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Thursday, March 27, 2008

Contact Patterns and the Flow of Infectious Diseases

Researchers in Europe have analyzed and modeled social contact patterns in order to better understand the spread of respiratory infections. The study is published in the open-access journal PLoS Medicine.

It is common to use mathematical modeling techniques to understand and predict the impact of infectious diseases. The models help policy officials and researchers make recommendations regarding intervention, treatment, and prevention of infectious and contagious diseases. In assessing the likelihood that a disease will spread among a population, researchers use contact rates - how many people that a person contacts in a day - as a key predictor. Contact rates are usually assumed to follow a certain pattern, but researchers have not yet directly observed these patterns. To improve our understanding of contact rates and to analyze patterns among age groups and social settings, researchers have decided to directly ask people how many contacts they have in a day and then generated models based on these data.

Joël Mossong (Microbiology Unit, Laboratoire National de Santé, Luxembourg and Centre de Recherche Public Santé, Luxembourg) and colleagues from several European nations recruited 7,290 people from Belgium, Germany, Finland, Great Britain, Italy, Luxembourg, The Netherlands, and Poland. The participants recorded basic demographic information and noted physical (kiss, handshake, e.g.) and nonphysical (two-way conversation without skin-to-skin contact) contacts for one day.

The researchers found that the participants averaged 13.4 contacts per day (97,904 contacts in total). The diversity of contact information is summarized below:

* Children had more contacts than adults
* People who live in large households had more contacts
* People had more contacts on weekdays than on Sundays
* Longer duration and higher frequency contact tended to be physical
* People of the same age tended to mix more frequently than people of different ages
* About 70% of daily contacts lasted less longer an hour
* About 75% of contacts with strangers lasted less than 15 minutes

"One of the most important findings of our study is that the age and intensity patterns of contact are remarkably similar across different European countries even though the average number of contacts recorded differed. This similarity implies that the results may well be applicable to other European countries, and that the initial phase of spread of newly emerging infections in susceptible populations, such as SARS was in 2003, is likely to be very similar across Europe and in countries with similar social structures," write the authors.

Mossong and colleagues then modeled the contact patterns using mathematical and statistical techniques. They were also able to create a model of how a respiratory infection epidemic may spread throughout a population. Results predicted that during the initial spread of a respiratory infection, 5- to 19-year olds would suffer the greatest burden. These younger people have many more contacts per day and more contacts within their own age group.

The authors conclude: "Our survey is, to our knowledge, the first population-based prospective survey of mixing patterns pertinent to the spread of airborne and close-contact infectious diseases performed in several European countries using a similar diary methodology. The quantification of these mixing patterns shows a remarkable similarity in degree of assortativeness, which likely results in similar patterns of spread in different populations. This finding represents a significant advance in our understanding of the spread of these infectious diseases and should help to improve the parameterization of mathematical models used to design control strategies."

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