"An unusual viral strain is dominating flu activity across the United States and may be one reason for the severe infections in children so far, according to a new report from the Centers for Disease Control and Prevention, and this season’s influenza vaccine is not a close match for the virus. There are different types of flu viruses, and the strain causing illnesses in most parts of the country is an influenza B virus. In most years, the A strain of influenza dominates during the early months, and the B strain shows up toward the end. Flu activity also started early this season, with some cases showing up by July. In more typical seasons, flu activity starts around October and lasts through March or April."