COVID-19 In Japan Research Articles

Family structure, gender, and subjective well-being: effect of children before and after COVID-19 in Japan

AbstractGrandparents were anticipated to participate in grand-rearing. The COVID-19 pandemic had detached grandparents from rearing grandchildren. The research questions of this study were as follows: How does the change in family relations impact the Subjective well-being (SWB) of grandparents and parents? Using independently collected individual-level panel data over 2016–2023, we examined how family structure influenced SWB before and after COVID-19. We focused on the effects of children, grandchildren, and their gender on grandparents and parents. We found that compared with the happiness level before COVID-19, (1) granddaughters increased their grandmothers’ SWB after COVID-19, (2) both daughters and sons reduced their fathers’ SWB after COVID-19, whereas neither daughters nor sons changed their mothers’ SWB, and (3) the negative effect of sons reduced substantially if their fathers had younger brothers. Learning from interactions with younger brothers in childhood, fathers could avoid the deterioration of relationships with their sons, even when unexpected events possibly changed the lifestyle of the family and their relationship.

Telework experience, frequency, and knowledge management during COVID-19 in Japan and the United States

Purpose This study investigates the relationship between telework frequency and knowledge management (KM) activity in Japan and the USA. By examining how telework impacts KM activity differently across these two countries, this study aims to provide insights into the design and implementation of effective telework policies tailored to specific cultural contexts. Design/methodology/approach Linear and quadratic regression analyses were conducted to explore the relationship between telework frequency and KM activity. Data was collected from samples in Japan and the USA, with respondents categorized based on prior telework experience. Various KM activities such as knowledge acquisition, sharing and application were measured using established scales. Findings In Japan, an inverted U-shaped relationship between telework frequency and KM activity was observed, with optimal activity occurring at four days of telework per week. Conversely, the USA demonstrated a linear relationship, indicating sustained KM activity across different telework frequencies. Notably, individuals with prior telework experience showed higher levels of KM activity compared to those without experience. Originality/value This study contributes to the understanding of how cultural differences influence the relationship between telework and KM activity, and ultimately learning. By highlighting the nuanced patterns observed in Japan and the USA, it emphasizes the importance of tailored telework policies and support mechanisms for effective KM in diverse cultural contexts. Future research could further explore additional explanatory variables and their impact on telework-related outcomes.

Modeling the dynamics of Covid-19 in Japan: employing data-driven deep learning approach

Modeling the dynamics of Covid-19 in Japan: employing data-driven deep learning approach

Delivery vehicle and route of administration influences self-amplifying RNA biodistribution, expression kinetics, and reactogenicity

Self-amplifying RNA (saRNA) is a next-generation RNA platform derived from an alphavirus that enables replication in host cytosol, offering a promising shift from traditional messenger RNA (mRNA) therapies by enabling sustained protein production from minimal dosages. The approval of saRNA-based vaccines, such as the ARCT-154 for COVID-19 in Japan, underscores its potential for diverse therapeutic applications, including vaccine development, cancer immunotherapy, and gene therapy. This study investigates the role of delivery vehicle and administration route on saRNA expression kinetics and reactogenicity. Employing ionizable lipid-based nanoparticles (LNPs) and polymeric nanoparticles, we administered saRNA encoding firefly luciferase to BALB/c mice through six routes (intramuscular (IM), intradermal (ID), intraperitoneal (IP), intranasal (IN), intravenous (IV), and subcutaneous (SC)), and observed persistent saRNA expression over a month. Our findings reveal that while LNPs enable broad route applicability and stability, pABOL (poly (cystamine bisacrylamide-co-4-amino-1-butanol)) formulations significantly amplify protein expression via intramuscular delivery. Notably, the disparity between RNA biodistribution and protein expression highlight the nuanced interplay between administration routes, delivery vehicles, and therapeutic outcomes. Additionally, our research unveiled distinct biodistribution profiles and inflammatory responses contingent upon the chosen delivery formulation and route. This research illuminates the intricate dynamics governing saRNA delivery, biodistribution and reactogenicity, offering essential insights for optimizing therapeutic strategies and advancing the clinical and commercial viability of saRNA technologies.

