Researchers Release New Findings on How Climate Drives Flu Outbreaks

People in the northern hemisphere know to brace for a flu season in the cold months of the year, but in temperate climates without seasonality, the correlation between influenza outbreaks and climate has not always been understood. A recent study explains the link between absolute humidity, temperature, and flu outbreaks.

In North America, flu season tends to begin in October and can last as late as May. According to an analysis by the Centers for Disease Control and Prevention of the 34-year period from 1982 through 2016, the flu season peaked 14 times in February, seven times in December, and six times in March. In the United States and other countries with temperate climates flu season generally occurs in months with certain temperature and humidity conditions, the relationship between temperature and humidity is less understood when it comes to outbreaks in tropical climates.

In a new study published in the Proceedings of the National Academy of Sciences of the United States of America, researchers from the University of California and Stanford University investigated climate-related drivers of influenza. “Flu epidemics are very complex phenomena that have been shown by various studies at various times to depend on many different kinds of variables,” said study author Ethan R. Deyle. “Not just environment, as we studied here, but human movement patterns, behavior, diversity across types or strains, and antigenetic drift.”

The flu is often transmitted by droplets that infected people release when they sneeze or cough, but the size of the droplets and their transmission in the air are affected by relative humidity. Likewise, viral shedding in mammals gets higher as the temperature falls.

By studying time series data, the research team found that absolute humidity — the amount of water vapor in the air regardless of temperature – is a driver of influenza across latitudes. In colder temperatures, high humidity reduces influenza incidence, while in higher temperatures, high humidity increases influenza incidence, showing that the effect of absolute humidity on influenza is U-shaped. The positive effect of absolute humidity on influenza, note the authors, was greatest within a limited range of temperatures, between 75 °F and 85 °F. Once the temperature gets higher, the effect of absolute humidity drops off, showing that the changing effect of humidity occurs between 70 °F and 75 °F.

“Our results point towards the most favorable transmission in cold, dry air or hot, humid air,” explained Deyle. “So unless you are someone that likes to turn their heat up to 80ºF in the winter, the evidence suggests that humidifying your home would be the way to go if you wanted to make your home climate control choices based solely on flu transmission.” He added that since influenza transmission is most likely to occur in public spaces, such climate adjustments in the home are not likely to impact a flu outbreak.

However, the authors noted that the results of their study may make the case for some public health initiatives which may help prevent flu transmission, such as placing humidifiers in schools and hospitals during cold, dry, temperate winters, and using air conditioners set above 75 °F or dehumidifiers in public buildings in the tropics. “Based on work that precedes ours, there has already been discussion of public health interventions based on an effect of absolute humidity on flu transmission,” said Deyle. “We hope that our study will help spark that action. However, those discussions have generally focused on temperate latitudes, where cold, dry air appears to favor transmission. Our work will hopefully broaden this discussion to include possible public health intervention in tropical countries where hot, wet air appears to favor transmission.”