Discussion
The results revealed the existing regional heterogeneity in the severity of COVID-19 cases at the time of admission in Japan. Our results suggest that the health policy of each prefecture might have an impact on the severity of hospitalized COVID-19 cases on admission.
A wide variety was observed in the median values of the posterior distribution of random effects for the different prefectures. That of Kanagawa prefecture in model A was 0.340 [95% Credible Interval (CrI) 0.269, 0.410], and that of Iwate prefecture was −0.431 [95% CrI −0.593, −0.268], indicating a substantial discrepancy in the severity of hospitalized COVID-19 cases between these two prefectures.
The posterior distribution of random effects in model B exhibited a similar trend to that in model A. The coefficient of Kanagawa prefecture in model B was 0.306 [95% CrI 0.213, 0.402], and that of Iwate prefecture was −0.413 [95% CrI −0.579, −0.249]. Other prefectures demonstrated similar results in both models, suggesting that factors other than the number of beds secured for COVID-19 patients (e.g., prioritization policy for COVID-19 case admission and number of physicians) had an impact on the severity of COVID-19 on admission.
We should take various factors into consideration when we interpret our results. The potential factors associated with the variation of the severity on admission could be viewed from two directions, namely, the local epidemiology of COVID-19 and the COVID-19 healthcare policy in each prefecture. From the former viewpoint, a regional surge in the number of COVID-19 patients or a rapid increase in the severity of disease due to new SARS-CoV-2 variants may contribute to the change in the severity on admission.8,14 However, our analysis did not include patients with new SARS-CoV-2 variants as the study period was limited to before March 2021. While it is possible that COVID-19 cases due to new variants carry a greater risk of serious illness, there was a substantial regional heterogeneity in the severity of COVID-19 cases at the time of admission, as previous studies have demonstrated.15–17 Presumably, variants are contributing factors to the severity of COVID-19 cases, but it should be noted that other factors also have a non-negligible influence on the severity of disease.
The latter point is relevant to policymakers (and healthcare providers) in the municipalities, mainly related to the on-site indication for hospitalization that is operated in each prefecture. The policy implemented to manage the waves of infection could be affected by the total medical capacity in the area. The number of beds secured for COVID-19 patients is one of the major resources.15,18The number of beds per population in each prefecture was included as one of the fixed effects in model B, and the random effect of each prefecture did not drastically change. Also, the number of physicians eligible for COVID-19 management may have an impact as this factor could create a bottleneck for patient acceptance.19 However, it is difficult to precisely grasp the number of physicians who are engaging in COVID-19 patient management. This is because the specialty of the medical doctor is not a good proxy for the number of physicians engaging in COVID-19 management under such an emergency period.20
The local government of each prefecture has established policy for the process of admission prioritization as a way of integrating the information from the regional medical situations, including the aforementioned factors, a strategy that may determine the variation in the severity on admission.21,22 This means that there may be differences in whether a patient can be hospitalized and how long it takes for a patient to be hospitalized, even for clinically similar cases, depending on the prefecture they belong to.
Two types of strategy for symptomatic COVID-19 patients can be assumed. The following are simplified examples: 1) moderate to severe cases are admitted to hospital, whereas mild cases are managed at home or in a dedicated facility (severity on admission will increase), and 2) all patients, whether with mild or severe disease, are admitted to hospital (severity on admission will decrease). Both strategies have their own risks and benefits, and the local government of each prefecture has adopted each strategy depending on the height of each wave of the epidemic.
The strategy adopted should be evaluated by the outcomes of COVID-19 patients, damage to the medical system for non-COVID-19 diseases, and the impact on hospital and regional healthcare economy. A previous study in China demonstrated that the number of beds and medical staff per COVID-19 patient was negatively correlated with mortality.23 Conversely, a recent study from Italy reported that greater access to hospitals did not affect the mortality of COVID-19 patients older than 80 years but affected that of patients younger than 80 years,24 suggesting that the best approach may be to change the strategy according to age group or other properties of the patients. Evaluation of the regional policy could provide important insights to construct a better healthcare system against COVID-19 and other emerging infectious diseases.
Our analysis had several limitations. First, we examined data solely from Japan; thus, it is not appropriate to generalize our results globally. Nevertheless, it is easy to imagine that, even in other countries, there would be differences in health policies among local authorities and that they might have an impact on the management of COVID-19 cases. Second, we could not include other confounding factors, such as human resources and variance of SARS-CoV-2; these aspects should be addressed in future studies.
In conclusion, our analysis revealed a possible association between prefecture and an increased/decreased risk of severe COVID-19 infection at the time of admission. In addition, the number of beds secured for COVID-19 patients in each prefecture is not a single cause of such heterogeneity; therefore, other factors could be significant for the management of COVID-19 cases in Japan. Countermeasures against COVID-19 will be more appropriate if we take these insights into consideration.