Sick Leave and Absence Rate of Hospital Workers During the Sixth to Eighth Wave of COVID-19 in Japan

Abstract Background There are several reports of breakthrough infections of severe acute respiratory syndrome coronavirus 2 in vaccinated individuals and reinfections in previously infected individuals. Although the coronavirus disease (COVID-19) pandemic has increased the incidence of infections at healthcare facilities, the association between the increase in community spread and absence rate of hospital workers due to COVID-19 infection-related sick leave has not yet been fully elucidated. Method We reviewed the case files of patients with COVID-19 diagnosed at the Jikei University Katsushika Medical Center, Tokyo, Japan, between January 1, 2022, and February 28, 2023. The obtained data was used to determine the number of COVID-19 admissions and absence rate of hospital workers due to COVID-19 infection-related sick leave during the sixth to eighth wave of the COVID-19 pandemic. Results Of 1899 patients with confirmed COVID-19, 397 (20.9%) were hospital workers. Of the 397 patients, 71, 156, and 119 contracted the infection during the sixth (January–March 2022), seventh (July–September 2022), and eighth wave (November 2022–February 2023), respectively. The absence rate of hospital workers due to COVID-19 was 6.3%, excluding leaves of absence due to close contact in the seventh wave, which had the highest number of infections. Conclusions Given the difficulty of maintaining a usual work schedule during the COVID-19 outbreak, it may be necessary to re-evaluate the backgrounds of healthcare workers at high risk of contracting COVID-19 and to make aggressive adjustments to their work schedules by distributing assignments and restricting leave.

Simple mathematical model for predicting COVID-19 outbreaks in Japan based on epidemic waves with a cyclical trend

BackgroundSeveral models have been used to predict outbreaks during the COVID-19 pandemic, with limited success. We developed a simple mathematical model to accurately predict future epidemic waves.MethodsWe used data from the Ministry of Health, Labour and Welfare of Japan for newly confirmed COVID-19 cases. COVID-19 case data were summarized as weekly data, and epidemic waves were visualized and identified. The periodicity of COVID-19 in each prefecture of Japan was confirmed using time-series analysis and the autocorrelation coefficient, which was used to investigate the longer-term pattern of COVID-19 cases. Outcomes using the autocorrelation coefficient were visualized via a correlogram to capture the periodicity of the data. An algorithm for a simple prediction model of the seventh COVID-19 wave in Japan comprised three steps. Step 1: machine learning techniques were used to depict the regression lines for each epidemic wave, denoting the “rising trend line”; Step 2: an exponential function with good fit was identified from data of rising straight lines up to the sixth wave, and the timing of the rise of the seventh wave and speed of its spread were calculated; Step 3: a logistic function was created using the values calculated in Step 2 as coefficients to predict the seventh wave. The accuracy of the model in predicting the seventh wave was confirmed using data up to the sixth wave.ResultsUp to March 31, 2023, the correlation coefficient value was approximately 0.5, indicating significant periodicity. The spread of COVID-19 in Japan was repeated in a cycle of approximately 140 days. Although there was a slight lag in the starting and peak times in our predicted seventh wave compared with the actual epidemic, our developed prediction model had a fairly high degree of accuracy.ConclusionOur newly developed prediction model based on the rising trend line could predict COVID-19 outbreaks up to a few months in advance with high accuracy. The findings of the present study warrant further investigation regarding application to emerging infectious diseases other than COVID-19 in which the epidemic wave has high periodicity.

Exploring viral infections' role in Kawasaki disease onset: A study during the COVID-19 pandemic.

During thecoronavirus disease 2019 (COVID-19) pandemic, known viral diseases declined in all ages. By using the current situation as a natural experiment, this study aimed to evaluate whether the change in the incidence of Kawasaki disease (KD) during the COVID-19 pandemic varies with age and whether a specific infectious disease mediates the occurrence of KD. Monthly number of KD patients were extracted from the nationwide inpatient database. Segmented regression analysis was conducted on the interrupted time series data. Additionally, causal mediation analysis was performed to examine the role of viral infections in the changes in the number of KD patients. After the first emergency declaration for COVID-19 in Japan, there was an immediate decrease in the number of KD patients per 100 000 population aged between 6 months and 4 years (immediate change = -2.66; 95% confidence interval [CI]: -5.16 to -0.16) and aged 5-15 years (immediate change = -0.26; 95% CI: -0.49 to -0.04). However, no immediate change was observed in patients under 6 months of age. In the causal mediation analysis for each viral infection, it was found that the decrease in the number of patients with KD was mediated by changes in the number of patients with pharyngoconjunctival fever and infectious gastroenteritis. The current results suggest that viral infections may be one of the etiological agents for KD, while they may not be the main cause in early infancy. Specifically, we found that adenovirus infection and gastroenteritis was closely related to the onset of KD in some areas of Japan.

