Contributed by Guest Blogger: Aaron Grober ’11

The H1N1 subtype of the Influenza type A virus, known colloquially as “swine flu,” was the most common cause of human influenza infection in 2009, and remained a major concern in sparking a pandemic throughout the 2009/2010 flu season.

This recent paper examines the role of grade, class, and social network in transmission of this virus in a school setting. Taking a closer look at the actual transmission pattern of this novel subtype of influenza is critical in developing models to better predict and combat pandemic spread. In the case of this school, closure due to outbreak did not significantly affect transmission among students, indicating that it may have occurred too late to be effective, stressing the importance of more exact models. The study encompassed 370 students from 295 households, surrounding an H1N1 pandemic that occurred in a Pennsylvania elementary school in April and May 2009.

The researchers found that the structuring of the school into grades and classes significantly affected the probability of transmission: 3.5% between students within a class, five times less than that between students of the same grade but different class, and five times less than that between students of different grades.

The researchers took an in-depth look at fourth-graders. They note that children are four times more likely to play with members of the same sex, and found that this behavior had a significant impact on disease transmission; the onset of epidemic transmission occurred among boys significantly before that of female classmates. In addition, they found no significant difference between recorded playmate transmission rates, and the expected proportion for if being a playmate was not a risk factor. The researchers used class seating charts to determine if proximity to an infected individual affects the risk of transmission; as it turns out, they found that sitting next to an infected individual did not significantly affect one’s risk.

In addition to school structure, the researchers looked at spread within households. The probability of a child to adult transmission within a household depended significantly on the household size, where probability of spreading the disease is much lower in larger households than smaller ones. The predominant means of adult infection was from outside the home.

These unique findings shed light on the extremely complex transmission pattern within structured populations. The biggest factors for transmission within school are grade and class, but not seating arrangement, sex, but not playmate transmission. A number of obvious questions remain: Why does sharing a class, but not a desk-space affect transmission? Why is one more likely to transmit the disease in a smaller household than a larger one? This study is an extremely insightful epidemiological tool to help explain transmission, but our knowledge of how this virus spreads remains incomplete; it seems that the flu is far more complex than we imagined.