key: cord-0686520-1vq8clpd authors: Kato, Mikiro; Sakihama, Tomoko; Kinjo, Yoshio; Itokazu, David; Tokuda, Yasuharu title: Effect of Climate on COVID-19 Incidence: A Cross-Sectional Study in Japan date: 2022-01-20 journal: Korean J Fam Med DOI: 10.4082/kjfm.20.0260 sha: 6382a07ca693eb07a9e5256bb2a76d94bfd85d3b doc_id: 686520 cord_uid: 1vq8clpd BACKGROUND: Effect of meteorological factors such as air temperature, humidity, and sunlight exposure on transmission dynamics of novel coronavirus disease 2019 (COVID-19) remains controversial. We investigated the association of these factors on COVID-19 incidence in Japan. METHODS: We analyzed data on reverse transcription polymerase chain reaction confirmed COVID-19 cases for each prefecture (total=47) in Japan and incidence rate was defined as the number of all reported cumulative cases from January 15 to March 17, 2020. Independent variables of each prefecture included three climatic variables (mean values of air temperature, relative humidity, and sunlight exposure), population elderly ratio, and the number of inbound travelers from China during February 2020. Multivariable-adjusted Poisson regression model was constructed to estimate COVID-19 incidence rate ratio (IRR) of independent variables. RESULTS: There was a total of 702 cases during the study period in Japan (population=125, 900,000). Mean±standard deviation values of meteorological variables were 7.12°C±2.91°C for air temperature, 67.49%±7.63% for relative humidity, and 46.77±12.55% for sunlight exposure. Poisson regression model adjusted for climate variables showed significant association between the incidence and three climatic variables: IRR for air temperature 0.854 (95% confidence interval [CI], 0.804–0.907; P<0.0001), relative humidity 0.904 (95% CI, 0.864–0.945; P<0.0001), and sunlight exposure 0.973 (95% CI, 0.951–0.997; P=0.026). CONCLUSION: Higher values of air temperature, relative humidity and sunlight exposure were associated with lower incidence of COVID-19. Public health interventions against COVID-19 epidemic in a country should be developed by considering these meteorological factors. Coronavirus disease is currently spreading globally despite strong public health measures such as travel restrictions and lockdowns. Epidemics of viral respiratory tract infections, such as influenza, occur more frequently in winters, characterized by low air temperatures and dry air. 1) Climatic conditions, including temperature, humidity, and sunlight exposure, are considered to be important factors affecting the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and several studies have suggested a relationship between infection spread and winter climate. [2] [3] [4] [5] [6] In a study using global data, Wu et al. 6) noted that low air temperature and low relative humidity were associated with the number of daily new cases and COVID-19 mortality. However, regional patterns of COVID-19 outbreaks differ depending on the number of inbound travelers from a global epicenter, elderly population ratio, and population density. Thus, the results of these studies have not been consistent, and the impact of meteorological factors on viral spread is controversial. Japan, located in the Northern Hemisphere, spreads across a wide range of longitudes. Thus, it is possible to examine the impact of climate on viral spread in Japan while considering the number of inbound travelers from a global epicenter, elderly population ratio, and population density. Therefore, in the current study, we aimed to investigate the impact of climate, specifically temperature, humidity, and sunlight exposure, on the spread of COVID-19 by using regional data from 47 prefectures during the initial phase (February and March 2020) of the COVID-19 outbreak in Japan. Data collection for each prefecture in Japan was conducted using published data from the public domains of the national government. As the dependent variable, the incidence of COVID-19 was defined as the cumulative number of reported cases in Japan, from the report of the first case in Japan on January 15, 2020 up to March 17, 2020 . According to the Japanese case definition, clinically suspected patients with positive reverse transcription polymerase chain reaction results from respiratory specimens were considered cases. Our observation period ended in March 23, 2020 because public health measures, such as travel restrictions from China, were implemented at that time to mitigate the epidemic throughout the country. Cases involving the Diamond Princess cruise ship or charter flights from Wuhan, China, were excluded from the current study. The independent variables