The undersecuritization of COVID-19 in Japan: Voluntary behavioral change as self-defense?

Facing the spread of the novel coronavirus (COVID-19), leaders of many countries attempted to securitize the COVID-19 issue. The Japanese government, however, neither attempted to securitize it nor implemented strong measures to control its circulation. Yet the behavior of Japanese citizens changed significantly to prevent the spread of COVID-19. What were the mechanisms that drove them to voluntarily change their behavior? By introducing the analytical concept of ‘undersecuritization’, this article tries to answer this question, as well as to broaden the scope of the securitization argument. It first analyzes why Prime Minister Shinzo Abe did not attempt to securitize COVID-19. It then examines the mechanism behind citizens’ behavioral change. When Japanese citizens perceived the measures taken by their government as too weak to address the life-threatening COVID-19 issue, they started to feel that it was undersecuritized. This article argues that such awareness was the reason behind the voluntary behavioral changes that the Japanese people made in self-defense. It also shows that such self-defense measures can be more formidable than the measures taken by the government in a situation of oversecuritization.

Nationwide epidemiology and health resource use among children with COVID-19 in Japan

BackgroundThe COVID-19 pandemic posed substantial challenges to healthcare systems. Understanding the responses of pediatric health services is crucial for future pandemic planning and preparedness, yet such data remains limited. MethodsIn this retrospective cohort study, we analyzed data from administrative databases developed by Japan Medical Data Center and DeSC Healthcare Inc. The dataset comprised records of 2,612,511 children, totaling 60,224,888 person-months, from January 2020 to May 2022. Multivariate generalized estimation equations were used to examine the incidence rates of COVID-19 and associated health resource use. ResultsOur analysis revealed that the incidence rates of COVID-19 gradually increased from Wave I (2.2 cases per 100,000 person-months) to Wave V (177.8cases per 100,000 person-months), with a notable elevation during Wave VI (2367.7 cases per 100,000 person-months). While nucleic acid amplification tests were primarily used during Waves I–V, the use of rapid antigen tests markedly increased in Wave VI. The hospitalization rates increased gradually from 0.2 in Wave I to 10.2 events per 100,000 person-months in Wave VI, and the case-hospitalization risk decreased from 14.9% in Wave II to 0.7% in Wave VI. Additionally, we observed decreasing trends in the use of antibiotics (Wave I, 31.8%; Wave VI, 9.0%), whereas antipyretic use rose from Wave I (56.1%) to Wave VI (86.6%). ConclusionsOur study highlighted essential changes in the nationwide pediatric healthcare system's response to the COVID-19 pandemic. These findings provide valuable insights into the future pandemic planning and preparedness.

Economic Repercussions of COVID-19 in Japan and China

This study seeks to explore and elucidate the economic repercussions of the COVID-19 pandemic on Japan and China. By comparing these two prominent Asian economies, the paper aims to offer insights into how different economic structures, policies, and external relations can influence the economic impacts of a global health crisis and determine what each country should do in the post-pandemic era economy. This research delves into various economic indicators such as GDP growth, employment rates, number of bankruptcies, and sectoral analyses. Using quantitative analysis shown in graphs, the paper concludes that the COVID-19 pandemic significantly impacted Japan and China's economies, affecting SMEs, supply chains, unemployment, and government finances; the study suggests both nations need a multifaceted approach from government and society to boost their post-pandemic economies. The study found that Japan faced a surge in SME bankruptcies despite financial support measures; aggregate supply experienced disruptions, and aggregate demand declined due to service restrictions, exacerbating fiscal challenges. China, facing a supply-side shock, should prioritize domestic demand, SME support, digital transformation, and healthcare investment, leading to the study's conclusion that strengthening supply chains and aligning recovery with sustainability is vital for recovery and growth in the post-pandemic era.

