February 3, 2011
What’s a Little Swine Flu Outbreak Among Friends?
By NICHOLAS BAKALAR

If you or your child came down with influenza during the H1N1, or swine flu, outbreak in 2009, it may not have happened the way you thought it did.

A new study of a 2009 epidemic at a school in Pennsylvania has found that children most likely did not catch it by sitting near an infected classmate, and that adults who got sick were probably not infected by their own children.

Closing the school after the epidemic was under way did little to slow the rate of transmission, the study found, and the most common way the disease spread was a through child’s network of friends.

Researchers learned all this when they studied an outbreak of H1N1 at an elementary school in a semirural community in spring 2009. They collected data in real time, while the epidemic was going on.

With this information on exactly who got sick and when, plus data on seating charts, activities and social networks, they were able to use statistical techniques to trace the spread of the disease from one victim to the next. Their report appears online in The Proceedings of the National Academy of Sciences.

The scientists collected data on 370 students from 295 households. Almost 35 percent of the students and more than 15 percent of their household contacts came down with flu. The most detailed information was gathered from fourth graders, the group most affected by the outbreak.

The class and grade structure had a significant effect on transmission rates. Transmission was 25 times as intensive among classmates as between children in different grades. And yet sitting next to a student who was infected did not increase the chances of catching flu.

Social networks were apparently a more significant means of transmission than seating arrangements. Students were four times as likely to play with children of the same sex as with those of the opposite sex, and following this pattern, boys were more likely to catch the flu from other boys, and girls from other girls.

The progress of the disease from day to day followed these social interactions: from May 7 to 9, the illness spread mostly among boys; from May 10 to 13 mostly among girls.

“Our social networks shape disease spread,” said Simon Cauchemez, the lead author. “And we can quantify the role of social networks.”

Thirty-eight percent of children 6 to 10 were infected, compared with 23 percent of 11- to 18-year-olds and 13 percent of those older than 18. Adults were only about half as susceptible as children, but when they got sick they were just as likely to transmit the virus to others.

The school closed from May 14 to 18, but there was no indication that this slowed transmission. It may already have been too late — May 14 was the 18th day of the outbreak, and 27 percent of the students already had symptoms.

The scientists found no difference in transmission rates during the closure and during the rest of the outbreak. This, they write, confirms earlier studies showing that a school has to be closed quite early in an epidemic to have any effect on disease transmission.

Only 1 in 5 adults caught the illness from their own children, and this goes against one of the most common arguments for closing schools: that it will prevent the disease from moving from the school to households.

“Here we find that most of the infected adults were not infected by one of the children in their household,” said Dr. Cauchemez, a research fellow at Imperial College London. “This information could be used to understand whether it might be better to close a school, or to close individual classes or grades.”

Other experts were impressed with the work. “I think it’s a nice step,” said Ira M. Longini Jr., a professor of biostatistics at the Fred Hutchinson Cancer Research Center in Seattle. “It’s a beautiful analysis of an important dataset. This virus spreads very fast among school-age children, so the topic is important.”

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