Practice of Epidemiology The Impact of Illness on Social Networks: Implications for Transmission and Control of Influenza

We expect social networks to change as a result of illness, but social contact data are generally collected from healthy persons. Here we quantified the impact of influenza-like illness on social mixing patterns. We analyzed the contact patterns of persons from England measured when they were symptomatic with influenza-like illness during the 2009 A/H1N1pdm influenza epidemic (2009–2010) and again 2 weeks later when they had recovered. Illness was associated with a reduction in the number of social contacts, particularly in settings outside the home, reduc-ing the reproduction number to about one-quarter of the value it would otherwise have taken. We also observed a change in the age distribution of contacts. By comparing the expected age distribution of cases resulting from trans-mission by (a)symptomatic persons with incidence data, we estimated the contribution of both groups to trans-mission. Using this, we calculated the fraction of transmission resulting from (a)symptomatic persons, assuming equal duration of infectiousness. We estimated that 66 % of transmission was attributable to persons with sympto-matic disease (95 % confidence interval: 0.23, 1.00). This has important implications for control: Treating sympto-matic persons with antiviral agents or encouraging home isolation would be expected to have a major impact on transmission, particularly since the reproduction number for this strain was low. behavioral change; contact networks; disease transmission; influenza; R0 ratio; social interactio