Now is the time to get your flu shot — and this vaccine is expected to be more effective than last season’s. Flu season is that time of year when more of your co-workers call in sick and more of your children stay home from school; it starts in October and typically runs through May. The Centers for Disease Control and Prevention doesn’t generally start to keep a running count of the new season’s cases until the end of the month, but doctors advise getting the shot now because it takes about two weeks for the body to build up immunity against the virus.