Physician characteristics associated with antiviral prescriptions for older adults with COVID-19 in Japan: an observational study

ObjectivesAlthough guidelines recommend antiviral therapy for outpatients with COVID-19 who are at high risk of progressing to severe conditions, such as older adults, many patients do not receive appropriate treatment....

EHealth Literacy and Web-Based Health Information-Seeking Behaviors on COVID-19 in Japan: Internet-Based Mixed Methods Study.

During the COVID-19 pandemic, much misinformation and disinformation emerged and spread rapidly via the internet, posing a severe public health challenge. While the need for eHealth literacy (eHL) has been emphasized, few studies have compared the difficulties involved in seeking and using COVID-19 information between adult internet users with low or high eHL. This study examines the association between eHL and web-based health information-seeking behaviors among adult Japanese internet users. Moreover, this study qualitatively shed light on the difficulties encountered in seeking and using this information and examined its relationship with eHL. This cross-sectional internet-based survey (October 2021) collected data from 6000 adult internet users who were equally divided into sample groups by gender, age, and income. We used the Japanese version of the eHL Scale (eHEALS). We also used a Digital Health Literacy Instrument (DHLI) adapted to the COVID-19 pandemic to assess eHL after we translated it to Japanese. Web-based health information-seeking behaviors were assessed by using a 10-item list of web sources and evaluating 10 topics participants searched for regarding COVID-19. Sociodemographic and other factors (eg, health-related behavior) were selected as covariates. Furthermore, we qualitatively explored the difficulties in information seeking and using. The descriptive contents of the responses regarding difficulties in seeking and using COVID-19 information were analyzed using an inductive qualitative content analysis approach. Participants with high eHEALS and DHLI scores on information searching, adding self-generated information, evaluating reliability, determining relevance, and operational skills were more likely to use all web sources of information about COVID-19 than those with low scores. However, there were negative associations between navigation skills and privacy protection scores when using several information sources, such as YouTube (Google LLC), to search for COVID-19 information. While half of the participants reported no difficulty seeking and using COVID-19 information, participants who reported any difficulties, including information discernment, incomprehensible information, information overload, and disinformation, had lower DHLI score. Participants expressed significant concerns regarding "information quality and credibility," "abundance and shortage of relevant information," "public trust and skepticism," and "credibility of COVID-19-related information." Additionally, they disclosed more specific concerns, including "privacy and security concerns," "information retrieval challenges," "anxieties and panic," and "movement restriction." Although Japanese internet users with higher eHEALS and total DHLI scores were more actively using various web sources for COVID-19 information, those with high navigation skills and privacy protection used web-based information about COVID-19 cautiously compared with those with lower proficiency. The study also highlighted an increased need for information discernment when using social networking sites in the "Health 2.0" era. The identified categories and themes from the qualitative content analysis, such as "information quality and credibility," suggest a framework for addressing the myriad challenges anticipated in future infodemics.

Cost-effectiveness of nirmatrelvir/ritonavir for high-risk adult outpatients with COVID-19 in Japan

IntroductionThis study aimed to evaluate the cost-effectiveness of nirmatrelvir/ritonavir (Nir/Rit) for adult outpatients with COVID-19 from the perspective of a Japanese public healthcare payer. MethodsA cost-effectiveness simulation was conducted comparing Nir/Rit for the outpatient treatment of high-risk COVID-19 patients to best supportive care (BSC) without antiviral or antibody drugs. The analytical model was divided into two phases: the treatment phase, lasting 35 days from the start of COVID-19 treatment, and the post-treatment phase. Patients who survived the treatment phase were assumed to follow a general population survival curve. Expected costs and expected quality-adjusted life years (QALYs) for both BSC and Nir/Rit were calculated for ages 40 to 80 to obtain the incremental cost-effectiveness ratio (ICER). The robustness of the results was evaluated through deterministic and probabilistic sensitivity analysis (PSA). ResultsThe ICERs for patients aged 40, 50, 60, 70, and 80 were 18,854,276 Japanese Yen (JPY)/QALY, 8,482,034 JPY/QALY, 4,976,612 JPY/QALY, 2,636,096 JPY/QALY, and 1,597,783 JPY/QALY, respectively. In the deterministic sensitivity analysis, both the mortality risk during the treatment phase and the relative mortality risk with Nir/Rit had a high impact on ICER across all ages. In the PSA, when the willingness-to-pay (WTP) threshold was set at 5 million JPY/QALY, the probability of the ICER being below the WTP threshold was 0%, 0.2%, 45.4%, 99.9%, and 100% at ages 40, 50, 60, 70, and 80, respectively. ConclusionNir/Rit is cost-effective for older individuals aged 60 and over but not for younger age groups.

