Intended for healthcare professionals


Social networks and collateral health effects

BMJ 2004; 329 doi: (Published 22 July 2004) Cite this as: BMJ 2004;329:184
  1. Nicholas A Christakis (, professor
  1. Department of Health Care Policy, Harvard Medical School, 180 Longwood Avenue, Boston, MA 02115

    Have been ignored in medical care and clinical trials, but need to be studied

    Since a patient or a clinical trial participant is connected to other people through social network ties, medical interventions delivered to a patient, quite apart from their health effects in that person, may have unintended health effects in others to whom he is connected. The cumulative impact of an intervention is therefore the sum of the direct health outcomes in the patient plus the collateral health outcomes in others (figure). These effects, in both the patient and in their social contacts, might be positive or negative. Doctors, trialists, patients, or policy makers might see reason to take them into account when choosing treatment or evaluating benefit.

    Collateral health effects of medical care in social networks. In the conventional perspective on medical care, the benefits and costs of health care are judged by the way in which they help to achieve a direct, intended outcome in a patient. However, since a patient is connected to others through social ties, health care delivered to one person, quite apart from its health effects on that person, may have health effects on others. The cumulative impact of the intervention is therefore a sum of the direct outcomes in the patient plus the collateral outcomes in others. These outcomes may be …

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