An in-depth study of how a swine flu outbreak carries between school children has revealed just how quickly the virus can spread - and the limited value of school closures.
The study, by Health Protection Agency officials, suggests that social events are just as likely as classroom-based activities to encourage the spread of the pandemic.
It found that one child who attended school for about four hours on a Friday while suffering from flu symptoms passed the virus on to four of the 23 pupils in the class - possibly on the previous day before any symptoms appeared. One pupil in the same year group, but a different class, was then infected by one of the classmates.
Two of the four classmates infected were in a large choir that met over the weekend. By the end of this gathering a further six choir members had been infected, including three children who were pupils at a second school.
Two of these three pupils did attend school while in the infectious but symptomless phase, but did not pass on the virus.
However, another two pupils from the original school went to a party during the same weekend, where they then passed the virus on to a pupil from a third school.
There were then three further cases at this school - and one of these passed the virus on to their sibling at a fourth school.
The officials also found that while in two cases pupils used buses while infectious, no one on the bus subsequently tested positive for swine flu.
"High attack rates were observed in the classroom, at a choir and a party," concluded Health Protection Agency officials. "Our findings add weight to the argument that social activities are important routes of transmission, which means that in the containment phase school closure alone may not be enough to interrupt transmission."
In the UK, schools were closed at the beginning of the outbreak, but that is now not considered necessary. In other countries, such as Mexico and India, closures have been more widespread, including cinemas and cafes.