COVID-19: High temperature and high humidity reduce the transmission of COVID-19.

COVID-19: High temperature and high humidity reduce the transmission of COVID-19. Spencer EA, Heneghan C.

Published on July 2, 2020

Reference Wang J, Tang K, Feng K, Lin X, Weifeng L, Chen K, Wang F. High temperature and high humidity reduce the transmission of COVID-19 Available at SSRN: 2020
Study type
Country China and USA
Setting Global
Funding Details Non reported
Transmission mode Meteorological, Urban environment
Exposures Temperature, Relative humidity, density

Bottom Line

Some of the variations in COVID-19 transmission between cities worldwide can be explained by higher temperature and humidity being associated with lower transmission rates.

Evidence Summary

69,498 cases in China and 740,843 in the USA were included in the analysis.

  • A 1°C increase in temperature was associated with a reduction in R of 0.023 (95% CI [-0.0395 to -0.0125) in China and 0.020 (95% CI -0.0311 to -0.0096) in the US. 
  • A 1% relative humidity rise was associated with a reduction in RE by 0.0078 (95% CI -0.011 to -0.0045) in China and 0.0080 (95% CI -0.015 to -0.0010) in the US.

After the lockdowns in China and the US, temperature and relative humidity were still associated with changes in transmission but appeared to have a lower impact.  

A one thousand people per square kilometre rise in population density was associated with a 0.12 (95% CI [0.057, 0.18]) increase in the R-value before lockdown.

What did they do?

Cases with symptom-onset dates 19th January to 10th February 2020 for 100 Chinese cities, and cases with confirmed dates from 15th March to 25th April for 1,005 US counties.

The authors assessed the relationship between the transmissibility of COVID-19 and the temperature/humidity, controlling for various demographic, socioeconomic, geographic, healthcare and policy factors and correcting for cross-sectional correlation.

Data sources 

  • Temperature and relative humidity data are obtained from 
  • Population density, from 
  • Baidu Mobility Indexes are obtained from 
  • US temperature and relative humidity data are from National Oceanic and Atmospheric Administration at 
  • Population data are obtained from 
  • GDP and person income in 2018 for each county are obtained from 
  • Data describing mobility changes,  are obtained from and r
  • Gini index, are obtained from American Community Survey data at The number of ICU beds for each county is obtained from 

Study reliability

This is an observational study; other confounding factors may be responsible for the associations observed. 

The R2 of the  regression is 30% in China and 12% in the .S., which means about 70% to 88% of cross-city R-value fluctuations cannot be explained by temperature and relative humidity

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Unclear No Yes Yes Yes

What else should I consider?

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

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.