Prevalence and trends in persistent symptoms following COVID-19 in Japan: A nationwide cross-sectional survey

IntroductionEven during the endemic phase of the COVID-19 pandemic, the impact of persistent symptoms on patients and healthcare systems remains significant. Thus, a more comprehensive understanding of these symptoms is essential. MethodsUsing data from the Japan Society and New Tobacco Internet Survey conducted in February 2023, this cross-sectional study investigated the prevalence of, and changes in, persistent COVID-19 symptoms. ResultsIn total, 21,108 individuals responded to the survey. Of these, 29.1 % (6143) had a history of COVID-19. Our analysis found that arm/leg/joint pain (adjusted odds ratio [aOR]: 1.17; 95 % confidence interval [95 % CI]: 1.03–1.33), back pain (aOR: 1.13; 95 % CI: 1.01–1.27), chest pain (aOR: 1.53; 95 % CI: 1.20–1.96), malaise (aOR: 1.14; 95 % CI: 1.02–1.28), loss of taste (aOR: 2.55; 95 % CI: 1.75–3.72), loss of smell (aOR: 2.33; 95 % CI: 1.67–3.26), memory impairment (aOR: 1.27; 95 % CI: 1.04–1.56), and cough (aOR: 1.72; 95 % CI: 1.38–2.13) were independently associated with a history of COVID-19 contracted more than two months but less than six months previously. Further, back pain (aOR: 1.24; 95 % CI: 1.04–1.47) and loss of taste (aOR: 2.28; 95 % CI: 1.24–4.21) showed independent association with COVID-19 contracted more than 12 months previously. ConclusionsVarious symptoms were independently associated with a history of COVID-19. While most patients tend to recover within a year after contracting COVID-19, certain symptoms, such as back pain and loss of taste, persist longer than a year, underscoring public health concerns and emphasizing the need for health care services to support patients suffering from persistent symptoms.

Real-world prescription of anti-COVID-19 drugs in hospitalized patients with COVID-19 in Japan.

Prescription trends and patterns of anti-COVID-19 drugs in hospitalized patients were examined based on real world data to understand the use of anti-COVID-19 drugs in clinical practice in Japan. The longitudinal and cross-sectional study was conducted utilizing data from January 1, 2019 to December 31, 2021 of the MID-NET® medical information database, which stored the electronic medical records, administrative claim data, and diagnosis procedure combination data of patients in Japan. Hospitalized patients with a COVID-19-related diagnosis who received at least one anti-COVID-19 drug between April 1, 2020 and December 31, 2021. The following 14 drugs were included in this study: remdesivir, baricitinib, combination product of casirivimab and imdevimab, favipiravir, dexamethasone, ivermectin, azithromycin, nafamostat mesylate, camostat mesylate, ciclesonide, tocilizumab, sarilumab, combination product of lopinavir and ritonavir, and hydroxychloroquine. We identified 5,717 patients hospitalized with COVID-19 and prescribed at least one anti-COVID-19 drug. The entire cohort generally included patients over 41-50 years and more males. The most common prescription pattern was dexamethasone monotherapy (22.9%), followed by the concomitant use of remdesivir and dexamethasone (15.0%), azithromycin monotherapy (15.0%), remdesivir monotherapy (10.2%), and nafamostat mesylate monotherapy (8.5%). However, an often prescribed anti-COVID-19 drug differed depending on the period. This study revealed the real-world situation of anti-COVID-19 drug prescriptions in hospitalized COVID-19 patients in Japan. A prescribed drug would depend on the latest scientific evidence, such as efficacy, safety, and approval status, at the time of prescription. Understanding the prescription of anti-COVID-19 drugs will be important for providing the most up-to-date treatments to patients and evaluating the benefit and/or risk of anti-COVID-19 drugs based on the utilization of an electronic medical record database.

