How The Flu Spreads

It’s shaping up to be one of the worst flu seasons in recent history, according to the U.S. Centers for Disease Control, which tracks the flu virus nationally. (This interactive map provides a visual of this year’s outbreak.) So far this season, 20 children have died from the flu, according to the CDC.

In light of these statistics, it’s useful to understand how the flu spreads from one person to another. A 2010 study in the Proceedings of the National Institutes of Sciencetook an in-depth look at how the influenza virus spreads.

Researchers from England and the U.S. studied an outbreak of H1N1 flu virus at an elementary school in Pennsylvania in spring 2009. They collected data in real time while the epidemic was going on, a unique method for studying the flu. In total, they collected information on 370 students from 295 households. Nearly 35 percent of the students and 15 percent of their family members came down with flu.

The interesting aspect of the study is that researchers collected data on exactly who got sick and when, plus information from seating charts, activities, and social networks at the school. They then used statistical methods to trace the spread of the disease from one child to the next.

Their findings were surprising:

  • Sitting next to a classmate with the flu did not significantly increase the risk of infection, but the social networks and the structure of classes certainly did.
  • Transmission was 25 times as intensive among classmates as between children in different grades. Boys were more likely to catch the flu from other boys, and girls from other girls. From May 7 to 9, the illness spread mostly among boys. From May 10 to 13, it spread mostly among girls.
  • Administrators closed the school from May 14 to 18, but there was no indication that this slowed transmission.
  • Only 1 in 5 adults caught the illness from their own children.

The researchers did point out some limitations of the study. Survey data were reported by the main caregiver in each household and focused on symptoms only. And the study did not take into account how the flu spread outside of the school environment, at gatherings like play dates or sports practice.

But the study does provide a unique snapshot at how a virus can spread, revealing definite patterns of what the researchers call “back-and-forth waves of transmission” between the school, the community, and the households. It is one, detailed piece in the complex puzzle of understanding how disease spreads.

Understanding how viruses like the flu spread is a first step. There are other evidence-based strategies for preventing the flu.

  • Get a flu vaccine. While not completely effective, the body of evidence suggests that flu vaccines reduce the likelihood you will get sick and can reduce the duration of your illness if you do contract the flu.
  • Avoid contact with sick people.
  • Wash your hands frequently with soap and water, especially before you eat and after using the bathroom.
  • Cover your nose and mouth with a tissue when you cough and sneeze.
  • If you are sick with a fever and body aches, stay home at least 24 hours after the fever has subsided.

The take-home message? Social networks facilitate the spread of the flu, but there are steps you can take to prevent the illness from spreading.

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