1.59 in Osaka during the fourth wave expansion period. The epidemic expansion period was short because of the state of emergency, so the data were scarce, resulting in overfitting. Overfitted models are unsuitable for predicting epidemics because they reproduce the peculiar characteristics of the data with which they are fitted . In the early expansion or convergence phase of an epidemic, the data show a common pattern. Data-specific fluctuations that cause overfitting are small. In addition, overfitting is suppressed by an objective function with a penalty term. The model deals with the COVID-19 epidemic expansion within megacities because the movement of people over a wide area has been reduced greatly as a result of public requests for self-restraint from travel across prefectures in Japan. The COVID-19 epidemic shows complex patterns in megacities. From April through June 2020, the 23 wards of Tokyo could be divided into three zones according to COVID-19 epidemic patterns: downtown, within the circumference of the Japan Railway Yamanote Line, and outer residential areas . For influenza, detailed simulations of Tokyo have been done using agent models to model people’s daily behavior [17, 18]. However, the necessary data for agent models, such as infection rates in homes, workplaces, restaurants, and commuter trains, are not yet available for COVID-19.
The effective reproduction numbers for Japan were reported as 0.6–0.8 in mid-February 2021 and 1.1–1.3 at the end of March . On March 22, 2021, the effective reproduction number reported for Osaka was 1.74 , which resulted from the rapid spread of infection because of an increase in people’s social activities from the middle of March . The generation-specific reproduction numbers of people in their 20s were 1.94 and 2.53 in Tokyo and Osaka, respectively, during the fourth wave, which were higher than for other generations. During both the Tokyo and Osaka epidemics, the proportion of infected people in their 20s increased during the expansion stage, whereas the proportion of infected people in their 60s and older increased during the convergence period. In the United States (US), the reported reproduction number for people under 20 years old was about half that of people in their 20s. Moreover, during the expansion stage of the epidemic, most cases were found among the 20–59-year age group, whereas after the peak of the epidemic, the numbers of infected people aged under 20 and over 60 years increased .
In Japan, the proportion of infected people in their 20s increased during the expansion period, indicating that this age group played a major role in the epidemic. The fourth wave was larger in Osaka than in Tokyo, as was the spread of infection among people in their 20s. The generation-specific reproduction number for people in their 20s was 1.94 in Tokyo and 2.53 in Osaka. When one specifically examines the night users of major stations in downtown areas of Tokyo and Osaka to specifically ascertain differences in behavioral changes in people in their 20s, the numbers of users at Shibuya and Shinjuku stations in Tokyo remained fundamentally constant, at about 40% of the 2019 population from early February to mid-March, with a gradual increase starting from the week of March 22. The station users at the Osaka and Namba stations in Osaka were nearly 60% of the 2019 station users in mid-February; they reached about 80% in March. According to a survey using mobile phones, about 40% of the people present at 21:00 in the period January 4–17, 2021, around Shinjuku and Shibuya stations were in their 20s . In addition, the downtown area was considered an infected area. The emergency declaration requested that restaurants with alcohol refrain from operating. A survey in Toyama, a regional city, showed that snack and karaoke bars each accounted for more than 20% of infected areas . In Japan, as of April 20, 2021, the nationwide trend of infection spread during the fourth wave was mainly driven by people in their 20s and 30s . By comparison, in the US, the contact rate during the epidemic was significantly higher among men younger than 45 years than in the remainder of the population . The epidemic persisted mainly because of people of 20–49 years .
Vaccination against SARS-Cov-2 virus is necessary to suppress COVID-19, but vaccination in Japan was not done in time to prevent or mitigate the fourth wave. How would the epidemic have changed if vaccination had been available in time? Because the epidemic is age-specific, the following two ideas have been proposed. The first is that the epidemic is sustained by young and middle-aged adults. Therefore vaccination of this generation would best control COVID-19 . The second idea is that vaccination of high-risk older adults is the best way to reduce the burdens of COVID-19 . Studies in the US showed that vaccinating the elderly population specifically, rather than a given group with numerous infected people, was superior in terms of protecting elderly people [26, 27]. In Japan, as in the US, the risk of COVID-19 infection was higher among elderly people , and elderly people were assigned priority for vaccination. Simulations using a simplified model of the fourth expansion wave showed that if vaccination rates were equal, the number of infected people in their 60s and older would be lower in Japan once people in this group had been vaccinated, as in the US. When vaccine supplies are sufficient, priority should be assigned to high-risk elderly people. However, if vaccine supplies are not sufficient, the idea of vaccinating a given group with numerous infected people must be considered. The numbers of people in their 60s and older are 2.2 and 2.9 times greater, respectively, than those of people in their 20s in Tokyo and Osaka. The amount of vaccine required to vaccinate 30% of people in their 60s and older would be sufficient to vaccinate 60% of people in their 20s. The simulation results show that vaccinating 60% of people in their 20s would reduce the total number of infected people more than vaccinating 30% of people in their 60s and older, although the number of infected people aged over 60 years would be about the same. However, vaccination of people of younger generations has the social benefit of mitigating silent transmission , although vaccination of infected people who are less likely to show symptoms might not be achievable in a voluntary manner. In a survey administered before vaccination began, among corporate workers in their 20s or younger in Japan, 49% intended to be vaccinated; 23% did not . After a vaccine scare, it is difficult to achieve adequate coverage via voluntary vaccination based on a general discussion of the personal benefits and shortcomings of vaccination .