In the quest for a universal influenza vaccine — one that elicits broadly neutralizing antibodies that can protect against most or all strains of flu virus — scientists have faced a sobering question: Does pre-existing immunity generated by prior exposure to influenza virus or vaccine hamper production of broadly neutralizing antibodies? If so, then a universal flu vaccine might work best (and perhaps only) in very young children who have had limited exposure to influenza viruses or vaccines.