Non-linear effects of meteorological factors on COVID-19: an analysis of 440 counties in the Americas

Background In the last three years, COVID-19 has caused significant harm to both human health and economic stability. Analyzing the causes and mechanisms of COVID-19 has significant theoretical and practical implications for its prevention and mitigation. The role of meteorological factors in the transmission of COVID-19 is crucial, yet their relationship remains a subject of intense debate. Methods To mitigate the issues arising from short time series, large study units, unrepresentative data and linear research methods in previous studies, this study used counties or districts with populations exceeding 100,000 or 500,000 as the study unit. The commencement of local outbreaks was determined by exceeding 100 cumulative confirmed cases. Pearson correlation analysis, generalized additive model (GAM) and distributed lag nonlinear model (DLNM) were used to analyze the relationship and lag effect between the daily new cases of COVID-19 and meteorological factors (temperature, relative humidity, solar radiation, surface pressure, precipitation, wind speed) across 440 counties or districts in seven countries of the Americas, spanning from January 1, 2020, to December 31, 2021. Results The linear correlations between daily new cases and meteorological indicators such as air temperature, relative humidity and solar radiation were not significant. However, the non-linear correlations were significant. The turning points in the relationship for temperature, relative humidity and solar radiation were 5°C and 23°C, 74% and 750 kJ/m2, respectively. Conclusion The influence of meteorological factors on COVID-19 is non-linear. There are two thresholds in the relationship with temperature: 5°C and 23°C. Below 5°C and above 23°C, there is a positive correlation, while between 5°C and 23°C, the correlation is negative. Relative humidity and solar radiation show negative correlations, but there is a change in slope at about 74% and 750 kJ/m2, respectively.