Seasonal UV radiation and global Covid-19 trends


The novel coronavirus has now seen all the seasons in both hemispheres, but the effect of seasonal factors such as temperature and humidity remains to be fully understood. 

  • New research has now looked at the influence of another seasonal variable - ultraviolet radiation from the Sun - and found evidence is that higher natural UV radiation reduces Covid-19 transmission.


The caveat

  • The seasonal changes in UV are just one of many causes of change in Covid-19 case rates. And the influence of UV is modest compared to policy measures such as travel restrictions, school closures, or event cancellations. 
  • On average across the northern hemisphere, we find that prior estimates of the effect of these social distancing policies are 3-6 times larger than our estimated effects of seasonal changes in UV.
  • Because other research has looked at how artificial UV light can inactivate the coronavirus, it is also important to note that this is not the same as the effect of the sun’s UV radiation. 
  • UV disinfectants currently in use rely on wavelengths in a range called UV-C. Such wavelengths do not reach us naturally from the Sun, as these are absorbed by the ozone layer
  • In sunlight that reaches the Earth, the wavelengths are in the range known as UV-A (and to an extent in the range UV-B).


The extent of variations

  • A change in UV exposure by 1 standard deviation, the analysis found, reduced the growth rate of new cases by around 1 percentage point over the following two weeks. 
  • A change of 1 percentage point, for context, is approximately equivalent to the change in UV between April and May in Delhi.
  • Based on changes in UV, the model predicted Covid-19 growth rates for the temperate zones north and south of the tropics. Between January and June, Covid-19 growth rates would increase by 7.3 percentage points in southern temperate locations and decline by 7.4 percentage points in the northern temperate ones. 
  • Indeed, infection rates did appear to have decreased in much of the northern hemisphere during the summer, the researchers noted, but observed that many parts of the northern hemisphere also relaxed their pandemic restrictions during the same period. 
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