Does extreme heat and humidity impact the SARS-CoV-2 viral transmission?

The role of relative humidity and temperature in SARS-CoV-2 viral transmission has not yet been studied in a robust way as of this writing and is currently poorly understood (Gupta, 2020).  However, several early studies indicate a reduction in transmission with higher temperatures and humidity.  None suggest that the virus would disappear.Research by Lui (2020) suggests that low temperature, mild diurnal temperature range and low humidity likely favored COVID19 transmission in China.

An observational study by Qi et al (2020) found an association between increased daily temperature and reduced daily number of cases. An association was also found with increased relative humidity and reduced daily number of cases. Ficetola and Rubolini (2020) found that case growth rates in 121 countries were highest in temperate regions. Islam et al (2020) looked at 310 geopgraphic regions and found that higher temperature and humidity were associated with a lower incidence of cases. For all studies, weaknesses include incomplete case ascertainment and possible uncontrolled confounding. Additional, clues as to how SARS-CoV-2 may behave can be gleaned from studies on similar respiratory viruses and their ability to remain viable in the air. Noti et al. studied influenza and indoor humidity and found that at usual indoor temperatures, increased humidity (>43%) was associated with decreased airborne infectivity of influenza at 60 mins: roughly 70% infectivity at 23% relative humidity dropping to 14% infectivity at 43% relative humidity. 

Other clues about the role of temperature in the transmission of viruses can be drawn from epidemiological studies that have demonstrated the seasonality of influenza infection.  Winter has consistently been the most common season when influenza peaks. Factors explored include seasonal host health, viral evolution, indoor heating, air travel, crowding (Lofgren, 2007).  Lowen et al. used the guinea pig model to show that cold and dry temperatures favored transmission of influenza.  Influenza infected guinea pigs had a much longer duration of peak shedding of virus at 5 C (41 F) than at 20 C (68 F). However, if temperature were an absolute predictor of infectivity we would not see viral outbreaks in the tropics which we absolutely do. Newly emerging  viruses  causing zoonotic disease have behaved in a less seasonably predictable way which may be due to persistent shedding in animal reservoirs (Dowell, 2004). More studies are needed to determine to what extent
SARS-CoV-2 viral transmission is impacted by extreme heat and humidity.


Noti JD, Blachere FM, McMillen CM, Lindsley WG, Kashon ML, Slaughter DR, et al. (2013) High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs. PLoS ONE 8(2): e57485.
Lowen AC, Mubareka S, Steel J, Palese P (2007) Influenza virus transmission is dependent on relative humidity and temperature. PLoS Pathog3(10): e151. doi:10.1371/journal.ppat.0030151
Lofgren, E., Fefferman, N. H., Naumov, Y. N., Gorski, J., & Naumova, E. N. (2007). Influenza seasonality: underlying causes and modeling theories. Journal of virology, 81(11), 5429–5436.
Dowell, S. F., & Ho, M. S. (2004). Seasonality of infectious diseases and severe acute respiratory syndrome-what we don't know can hurt us. The Lancet. Infectious diseases, 4(11), 704–708.
Gupta SRaghuwanshi GSChanda A. Effect of weather on COVID-19 spread in the US: A prediction model for India in 2020. Sci Total Environ. 2020 Apr 21;728:138860. doi: 10.1016/j.scitotenv.2020.138860. [Epub ahead of print] [I]
Liu JZhou JYao JZhang XLi LXu XHe XWang BFu SNiu TYan JShi YRen XNiu JZhu WLi SLuo BZhang K1. Impact of meteorological factors on the COVID-19 transmission: A multi-city study in China. 
 2020 Apr 9;726:138513. doi: 10.1016/j.scitotenv.2020.138513. [Epub ahead of print] [C]
Qi HXiao SShi RWard MPChen YTu WSu QWang WWang XZhang Z. COVID-19 transmission in Mainland China is associated with temperature and humidity: A time-series analysis. Sci Total Environ. 2020 Apr 19;728:138778. doi: 10.1016/j.scitotenv.2020.138778. [Epub ahead of print [I]
Ficetola, G. F., and D. Rubolini. 2020. Climate affects global patterns of COVID-19 early outbreak dynamics. 2020 Apr 20.

Islam N., S. Shabnam, and A. M. Erzurumluoglu. Temperature, humidity, and wind speed are associated with lower Covid-19 incidence. 2020 Mar 31.

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