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Any parent knows that colds spread like wildfire, especially through schools. New research may
give a clearer picture of just how, exactly, infectious diseases such as the common cold,
influenza, whooping cough, and SARS can spread through a closed group of people. Marcel Salathe,
a faculty member in the Department of Biology at Penn State University, developed a new
technique to count the number of possible disease-spreading events that occur in a typical
day.
Theoretically, scientists know that every day people come into contact with many other
people, that interactions vary in length, and that each contact is an opportunity for
a disease to spread. However, Salathe decided to figure out the number of person-to-person
contacts systematically. Using a population of high-school students, teachers, and staff
as a model for a closed group of people, Salathe and his team designed a method to count how
many times possible disease-spreading interactions occurred during a typical day.
"So it's important to know with how many people one interacts and for how long because with
the more people you interact with and the longer you interact with them, the more likely
you are to spread a disease if you're infectious."
Volunteers were asked to spend one school day wearing matchbox-sized sensor devices
-- called motes -- on lanyards around their necks. Like a cell phone, each mote was equipped
with its own unique tracking number, and each mote was programmed to send and receive radio
signals at 20-second intervals to record the presence of other nearby motes. Volunteers
then were asked to simply go about their day by attending classes, walking through the
halls, and chatting with other people. At the end of the day, Salathe's team collected
the motes and recorded how many mote-to-mote interactions had occurred, and how long each
interaction had lasted.
In Salathe's study, an interaction was not defined as just a face-to-face conversation.
He explained that when people aren't talking, they might be sneezing and coughing in each
other's direction, bumping into each other, and passing around pathogens.
"In our study close proximity was defined as a maximal distance of three meters because
we know from other studies that if you exhale droplets and there are virus particles in
those droplets, those can only travel so far."
Salathe and his team found that the total number of close-proximity events was 762,868.
He explained that the same two people might have had many short interactions. However,
his team counted each brief interaction individually, since each event is a new opportunity for
a pathogen to jump from person to person.
Salathe and his team also found that, at the end of the day, most people had experienced
a fairly high number of person-to-person interactions, but they also found very little variation
among individuals. Strikingly, they did not find any individuals who had an extraordinarily
high number of contacts when compared with the rest of the group. Salathe said that while
schools may indeed be "hot beds" for colds and the flu, individual students do not seem
to vary with regard to exposure risk due to their contact patterns.
Salathe also said that data from his motes confirmed an important social-networking theory
-- that contact events are not random because many "closed triangles" exist within a community.
"A closed triangle is a situation where we have three people -- person A, person B, person
C. If person A interacts with person B, and person A also interacts with person C, then
if person B interacts also with person C, that's a closed triangle."
Salathe hopes that networking data such as his may help guide public-health initiatives
such as vaccination strategies and prevention education.
For ScienceCast, I'm Katrina Voss.