Effect of Climate on COVID-19 Incidence: A Cross-Sectional Study in Japan

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 RT-PCR 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. Findings: There was a total of 702 cases during the study period in Japan (population; 125, 900,000). Mean values (SD) of meteorological variables were 7.12 (2.91) Celsius degree for air temperature, 67.49 (7.63) percent for relative humidity, and 46.77 (12.55) percent 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% CI, 0.804-0.907; p-value <0.0001), relative humidity 0.904 (95% CI, 0.864-0.945; p-value <0.0001), and sunlight exposure 0.973 (95% CI, 0.951-0.997; p-value=0.026). Interpretation: 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.Funding Statement: The authors received no funding for this work.Declaration of Interests: The authors declare there are no conflicts of interest.Ethics Approval Statement: Ethical approval was considered as waived since all data belonged to public domains of the Japanese government and personal data were not identified.