If you’re wondering when you might get the flu, a new study indicates you should keep an eye on your local weather report.

According to research published in the Journal of Clinical Virology, a season’s first cold snap below 37 degrees Fahrenheit (or zero degrees Celsius, as defined in the study) preceded a mass spread of the flu in Gothenberg, a large metropolitan area of Sweden.

The researchers suggest that if you keep your eye on the weather and watch for the first major dip in the temperature, you can essentially mark your calendar in anticipation for an influx of the illness. 

“We believe that this sudden drop in temperature contributes to ‘kickstarting’ the epidemic,” lead study author Nicklas Sundell, an infectious diseases specialist at Sahlgrenska University Hospital in Sweden, said in a statement. 

To figure out how the weather and flu outbreak may be connected, researchers analyzed nasal swabs of 20,000 people in the Gothenberg area over three seasons. The number of people who caught the flu was then compared with weather data from the Swedish Meteorological and Hydrological Institute.

After each season, Sundell’s team noticed one consistent finding: The first really cold period with low outdoor temperatures and low humidity was always followed by a week of a mass influenza outbreak. 

Why cold weather can activate the spread of the flu

The researchers say that aerosol particles containing virus and liquid are more able to spread in cold and dry weather. So, in theory, if you sneeze or cough and the surrounding air is very dry, the air will absorb the moisture and the particles containing the virus remain airborne for a longer period of time. 

While cold weather can make the flu spread more easily, it can also spur other viruses such as RS-virus and coronavirus, which cause respiratory tract infections, the researchers wrote.

“If you can predict the start of the annual epidemics of the flu and other respiratory viruses, you can use this knowledge to promote campaigns for the flu vaccine and prepare emergency wards and hospital staff in advance for an increased number of patients seeking care,” Sundell said. 

Since the virus changes each year, it is hard to estimate the number of people who will contract the illness each season. And while the U.S. Centers for Disease Control and Prevention does not count the number of deaths related to the flu, the number of people who do get it ranges between 3,000 to 49,000 per year. 

Luckily, there are ways to ward off the illness regardless of the weather. Washing your hands, coughing into the crook of your arm and getting the vaccine are the best ways to prevent the flu from spreading. People over the age of 65, pregnant women and young children are the most high risk for developing complications related to the virus, according to the CDC. 

Although the study seems like an interesting way count down to flu outbreaks, it is not without its caveats. The research only identifies there’s a correlation between the two; it has yet to be confirmed if one causes the other. The researchers also only tested for this effect in Sweden, so it is unclear if the phenomenon would occur in other cities. And the virus still spreads aggressively in warmer climates as well ― so just because you live in a tropical area doesn’t mean you’re immune. 

In other words, get the vaccine and wash those hands. You’ll thank yourself later.

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