Public holidays increased the transmission of COVID-19 in Japan, 2020-2021: a mathematical modelling study.

Although the role of specific holidays in modifying transmission dynamics of infectious diseases has received some research attention, the epidemiological impact of public holidays on the transmission of coronavirus disease 2019 (COVID-19) remains unclear. To assess the extent of increased transmission frequency during public holidays, we collected COVID-19 incidence and mobility data in Hokkaido, Tokyo, Aichi, and Osaka from February 15, 2020 to September 30, 2021. Models linking the estimated effective reproduction number (Rt) with raw or adjusted mobility, public holidays, and the state of emergency declaration were developed. The best-fit model included public holidays as an essential input variable, and was used to calculate counterfactuals of Rt in the absence of holidays. During public holidays, on average, Rt increased by 5.71%, 3.19%, 4.84%, and 24.82% in Hokkaido, Tokyo, Aichi, and Osaka, respectively, resulting in a total increase of 580 (95% confidence interval [CI], 213 to 954), 2,209 (95% CI, 1,230 to 3,201), 1,086 (95% CI, 478 to 1,686), and 5,211 (95% CI, 4,554 to 5,867) cases that were attributable to the impact of public holidays. Public holidays intensified the transmission of COVID-19, highlighting the importance of considering public holidays in designing appropriate public health and social measures in the future.

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.

MSR133 What Factors Influence the Prescription of Oral Antivirals for COVID-19 in Japan? – a State Transitional Agent-Based Model Analysis

MSR133 What Factors Influence the Prescription of Oral Antivirals for COVID-19 in Japan? – a State Transitional Agent-Based Model Analysis

Questionnaire survey of the frequency of dietary intake during hemodialysis and the impact of COVID-19 in Japan.

There have been no coherent reports on the effects of dietary intake during hemodialysis in Japan. Furthermore, few studies have reported the impact of the COVID-19 pandemic on them. This study aimed to investigate dietary intake during hemodialysis and its impact on the spread of COVID-19. This consecutive cross-sectional study included a survey of all hemodialysis facilities in the Niigata Prefecture. The survey form was sent via e-mail or fax. Fifty four facilities (98%) responded to the 2018 survey. Twenty-eight facilities (52%) provided meals, with 18% of all patients undergoing dialysis and 41% of patients undergoing nocturnal dialysis consuming meals during hemodialysis. In the 2020 survey, the number of facilities decreased to 17 (33%), and the number of all patients undergoing dialysis and nocturnal dialysis consuming meals decreased to 13% and 32%, respectively. In the 2022 survey, the number of facilities decreased to 14 (27%), and the number of all patients undergoing dialysis and nocturnal dialysis decreased to 9% and 19%, respectively. The COVID-19 pandemic has caused a decrease in both facilities serving meals and patients' food consumption during hemodialysis. To prevent the loss of meal opportunities, establishing safe methods for food intake and alternatives in hemodialysis facilities is necessary.

Association of Ambient Temperature and Absolute Humidity with the Effective Reproduction Number of COVID-19 in Japan.

This study aimed to quantify the exposure-lag-response relationship between short-term changes in ambient temperature and absolute humidity and the transmission dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in Japan. The prefecture-specific daily time-series of newly confirmed cases, meteorological variables, retail and recreation mobility, and Government Stringency Index were collected for all 47 prefectures of Japan for the study period from 15 February 2020 to 15 October 2022. Generalized conditional Gamma regression models were formulated with distributed lag nonlinear models by adopting the case-time-series design to assess the independent and interactive effects of ambient temperature and absolute humidity on the relative risk (RR) of the time-varying effective reproductive number (Rt). With reference to 17.8 °C, the corresponding cumulative RRs (95% confidence interval) at a mean ambient temperatures of 5.1 °C and 27.9 °C were 1.027 (1.016-1.038) and 0.982 (0.974-0.989), respectively, whereas those at an absolute humidity of 4.2 m/g3 and 20.6 m/g3 were 1.026 (1.017-1.036) and 0.995 (0.985-1.006), respectively, with reference to 10.6 m/g3. Both extremely hot and humid conditions synergistically and slightly reduced the Rt. Our findings provide a better understanding of how meteorological drivers shape the complex heterogeneous dynamics of SARS-CoV-2 in Japan.