Not all flu seasons are created equal. Sure, the 2017-2018 season was especially bad in the United States, sending about 30,453 people to the hospital. But once you zoom out, it appears that the length of the flu season varies in terms of length depending on what city you’re in. As researchers reveal in a study published in Science on Thursday, larger metropolises have very different flu seasons than smaller cities, though the reason why might not be what you expect.

The study, led by Oregon State University population biologist Benjamin Dalziel, Ph.D., shows that in large cities, the incidence of flu is spread out over a longer period of time, whereas in smaller cities, there are spikes in flu cases in a shorter period of time. Dalziel acknowledges the findings sound a bit unexpected.

“It was counterintuitive to find that the larger cities had the more spread-out epidemics,” he tells Inverse.

flu season map
A CDC animated map showing the spread of the flu during the 2017-2018 flu season.

Anyone who has ever caught a cold from someone else intuits to some degree that infections spread more efficiently when people are close together and well connected, and so it would make sense that epidemics in larger cities would be bigger and more explosive because everyone is all up in each others’ space. “And yet, we found that in these larger cities we have these more diffuse epidemics,” says Dalziel.

It was a conundrum he and his colleagues resolved by digging a little deeper into the mechanics of a spreading virus, using weekly flu incidence data from doctors in more than 600 U.S. regions. Turns out that the spread of the flu isn’t as simple as people forgetting to sneeze into their elbows and wash their hands. Virus particles hang in the air around a sick person like a cloud, says Dalziel, and the “specific humidity” (a measure of the amount of moisture in the air) of the air in a city greatly influences the size of that cloud and its ability to infect others. But the size of a “cloud of risk,” he continues, matters less in certain cities.

The size of an infected person’s cloud of risk gets bigger as the specific humidity drops, as it does in the winter, especially so during “peak” flu season. This is pretty much consistent, no matter what city you’re in. But key to solving the conundrum of the long, “smoldering” flu season in big cities was realizing that in those tightly packed metropolises, it doesn’t matter how big or how small a person’s cloud of risk is.

If you’re crammed next to a dozen sick passengers on a subway or bus, your risk of getting infected is higher, even if everyone’s clouds are tiny. And so, even at the tail end of the flu season, when specific humidity isn’t ideal for big clouds, people in cities tend to infect each other anyway — hence, the unexpectedly drawn-out flu season.

cloud of risk
To better imagine how the flu spreads, it helps to imagine infected people with a "cloud of risk" around their heads.

“The role of the climate drivers in flu transmission matters less as the contact becomes closer and closer in space and time, “says Dalziel. “That’s where city structures come in.”

Though people in all cities should stay vigilant of the flu, this study suggests that people in larger cities should be aware that their flu season will be longer than they might expect. “Influenza transmission is systematically different in large cities and maybe we can leverage that for surveillance,” he says. Likewise, healthcare systems in smaller cities could use this information to prepare to hit “surge capacity,” seeing as those places tend to experience shorter, more explosive flu seasons.

Dalziel is careful not to make any judgments about whether people in big cities or small towns will be better off this flu season, though he can’t help thinking these findings might come as an unwelcome surprise to big city dwellers.

“People talk about the experience of living in New York City, or L.A., or a metropolis as: ‘What an amazing thing it is to be a part of whatever it is that that city is,’” he says. “Part of what’s going on in a big city that’s systematically different, we’ve shown, is differences in how diseases spread.”

However, he’s careful to point out: “Our study doesn’t show that what’s happening in metropolises is necessarily a negative thing. It’s just different.”

In the paper, his team hesitates to say yet whether any cities are safer than others and doesn’t make any comment on the effectiveness of vaccination. Nevertheless, public health officials maintain that the best way to protect yourself — as well as the general population — is to get vaccinated, no matter where you live.