COVID-19: Effects of temperature and humidity on the daily cases and deaths

Effects of temperature and humidity on the daily cases and deaths. Jefferson T Heneghan C. 27/05/2020.

Published on May 27, 2020

Reference Wu Y, Jing W, Liu J, et al. Effects of temperature and humidity on the daily new cases and new deaths of COVID-19 in 166 countries. Science of The Total Environment 2020;729:139051. 2020
Study type
Country 166 countries (excluding China)
Funding Details National Key Research and Development Program of China and The National Natural Science Foundation of China.
Transmission mode Person to Person
Exposures Symptomatic and Asymptomatic exposures

Bottom Line

Temperature and relative humidity were negatively correlated with daily new cases and daily new deaths of COVID-19.

Evidence Summary

Adjusted data for potential confounders show that temperature and relative humidity were negatively associated with daily new cases and daily new deaths:

  • A 1 °C increase in temperature was associated with a 3.1% (95% CI: 1.5% to 4.6%) reduction in daily new cases, and a 1.2% (95% CI: 0.44%  to 2.0%) reduction in daily new deaths.
  • A 1% increase in relative humidity was associated with a 0.9% (95% CI: 0.5% to  1.2%) reduction in daily new cases and a 0.51% (95% CI: 0.34% to 0.67%) reduction in daily new deaths.

What did they do?

The study assessed the relationship between relative humidity (definition reported) and temperature (in degrees Centigrade) and daily new cases and deaths in 166 countries excluding China.

The data was sourced from daily WHO situation reports and the National Oceanic and Atmospheric Administration Center as at the 27th of March 2020. The authors carried out correlation testing adjusting for five potential confounders known to affect the transmission of respiratory viruses and our response to them: wind speed, median age of the national population, Global Health Security Index, Human Development Index, and population density. The results are expressed as % changes in daily new cases and deaths.

The authors carried out a sensitivity analysis accounting for the lag in accumulation cases in countries at different stages of the cure. The bulk of the data as of March 27 (509,164 cumulative confirmed cases and 23,335 deaths) came from Italy, the USA and Spain:  states where their index case was >10 days before data collection and countries with over 100 cumulative cases.

Temperature ranges were −5 to 34°C, relative humidity was 11% to 89%.

The study excludes China and its dataset for unclear reasons, even though the study is state-funded. In addition, the optimum conditions for transmission are not defined, although visual inspection of Figure 1 in the paper shows that temperature of 5 to 15°C and relative humidity of 40 to 60% are associated with the heaviest burden.

Study reliability

The number of confirmed cases will be an underestimate of the true number, especially in low-income regions. The effects of policies and measures on COVID-19 transmission were not assessed. The lack of other countries and the timings are a limitation. Finally, the temperature and humidity may not reflect the temperature and humidity of exposed individuals.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Unclear * No Yes Yes No
* Partly – a global ecological study

What else should I consider?

This evidence from this study needs replicating. Making inferences based on one study is prone to errors and the ecological fallacy. In the absence of a predefined protocol, studies are subject to significant uncertainties due to changes in the analysis that may not be clear. Studies with  a longer follow-up that varies across the seasons will increase our understanding of the play of the variables.

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About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Tom Jefferson

Tom Jefferson, epidemiologist.