Flu outbreaks are subject to humidity – not just heat

The flu’s not too fussy, it seems: whether it’s warm and muggy or cold and dry, the virus will spread – no matter where you are in the world.

Ethan Deyle from the University of California, San Diego and colleagues in the US used modelling to determine whether outbreaks of the disease across the globe could be predicted from factors such as absolute humidity, temperature, relative humidity and precipitation.

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Reporting in the Proceedings of the National Academy of Sciences, they found absolute humidity affects influenza rates across all latitudes – and whether this is a positive or negative effect is determined by temperature.

In other words, in temperatures above 21 to 24 °C, wetter conditions – high absolute humidity – mean a higher likelihood of an influenza outbreak, as do drier conditions (low absolute humidity) in cooler temperatures.

This could help forecast the spread of the disease in our warming world.

While the importance of environmental variables such as temperature and humidity has been known a while, says Allen Cheng, an epidemiologist at Monash University in Melbourne, Australia, this research shows how they work together across the globe to drive influenza incidence rates. 

It sounds simple, but modelling disease transmission is no easy feat.

Researchers must obtain data – and flu is often underreported in the tropics, Cheng says – then stitch together the complex relationships between a huge number of variables, such as human population dynamics, host-pathogen dynamics and environmental variables. 

Rob Moss, a mathematical biologist at the University of Melbourne in Australia, says Deyle and his crew did just that. Their modelling “would appear to be the best way to try and understand what this relationship between climate and influenza might look like”.

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The relationship also fits with what we know about how the influenza virus acts in different temperatures and humidity levels. 

Low humidity in the cool of winter may increase virus survival – as dry air means we have drier nostrils, it can efficiently hop between hosts.

In the tropics, though, where winter is much milder, the mechanism is less clear. Perhaps flu virus particles survive longer in water droplets.

Forecasting influenza outbreaks could be particularly beneficial for public health initiatives. Having more information about how environmental factors and influenza interact together will help refine current predictive models, Cheng says.

And in the face of global warming and associated climate swings, Moss and Cheng believe such work might also help us understand and deal with changes to patterns of influenza outbreaks that will stem from climate change.

“If we think we’re in for a particularly hot winter, or a particularly humid winter, it might improve our ability to expect what effect that will have,” Moss adds.