New research reports that temperature and humidity do not play a significant role in the spread of the coronavirus. The study was led by members from The University of Texas at Austin.

The authors aimed to better understand how temperatures influence the spread of the coronavirus. At the onset of the pandemic, it was hoped that the hot summer days would reduce its spread. The findings now, however, suggest that neither temperatures nor humidity play any significant role in the virus’ activity — its spread depends almost entirely on human behavior.

However, temperatures do have an influence on how people act, the team notes, so it can indirectly influence the spread.

All-weather bug

“The effect of weather is low and other features such as mobility have more impact than weather,” said team leader Dev Niyogi, a professor at UT Austin’s Jackson School of Geosciences and Cockrell School of Engineering. “In terms of relative importance, weather is one of the last parameters.”

The team lumped temperature and humidity together to form a single value, the “equivalent air temperature”. They then analyzed how variations in this value influenced the spread of the virus in different areas of the US and elsewhere between March and July of 2020. They also looked at the relationship between human behavior and coronavirus spread using cellphone data on a countrywide and statewide scale.

Across scales, they found that weather had nearly no influence on the spread of the virus. Compared to other factors, from a statistical point of view, weather had an influence (‘relative importance’) over the virus’ spread of less than 3%, the team reports, and they found no reason to believe that any type of weather was more conducive to infections than any other.

On the other hand, human behavior had an especially high influence. Taking a trip or spending time away from home were the two single largest contributing factors to COVID-19 spread, having a relative importance of around 34% and 26% respectively. Next were population and urban density, with a relative importance of about 23% and 13% respectively.

“We shouldn’t think of the problem as something driven by weather and climate,” Jamshidi said. “We should take personal precautions, be aware of the factors in urban exposure.”

Previous assumptions on the effect of weather on the virus were largely based on studies carried out in a laboratory setting or on related viruses, which may have skewed the results, according to the author.

Baniasad, a biochemist and pharmacist, said that assumptions about how coronavirus would respond with weather are largely informed by studies conducted in laboratory settings on related viruses. She said that this study illustrates the importance of studies that analyze how the coronavirus spreads through human communities.

“When you study something in a lab, it’s a supervised environment. It’s hard to scale up to society,” said Maryam Baniasad, a doctoral candidate at Ohio State University and co-author of the paper. “This was our first motivation to do a more broad study.”

One of the main lessons we should derive from the pandemic and this study is that we need to analyze phenomena on a “human scale” — the scale at which humans live their day-to-day lives — in order to properly understand them, says Niyogi.

“COVID, it is claimed, could change everything,” Niyogi said. “We have been looking at weather and climate outlooks as a system that we scale down, down, down and then seeing how it might affect humans. Now, we are flipping the case and upscaling, starting at human exposure scale and then going outwards. This is a new paradigm we will need for studying virus exposure and human environmental modeling systems involving new sensing and AI-like techniques.”

The paper “Global to USA County Scale Analysis of Weather, Urban Density, Mobility, Homestay, and Mask Use on COVID-19” has been published in the International Journal of Environmental Research and Public Health.