Humidity (not temperature) influences SARS‐CoV‐2 transmission

A recent study conducted in New South Wales, Australia, demonstrates that there is an inverse relationship between relative humidity and transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The study is published in the journal Transboundary and Emerging Diseases.

Since the emergence of coronavirus disease 2019 (COVID-19) December 2019 in Wuhan, China, many studies have investigated whether climatic factors, such as temperature, humidity, wind speed, etc., can influence the spread of the SARS-CoV-2 virus. Since SARS-CoV-2 is predominantly spread via respiratory droplets and aerosols, atmospheric temperature and relative humidity will likely affect the viral spread by challenging the sustainability of these particles in the air.

SARS-CoV-2 viruses binding to ACE-2 receptors on a human cell, the initial stage of COVID-19 infection. Image Credit: Kateryna Kon / Shutterstock
SARS-CoV-2 viruses binding to ACE-2 receptors on a human cell, the initial stage of COVID-19 infection. Image Credit: Kateryna Kon / Shutterstock

Current study design

In the current study, the researchers aimed at investigating the effects of several climatic factors on SARS-CoV-2 transmission during the ascending and descending phases of the COVID-19 pandemic in the eastern state of New South Wales, Australia.    

The researchers included all reported cases of COVID-19 in New South Wales between January and May 2020 for the analysis. They used postcode data to associate case reports with the closest weather station. For comparison analysis, they gathered information on several climatic factors, including temperature, relative humidity, rainfall, and wind speed, which were recorded by the Bureau of Meteorology at 9 a.m. and 3 p.m.

Relative humidity is negatively correlated with viral spread

By comparing each climatic factor with COVID-19 incidences, the researchers observed that during the ascending phase of the pandemic, the mean temperature was higher, and the relative humidity was lower compared to that during the descending phase. Of all climatic factors, relative humidity was found to be negatively correlated with COVID-19 cases throughout the pandemic. Precisely, with a 1% reduction in relative humidity, there was a 7.7% and 6.8% increase in daily COVID-19 cases in the two phases of the pandemic, respectively. This effect was more prominent when the relative humidity was more than 79% and 75% in the ascending and descending phases of the pandemic, respectively.     

However, the researchers did not observe any correlation between COVID-19 cases and other climatic factors, such as temperature, rainfall, and wind speed.

How relative humidity affects viral spread?

According to the researchers, in a highly humid environment, aerosol particles containing the virus become larger like droplets, and thus, cannot remain suspended in the air. In contrast, when the air is less humid, these particles tend to shrink, and thus, can stay in the air for a prolonged period of time.

However, this theory will be feasible only in the case of airborne transmission of SARS-CoV-2. Recently, the World Health Organization admits that aerosol-based airborne transmission of SARS-CoV-2 may be possible, particularly in closed settings.

Take-home message      

As mentioned by the researchers, the current study findings raise an important question: is it possible to restrict the spread of SARS-CoV-2 by merely increasing the relative humidity?

According to the researchers, the link between humidity and COVID-19 cases is not that straightforward. In the current study, the climatic data were obtained from meteorological stations, and an assumption was made that people were mostly infected in outdoor settings. However, in reality, most transmissions are believed to be happening in indoor settings. Since it is difficult to measure the indoor climatic condition and compare it with case reports, determining the actual impact of relative humidity on COVID-19 cases is challenging.    

Although the current study predominantly involved COVID-19 cases that occurred during the autumn in the southern hemisphere, the same negative correlation between relative humidity and COVID-19 cases was observed in the northern hemisphere countries like China during the winter season. This observation indicates that the correlation may exist worldwide and that SARS-CoV-2 is more sensitive to humidity than any other climatic factors.

Although the study findings suggest that aerosol-based transmission of SARS-CoV-2 can increase in low humid conditions, it is always important to remember other modes of viral transmission (respiratory droplets) before directly jump into any conclusion.

Overall, the observations made in this study can help authorities deploy effective public health interventions at the right time to stop the pandemic successfully.     

Journal reference:
  • Ward, MP, Xiao, S, Zhang, Z. Humidity is a consistent climatic factor contributing to SARS‐CoV‐2 transmission. Transbound Emerg Dis. 2020; 00: 1– 6.
Dr. Sanchari Sinha Dutta

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.


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