Temperature and COVID-19 Incidence: An Ecologic Study

Background: It has been hypothesized that COVID-19 is less prevalent in regions with warm climates. Contradictory results led us to investigate the correlation between temperature and the cumulative COVID-19 incidence rate. Method: We obtained COVID data from CRONALAB, COVID-DASHBOARD, and MCMC databases of Fars province, linked data, and finalized daily COVID-19 cases. Daily data on the temperature was gotten from meteorological stations’ reports from March 21, 2020, to March 21, 2021, for each county of Fars province, Southern Iran. The daily weighted cumulative incidence rate of COVID-19 cases was calculated for all of the counties, separately. Initially, for a uniform data visualization, average air temperature data were transformed into the ranked percentiles. Then, to visually assess the study hypothesis, the distribution of COVID-19 cumulative incidence was visualized on percentiles of temperature. Because of the non-linear distribution of data, we performed exploratory analyses using the generalized additive models (GAMs) and locally weighted (polynomial) regressions (LWRs) to choose the best response function. Then, the generalized linear models (GLMs) were used to build the model, parametrically. Results: GAMs showed a small decreasing – near-to-horizontal – linear pattern for COVID-19 incidence rate as the function of temperature (pseudo R2: 0.001, deviance explained: 0.13%, coefficient: -0.02). The GLMs showed head-to-head results (deviance explained: 0.13%, coefficient: -0.02], supported by similar Akaike information criteria (AICs) (34945). However, according to the LWRs model’s curve, lower COVID-19 incidence rates were recorded in days with the temperatures ranged 60-80 percentiles, equal to 20-25°C in cold climate and 25-35°C in warm climate; while the rates were increased at lower and upper temperatures. Conclusion: Daily COVID-19 incidence rate cannot be explained as a function of daily temperature, in Southern Iran. Apparently, higher rates of disease transmission out of the range of 20-25°C for cold temperatures and 25-35°C for warm climates might be linked to people’s indoor gatherings, coupled with insufficient ventilation.