Contacts in context: large-scale setting-specific social mixing matrices from the BBC Pandemic project

Petra Klepac; Adam J Kucharski; Andrew JK Conlan; Stephen Kissler; Maria Tang; Hannah Fry; Julia R Gog

The matrices we present can be directly incorporated into mathematical models of transmission to predict the dynamics of infection with and between key demographic groups and settings [18, 23, 2, 17] . The large scale nature of the BBC data presented, with over 36,000 users and over 378,000 contacts, made it possible to generate fine-scale age-specific social contact matrices across different contexts, and by week and weekend. These could be used to explore the impact of different intervention strategies that rely on social distancing to reduce the amount of mixing in the population (such as school closures, and working from home) on flattening and postponing the peak of an outbreak. There were some notable differences between these matrices and those presented in the POLYMOD study, which surveyed 1,012 people in Great Britain. In particular, POLYMOD participants report higher overall mean contacts than BBC participants (11.7 compared to 10.5 people on average), although this difference is reduced when both dataset are calibrated to the same population structure. More than a decade has passed since the POLY-MOD study, and it is possible that the average number of contacts in the UK may have dropped. This is particularly evident in the reduction of teenage contacts, which could be the signature of a real change in how teenagers interact with one-another and their preferred way of interacting with their friends shifting from face-to-face to social media over last several years [24] . While the mean number of conversational contacts reported by participants in the BBC study was higher than in 10 POLYMOD (7.9 and 5.9 respectively), the mean number of physical contacts was significantly lower, and mostly limited to contacts at home. Moreover, POLYMOD by design oversampled children, whereas the BBC data oversampled adults, and hence may have captured more of the tail of the contact distribution in older age groups.