Contact tracing is a non-pharmaceutical social distancing measure for limiting the spread of influenza by identifying people who may have come into contact with an infected person, and giving them prophylaxis or issuing them with quarantine orders. It has been shown theoretically that through contact tracing, major outbreaks can be reduced significantly at a small additional cost (Huerta & Tsimring 2002) . We divided the contacts of each patient into one of the following three categories: household, workplace and friends. When contact tracing for a class has been activated, i.e. the index case is being tested positive for pandemic influenza, all the contacts in that class will be traced successfully. To emulate logistical delays in tracing them, we set the time for successful traces to be uniformly distributed between 5 and 7 h after positive diagnosis of the index case.