A new study examines how the flu spreads:
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.