Flu seems like an inevitable scourge of winter. Even if you manage to avoid it yourself, plenty of those around you won’t. Now a new study offers clues to where and when flu will be worst…and helps you figure out the best way to protect yourself and your family.

Researchers at Oregon State University studying flu cases in 603 US cities over six years, observed that the same cities had the same flu patterns year after year. Not surprisingly, the most densely populated cities in the study had the highest proportion of flu cases. What did surprise the researchers was that the highest proportion of flu epidemics was in the smaller cities.

The researchers explain that cities are the hub of flu epidemics, which then spread out to affect surrounding areas. The virus is spread through droplets expelled into the air from a coughing or sneezing person who is infected, creating a “cloud of risk” around the person. Since flu viruses live longer in cold, dry air and die more quickly in humid air, the cloud lasts longer and spreads farther when air is cold and dry.

However, in large, densely populated cities where people typically live, work and walk around shoulder to shoulder, whether air is cold and dry matters less. The cloud doesn’t have to survive long before it reaches someone else. In these cities, flu season starts early in the fall and stays at a steady high level straight through to spring.

In smaller cities, where the population is less dense, the size and longevity of the flu cloud matters. So in those cities the virus is more dependent on cold, dry air—and virulent and violent flu cases tend to peak to epidemic proportions in mid-winter, when temperatures plunge.

This information is of course useful for epidemiologists and public health planners. Knowing when and where an epidemic is likely, for instance, can guide where and when to concentrate availability of flu shots…and help health-care systems prepare for an onslaught of patients.

This information is also helpful for you. If you live in or near a big city, it would be wise to get your flu shot early (for instance, in September). If you live in or near a small city, you may want to wait until later in the fall so that you have peak protection through the frigid winter months. And wherever you live, avoid crowds when you can, especially on cold, dry days.

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