Researchers wanted to know more about how social contact impacts the spread of epidemics, and studied the outbreak of Severe Acute Respiratory Syndrome (SARS) in Hong Kong from 2002 to 2003. It began with a single sick hotel guest in one of the most densely populated cities in the world; seven other infected people then went on to take flights; nearly 800 people died as a result. This data will help us understand the dissemination of illness in the future.
For this work, the scientists measured how many social encounters 1,450 Hong Kong residents had, as well as the kind of interactions they were. Dr. Jonathan Read of Lancaster University in the UK and Dr. On Kwok from the Chinese University of Hong Kong led this study.
"It's the first time anyone has measured social contact patterns for a large representative sample of people for more than a single day per person, so will help inform mathematical models of epidemics, particularly for Hong Kong - where SARS emerged into the world - but also how those patterns change over time,” noted Dr. Read.
They determined that the average person met twelve or thirteen others every day in different locations around the city. More encounters happened between Monday and Thursday than on weekends, while the average time spent in contact with others was about nine hours every day. Age was a factor, however.
It seemed that the highest rate of interactions with others was among those aged ten to 20 and 40 to 50; a sharp decrease was seen in those over 60. While some people had more encounters with others, the researchers found that the individuals in that group were not necessarily more likely to spread the disease or contract it.
"The number of contacts you make is often thought of as an important indicator of how likely you are to get infected or to pass infection on to others. People who make a lot of contacts, so-called super-mixers, could perhaps be targeted to help control an epidemic. As this study is the first of its kind to follow people over several days, we can see how different people are to each other, compared to their own day-to-day pattern of encounters. Our study shows that there is surprisingly large day-to-day variation in the rate at which individuals interact. This means it may not help to target 'super-mixers,' as they may well be making a reduced number of contacts the next day or while they are infectious.
"This may help to explain why contact tracing to prevent a disease, such as flu or Ebola or TB, from spreading is so hard."