It’s not the heat — it’s the humidity.
Thanks to a recently released study from a team of Western North Carolina scientists, the old chestnut about miserable summer weather may get a new interpretation for the COVID-19 era. The research, published in the journal Science of The Total Environment, suggests that humidity plays a greater role than does the temperature in the spread of the novel coronavirus.
Lead author Jennifer Runkle, an environmental epidemiologist with the Asheville-based North Carolina Institute for Climate Studies at N.C. State University, emphasizes that the results are preliminary due to the limited available data for a disease with no recorded U.S. cases until January. But she says the findings could nonetheless help inform health officials about what to expect from COVID-19 and manage the ongoing pandemic.
“There are so many factors at play: testing capacity, human behavior like handwashing, even access to care,” Runkle explains. “Weather is just another factor that we need to be incorporating in our infectious disease modeling. That’s what our pitch is to the scientific community.”
Coast to coast
The study analyzes weather conditions and coronavirus transmission in eight U.S. cities where viral spread was apparent early in the pandemic, such as Seattle and New York City. By comparing temperature, absolute humidity and sunlight for each city on a given day with the number of cases later reported, the researchers were able to find associations between the weather and COVID-19.
In three of the study cities — Chicago, New Orleans and Albany, Ga. — a day of low humidity was a significant predictor of new COVID-19 cases for the following two weeks. The risk of viral transmission was up to twice as high as normal in those cities when humidity fell within a specific low range. In contrast, neither temperature nor sunlight were strongly associated with coronavirus spread.
Study co-lead author Maggie Sugg, an assistant professor of medical geography at Appalachian State University in Boone, says that the humidity finding is consistent with patterns observed for other viral diseases such as the flu, which generally spreads most during dry winter conditions. “This is because the virus itself lives longer in low-humidity environments, and people’s respiratory systems are also more vulnerable to disease,” Sugg explains.
Runkle notes that the results also match up with early findings from China, where the coronavirus that causes COVID-19 (SARS-CoV-2) originated. She says her team’s work is among the first research to examine the ties between weather and COVID-19 transmission in the U.S., an important piece of the global puzzle for a disease that hasn’t been seen before.
While infection rates of coronaviruses and other respiratory viruses generally vary by season, Runkle says, that pattern won’t necessarily be the case for SARS-CoV-2, which is spreading into a world with no existing immunity. “What makes it complicated when looking at COVID-19 is that it’s a novel coronavirus,” she says. “These viruses typically don’t behave like their normal selves until they become established in the population, so scientists around the country and globe have been tackling different aspects of it.”
Sugg says the team plans to follow up on the research later in the year, when they will be able to draw on more months of data and better adjust their mathematical models for the effects of different social distancing measures on case counts. “Typically when we do these types of studies, we work with several years of data,” she points out. “We’re working with not even a full season.”
One important question to be resolved is whether higher humidity in the coming months will hinder COVID-19 transmission. The study’s authors say it’s too soon to draw that conclusion from the data, and they emphasize that limiting social interactions and promoting good public health practices like mask wearing will go much further toward slowing the spread of the virus than will changes in humidity.
But as North Carolina moves into the summer and fall, notes Sugg, a different aspect of the weather could become considerably more important than humidity. Hurricane season, which runs from June through November, could drive mass evacuations from the coast, bringing people from different parts of the state into closer contact. “Any time you have greater mobility, you’re going to have a greater introduction of a new disease,” she says.
Climate change, Sugg adds, is making extreme weather events such as hurricanes more dangerous, especially for the state’s residents of color and those living in poverty. According to the N.C. Climate Risk Assessment and Resilience Plan published in June by the state Department of Environmental Quality, “vulnerable communities will be most at risk of flooding occurrences due to hurricanes; with hurricanes happening in short succession, vulnerable communities will struggle to recover between hurricanes.”
“These are just compounding risks for populations that are already really vulnerable,” Sugg says. “COVID and climate change working together are going to amplify climate vulnerability for populations that are already struggling.”