in each prefecture included climate, particulate matter sized ≤2.5 µm (PM2.5), elderly population ratio, and the number of inbound travelers based on government records in February 2020. We also included the population density of each prefecture in 2015. Data on the population density in 2020 were not available, as they have not yet been officially announced. Climate variables included mean air temperature (°C), mean relative humidity (%), and mean relative sunshine duration (the ratio of actual to potential sunshine duration). Based on the methods described by Ujiie et al., 7) the elderly population ratio (the ratio of the number of people aged ≥65 years to the number of individuals aged 15-64 years) and the number of inbound travelers per 1,000 individuals in each prefecture were added as explanatory variables. Ujiie et al. 7) allotted the number of travelers based on the people traveling from Tokyo International Airport (Chiba prefecture) to Chiba (the prefecture east to Tokyo prefecture), where the airport is located. However, in the current study, this number was allotted based on the number of people traveling to Tokyo, because most travelers were considered to travel directly to Tokyo from the airport through public transport (trains or buses) rather than through Chiba. A multivariable-adjusted Poisson regression model was constructed to estimate the association between the dependent and independent variables. The incidence rate ratio (IRR) was determined. Two-tailed P-values less than 0.05 were considered statistically significant. STATA ver. 14.0 (Stata Corp., College Station, TX, USA) was used for all statistical analyses. The need for ethical approval was waived because all data belonged to the public domains of the Japanese government and personal data were not identified. There were a total of 702 confirmed cases throughout Japan during the study period (population=125,900,000). The highest number of cases per 1 million people was identified in Hokkaido prefecture (26.7), followed by Aichi (15.8) . No cases were reported in 13 prefectures. Figure The lowest mean air temperature, relative humidity, and sunlight exposure were observed in Hokkaido, Chiba, and Niigata, respectively. study on SARS-CoV, a beta-coronavirus that is genetically close to SARS-CoV-2, showed that high temperature and high relative humidity could synergistically inactivate SARS-CoV. Conversely, a low temperature and low relative humidity could prolong the viability of the virus on contaminated surfaces. 10) In the current study, sunlight exposure was associated with a lower incidence of COVID-19 in Japan. A study using global data by Gunthe et al. 5) reported that the ultraviolet index greatly lowered the spread and survival of the SARS-CoV-2. In SARS, ultraviolet irradiation, especially ultraviolet C irradiation, efficiently eliminated infectivity. 11, 12) The indirect interaction between sunlight exposure and severity of COV-ID-19 has also been shown. 13 20) suggested that air-conditioning played little role in the spread of COVID-19 on the ship and that most transmissions occurred through close contact in rooms without air-conditioning. In conclusion, our study showed a possible association between a lower incidence rate of COVID-19 and specific climactic conditions, including higher values of air temperature, relative humidity, and sunlight exposure. The mechanisms underlying these associations may include reduced viral activity and transmission under such climatic conditions. To consolidate this finding, it is necessary to validate this relationship of COVID-19 in other countries. Healthcare workers, public health specialists, and policy makers may need to consider climate factors for developing a comprehensive plan for public health interventions against COVID-19 and increasing opportunities for economic activities. No potential conflict of interest relevant to this article was reported. Mikiro Kato: https://orcid.org/0000-0003-4545-0745 Tomoko Sakihama: https://orcid.org/0000-0002-1160-1704 Yoshio Kinjo: https://orcid.org/0000-0002-6965-901X Yasuharu Tokuda: https://orcid.org/0000-0002-9325-7934 Seasonality of respiratory viral infections No association of COVID-19 transmission with temperature or UV radiation in Chinese cities COVID-19 transmission in Mainland China is associated with temperature and humidity: a time-series analysis Correlation between climate indicators and COVID-19 pandemic On the global trends and spread of the COVID-19 outbreak: preliminary assessment of the potential relation between location-specific temperature and UV index. 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