Electron microscope view of COVID-19 virus

Heat, Humidity, UV Rays Linked to COVID-19 Spread

June 16, 2021

While physical proximity and the use of face masks are well-known factors affecting the spread of COVID-19, new research links meteorological variables such as temperature, humidity, and ultraviolet (UV) radiation to the transmission of SARS-CoV-2, the virus that causes COVID-19, a study by Columbia University Mailman School of Public Health and the Yale School of Public Health finds.

The researchers investigated 2,669 counties throughout all regions and states in the United States from March 15 to December 31, 2020, to determine how readily the virus was transmitted. The results are published in the journal Nature Communications.

They found that warmer temperatures (above 20° Celsius), 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 played the greatest role.

“Our findings suggest that in the wintertime, the virus will be more transmissible,” says co-author Jeffrey Shaman, PhD, professor of environmental health sciences at Columbia Mailman School. “In the long run, the virus may persist as a wintertime ailment. We need to plan for this, especially in the northern regions of the U.S., where we could see simultaneous seasonal outbreaks of COVID-19 and influenza.”

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

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 study is 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.

The study’s senior author is Kai Chen, a Yale environmental health scientist. Its first author is Yiqun Ma, a Yale doctoral student. Sen Pei and Jeffrey Shaman at Columbia University Mailman School of Public Health are co-authors.

The study was funded by the China Scholarship Council, National Institutes of Health, the National Science Foundation, Morris-Singer Foundation, and the High Tide Foundation.