Flu shots have become a $3 billion annual enterprise in the United States, and during the 2011-12 flu season, drug stores and retailers sold more than 20 percent of flu shots nationwide. "Certainly, (drug stores) like it whenever people go into the store, get a flu vaccine, take a peek around and fill up on groceries and other things you might need," said Assistant Professor Eric Keuffel, a health economist at Temple's Fox School of Business.