By
msnbc.com contributor
updated 2/10/2011 8:33:24 AM ET 2011-02-10T13:33:24

If your 7-year-old son comes home from school with flu, he probably caught it from another boy rather than one of the girls, says new research that sheds light on how the flu virus spreads.

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Scientists researching the spread of H1N1 in an elementary school classroom found that boys typically transmit the infection to other boys and girls pass it on to girls. In fact, grade-school guys are three times more likely to spread flu to classmates of the same sex than the opposite sex, according to a recent study in the journal Proceedings of the National Academy of Sciences.

While the researchers were studying H1N1 transmission specifically, the findings apply to seasonal flu strains as well, said Dr. Simon Cauchemez, lead author of the study.

Public health researchers from Britain and the United States looked at one rural elementary school in Pennsylvania forced to close its doors for a week in 2009 because of a swine flu outbreak. More than a quarter of the kids had flu symptoms. They studied 370 grade-school pupils (81 percent of students) from 295 households. 

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The researchers found that the H1N1 transmission rate was about five times higher between students in the same classroom than between kids in a different class in the same grade, and was roughly 25 times higher than between children at other grade levels.

"This study helps us to better understand how influenza spreads at school, in the community, and in households," said Cauchemez. "And it's important to support decision making, such as whether it's appropriate to close a school or if it's better to close individual classes or grades."

Sitting next to a kid in class with swine flu or having an infected pupil as a playmate did not significantly increase a child's odds of illness.

"I was surprised that sitting next to a sick classmate was not found to be a risk factor for infection," said Cauchemez, an infectious disease researcher at Imperial College London. But in elementary school, "children actually spend a lot of time doing activities where they do not sit at their desk," he explained.

Molly McMahon has spent nearly a decade in the classroom and taught third grade. It makes sense to her that boys transmit flu to boys and girls infect girls.

"The results reflect this age group," said McMahon, who is currently an assistant principal at the Fitzgerald Elementary School in Waltham, Mass. "Girls are more likely to share a toy, a pencil, or even part of a snack with another girl, as do boys."

And although elementary school teachers often encourage more social interaction by pairing boys and girls for projects or group work, at this stage in their development it's not unusual for girls to think boys are gross or for boys to not want to play with girls. 

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