Researchers used computer simulations and U.S. census data to come to the conclusion that schools might need to stay closed for at least eight weeks in order to put a stop to the flu. They found that closing schools at the beginning of an outbreak was not as important as keeping them closed for a long enough period. The researchers reason that short-term closures could increase the spread of the disease because students who haven’t yet caught the flu return at their most vulnerable, in the midst of the epidemic. Computer simulations used in the study were based on a model of Allegheny County, Pennsylvania. They followed the movements of residents each weekday from home to work or school to simulate an epidemic.