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Why weather may be a missing link in flu prediction

People walk during cold temperature and high winds in Manhattan, as deep cold spread across the northeast United States in New York City on Feb. 4. (Jeenah Moon/Reuters)
4 min

It’s no mystery that the weather affects our health. From achy joints to sinus flare-ups, weathering the weather often means more than just deciding what to wear. With the flu raging this winter, unlocking how the weather influences the virus could go a long way in helping to develop measures to prevent its spread.

In recent years, scientists have identified intriguing connections between weather and flu. They’ve found, for example, that flu tends to spread fastest when the air is cold and dry. A study in 2020 also linked big weather swings in the fall to spikes in flu during winter. But these linkages are still somewhat tentative and clouded by other influences. Weather is only one piece of this intricate puzzle of predicting flu transmission. Even so, scientists are working to one day integrate weather forecasts into disease-monitoring models.

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The flu follows a predictable pattern and hits its peak in January and February annually in the United States. Scientists have proposed several hypotheses for the wintertime peak including weather fluctuations, an increase in indoor activity and changes in our immune system. The connections are very involved, said Jeffrey Shaman, who studies viral spread and weather at Columbia University.

Shaman’s work has revealed a strong correlation between the flu virus and humidity. He notes that most wintertime flu transmission occurs indoors, which is probably linked to the drier and heated inside air.

In his analysis of lab studies, he found a significant connection with humidity changes and how long the flu virus survived.

The “virus doesn’t stay viable as long when conditions are more humid, and stays more viable when conditions are drier,” Shaman said in an interview.

“It seems a little counterintuitive — we often think if things dry out they’re not going to survive, but that’s for bacteria and other cells. Viruses are different,” said Linsey Marr, an expert in the transmission of particles in the air at Virginia Tech.

But it gets complicated. Marr’s research also shows flu survival is elevated at both low and high extremes of humidity. At high humidity, she said, minimal evaporation means respiratory droplets and aerosols survive intact, so there is little trigger to displace the virus. So the sweet spot for the least viability for the flu bug to survive may be the middle range of humidity.

This theory was confirmed to some degree in a real-world 2018 study by Jennifer Reiman et al. that found humidification of classrooms resulted in lower numbers of flu cases and detectable flu virus on surfaces. The caveat in this study was its small size, and Marr suggests more, larger-scale studies like this one are needed.

Research by Robert Davis, a climatologist at the University of Virginia, affirms a connection between flu transmission and cold, dry air.

“There is reasonably convincing evidence from the biometeorological community that influenza transmission is affected by cold and or dry weather,” Davis said in an email.

But Davis is reluctant to focus too much on the role of humidity. “I also believe the role of humidity is still unsettled, mostly because of the difficulty in separating out humidity and temperature effects in many U.S. climates,” he said.

Then there’s the question of how humidity may affect our immune response separately from impacting the virus. Humidity “could mediate transmission through effects on our immune response, too,” Marr said. “Studies have shown we have stronger, better immune response when the air is more humid compared to when it’s dry.”

Researchers are working to one day include weather forecasts in flu prediction models, but work remains to better isolate the links.

“Predicting peaks in flu activity can be tricky, but the weather forecasting aspect is the easy part,” Davis said.

For his part, Shaman acknowledged the importance of controlled experiments to figure it all out. “But how that maps over into the real world, oh boy … that’s a real mess!” he said

Jim Duncan recently retired from his 40-year career as chief meteorologist with NBC12 WWBT-TV in Richmond. He runs his own meteorological consulting firm, Jim Duncan, LLC, serving clients in media, education and other industries.