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Peer-to-peer interaction influences spread of H1N1

Cauchemez S. Proc Natl Acad Sci. 2011;doi:10.1073/pnas.1008895108.

Peer-to-peer interaction influences how H1N1 influenza outbreaks spread across schools, communities and households, researchers have found.

Using data collected during an H1N1 pandemic influenza outbreak at a semirural Pennsylvania elementary school, researchers studied 370 students (81% of the school’s enrollment) from 295 households. Data collected included seating charts, bus and bell schedules, nurse logs, attendance records and questionnaires regarding social network factors such as who the students’ playmates were. The information was interpreted using the Markov chain Monte Carlo sampling and data augmentation techniques to create probability models for how the virus spread.

In accordance with observed assortative mixing among boys and girls, researchers found that boys were three times more likely to transmit influenza to other boys than to girls; the same was true for the girl-to-girl transmission. The rate of transmission between classmates was five times higher than between children in the same grade but in different classrooms and 25 times higher than between students not in the same grade; however, sitting next to an infected student in class did not significantly raise a student’s risk of infection.

This is an interesting study, with results that should probably surprise no one. Nonetheless, it’s nice to finally see some data on what has been widely assumed to be the case. Nothing has been said about how influenza was diagnosed; nonetheless, if the study was carried out during the pandemic in the fall of 2009, and absenteeism in the school was so great that the school was closed, it’s a safe bet that a substantial majority of the respiratory illnesses were indeed pandemic H1N1 influenza.

Note that these were elementary school children, and hormones had not yet started to flow in most of them. It would be interesting to see how these transmission patterns changed as children got older and entered the post-pubertal years.

It’s perhaps a bit surprising that sitting next to an infected student did not apparently increase the risk of infection. Some assessment of “respiratory hygiene” among these children would have been helpful in understanding and interpreting the results.

—Theodore C. Eickhoff, MD
Infectious Diseases in Children Editorial Board member

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