Coronaviruses are sensitive to humidity and are less likely to thrive at moderate levels of humidity, a series of experimental studies over the years have shown.
COVID-19 | Interactive map of confirmed coronavirus cases in India
Earlier this month, a study by researchers at the Massachusetts Institute of Technology (MIT) in the United States said that regions experiencing a monsoon might see a slowing — but not a stop — to the transmission of the virus and, therefore, could help mitigation strategies in tropical countries such as India.
The maximum number of cases were reported in regions with an average humidity range of 3-9 g/cubic metre, which comprised the United States and several European countries that are inundated by COVID-19 infections.
Tests or temperature?
However, while warmer and humid countries such as Singapore, Australia, Taiwan and Qatar had comparatively fewer infections — attributable to higher testing — it was possible that warmer temperatures retarded the spread of the virus, the authors — Qasim Bukhari and Yusuf Jameel — contend.
The study isn’t yet peer-reviewed and is available at the open access repository Social Science Research Network.
A 2018 study in the peer-reviewed Applied and Environment Microbiology concluded that influenza and coronaviruses thrived when the relative humidity (RH) exceeded 85% or dipped below 60%. RH is the proportion of water vapour in the air compared to what’s the maximum possible. There is a “significant decrease in infectivity at mid-range RHs (60 to 85%),” the paper notes.
It is important to note that these studies didn’t study actual coronaviruses but a bacteriophage Phi6, which is considered a safer surrogate.
A similar experiment reported in the same journal in 2010, that tested the ability of viruses to survive on stainless steel surfaces, found that at 4°C, infectious viruses persisted for as long as 28 days, and the lowest level of inactivation occurred at 20% RH.
Inactivation was quicker at 20°C than at 4°C at all humidity levels; the viruses persisted for 5 to 28 days, and the “slowest inactivation” occurred at low levels of RH.
