Heat, humidity and UV rays linked to COVID-19 spread, YSPH finds

While physical proximity and the use of face masks are well-known factors in 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, the Yale School of Public Health (YSPH) finds.

A research team led by Assistant Professor Kai Chen 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. 

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.

The study, published today in the journal Nature Communications, 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 non-meteorological factors, including using the reproductive number adjusted for public health interventions and simultaneously controlling for spatially and temporally heterogenous confounders,” said Chen, a member of YSPH’s Department of Environmental Health Sciences and the Yale Center on Climate Change and Health (CCCH). “Accounting for the substantial differences in the epidemic curves and confounding factors among states and counties, we were able to quantify the nonlinear associations between meteorological factors and SARS-CoV-2 transmission in the United States in 2020.”

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

Yale Professor Robert Dubrow, director of CCCH and a co-author of this study, said the findings suggest that the role of meteorological factors in COVID-19 dynamics is meaningful but not dominant. “Public health measures, including vaccination, mask wearing, and social distancing, represent the primary strategies for mitigating transmission of SARS-CoV-2,” he said. 

Yiqun Ma, a doctoral student in the Department of Environmental Health Sciences, is the first author of this study. Sen Pei and Jeffrey Shaman at Columbia University Mailman School of Public Health are co-authors.


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