Heat, Humidity & UV Rays Linked to Covid-19 Transmission

While physical proximity and not wearing face masks are well-known factors in the spread of COVID-19, meteorological variables such as temperature, humidity and ultraviolet (UV) radiation also appear to aid the transmission of SARS-CoV-2, the virus that causes COVID-19.

A research team led by Kai Chen, Ph.D., assistant professor in the Department of Environmental Health Sciences and a member of the Yale Center on Climate Change and Health, investigated 2,669 counties throughout all regions and states in the United States from March 15 to Dec. 31, 2020, to determine how readily the virus was transmitted.

They found that warmer temperatures (above 20 degrees Celsius, or 68 degrees Fahrenheit), increased humidity and higher levels of UV radiation were moderately associated with a lower reproductive number (a measurement of how many new infections are caused by a single infected person in a fully susceptible population), meaning that these factors were likewise associated with decreased person-to-person transmission. Of the three factors, absolute humidity (the actual amount of moisture in the air, regardless of the air’s temperature) played the greatest role.

The study provides one of the most robust bodies of scientific evidence yet linking weather conditions to the transmission of SARS-CoV-2. Early epidemiological studies of COVID-19 and meteorological factors offered uncertain and contradictory findings, mainly due to short study periods, inadequate control for confounding, and inappropriate COVID-19-related outcome variables and statistical methods.

“A unique strength of our study was its comprehensive control for the space- and time-varying nonmeteorological factors, including using the reproductive number adjusted for public health interventions and simultaneously controlling for spatially and temporally heterogenous confounders,” said Chen.

In total, Chen and the members of his research team determined 17.5% of the virus’s reproductive number was attributable to meteorological factors. Specifically, they found that temperature accounted for 3.73%, humidity for 9.35% and UV radiation for 4.44%.

The fractions attributable to meteorological factors generally were higher in northern counties than in southern counties, meaning the people living in regions such as New England may need to be especially vigilant about the increased transmissibility of SARS-CoV-2 in winter, when cold and dry weather and low levels of UV radiation offer favorable conditions for COVID-19 spread, the researchers said.

The study is published in the journal Nature Communications.