This past summer when influenza experts at the Centers for Disease Control and Prevention (CDC) went over all their data on flu vaccine effectiveness (VE) during the 2013-14 flu season, they found a big surprise: The nasal-spray vaccine was ineffective against H1N1 viruses in young children—the group in which the vaccine was expected to be most protective.

Past studies had suggested that the intranasal vaccine, also known as live-attenuated influenza vaccine (LAIV) and commercially as FluMist, worked especially well in children 2 to 8 years old. Those findings prompted the CDC in June of this year to preferentially recommend LAIV over inactivated influenza vaccine (IIV, the injected kind) for children in that age group.

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