Temperature and COVID-19 Incidence: An Ecologic Study

Background: According to the hypothesis, COVID-19 is less prevalent in regions with warm climates. Contradictory results have led us to investigate the correlation between temperature and the cumulative COVID-19 incidence rate. Methods: We obtained COVID-19 data from CRONALAB, COVID-DASHBOARD, and MCMC databases of Fars Province, Iran, linked the data and finalized daily COVID-19 cases. The 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 counties, separately. Initially, for uniform data visualization, the average air temperature data were transformed into ranked percentiles. Then, to visually assess the study hypothesis, the distribution of COVID-19 cumulative incidence was visualized on percentiles of temperature. Given the non-linear distribution of the data, we performed exploratory analyses using the generalized additive models and locally weighted (polynomial) regressions to choose the best response function. Then, the generalized linear models were used to parametrically build the model. Results: The generalized additive models showed a small decreasing, near 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 locally weighted regressions model’s curve, lower COVID-19 incidence rates were recorded on days when the temperatures ranged from 60 to 80 percentiles, equal to 20°C to 25°C in a cold climate and 25°C to 35°C in a warm climate. This is while the rates increased at lower and upper temperatures. Conclusion: Daily COVID-19 incidence rate cannot be explained as a function of daily temperature in Southern parts of Iran. Higher rates of disease transmission out of the range of 20°C to 25°C for cold temperatures and 25°C to 35°C for warm climates might be linked to people’s indoor gatherings, coupled with insufficient ventilation.